Polarographic Investigation of the Oxidation Levels of Rhenium

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PURDUE UNIVERSITY

t h is is t o

c e r t if y t h a t t h e t h e s is p r e p a r e d

BY

C h a rle s

E N T IT L E D

L e s lie

under

MY S U P E R V IS IO N

R u l f s ------------------------------------ -------------- -------------

POLAROGRAPHIC IM 5STIG-ATI0N OF THE OXIDATION,

LEYSLSOFRHSNTD^

C O M P L IE S W IT H T H E U N IV E R S IT Y R E G U L A T IO N S O N G R A D U A T IO N T H E S E S

A N D IS A P P R O V E D B Y M E A S F U L F IL L IN G T H IS P A R T O F T H E R E Q U IR E M E N T S

FO R TH E D EG REE O F

D o c to r

^

^

O f P h ilo s o u h v

G

____________________ _____

P r o f e s s o r in C h a r g e o f T h e s is

-

htcat ) o f

S cho ol or D epartm ent

A u g u s t f_____ 6 ______i » 4 9 _

T O T H E L I B R A R I A N : ----T H IS T H E S IS IS ^ T

T O B E R E G A R D E D A S C O N F ID E N T IA L .

j ^ O F B S S O R 1ST

R E G IS T R A R F O R M 1 0 —7 - 4 7 —IM

POLAROGRAPHIC INVïSTIGATION OP THE OXIDATION LEVELS OP RHENIUM A T hesis Submitted to the F a cu lty of Purdue U n iv e r sity by Charles L e s lie R u lfs In P a r t ia l P u lfillJn en t of th e Requirements fo r th e Degree of Doctor o f P hilosophy August, 1949

ProQuest Number: 27712216

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uest ProQuest 27712216 Published by ProQuest LLO (2019). C opyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C o d e M icroform Edition © ProQuest LLO. ProQuest LLO. 789 East Eisenhower Parkway P.Q. Box 1346 Ann Arbor, Ml 4 8 1 0 6 - 1346

ACKNOWLEDGMENT The author w ishes to thank th e Atomic Energy Commission fo r th e grant o f a p red o cto ra l fe llo w s h ip in support o f t h i s work. He a ls o w ishes to thank the Purdue Research Foundation which adm inistered the fello v /sh ip and the R esearch Corporation fo r th e use o f equipment purchased on a F rederick Gardner C o t tr e ll g r a n t.

He i s

g r e a tly indebted to P r o fe sso r P h ilip J . E lvin g fo r h is generous a s s ista n c e and nunerous h e lp fu l su g g estio n s throughout th e course of th is in v e s t ig a t io n . Gantz fo r h is help*

He a ls o w ish es to thank P ro fesso r E. S t . C lair

TABLE OF CONTENTS Page i ABSTRACT ...................................... iii P art 1.

R e d u c tio n o f

......................................

P a rt 2.

O x id a tio n o f R hen id e ...........................................

P art 3.

The R henide S p e c ie s i n H y d ro h a lid e A cid .

iv vi 1 INTRODUCTION...........................................................................

PMTi

1

TrnKKAMauRacRaMenMWHBRmNMB. .

1 I n t r o d u c t i o n ....................................................

1 E xp erim en ta l................................................................................

7 D is c u s s io n ....................................................................................

7

D if f u s i o n C o e f f i c i e n t f o r th e ^ ^ r r h a n a te I o n . . .

: : : : :

:

: : : : : :

8

12 14 15

: : E

P e r r h e n a te R e d u c tio n s i n O th e r M edia ...................

16 Summary PART I I

IHE OXIDATION OF RHENIDE AT THE DROPPING MERCURY ELECTRODE...................................................

21 21

In tr o d u c tio n . 22 E xp erim en tal, 23 D is c u s s io n . , G e n e ra l C h a ra c te r o f A nodic R henide W aves.................................

:::

-

30

34 Summary PART I I I

THE NATURE OF THE RHENIDE SPECIES ID ................... HYDROHALOGEN A C ID S ............................................................. 43

I n tr o d u c tio n . 43 E xp erim en tal.

D is c u s s io n . . . • • ...............................................

,

.

•

Evidence and Nature o f Rhenide Complex in

•

•

•

•

•

•

44

........................... ^

R henide i n t h e F o u r H y d ro h a lid e A cid s ........................................... R a te o f F o rm a tio n o f ....................................................................... A b s o rp tio n S p ectru m o f ReCl ..................................................................

............................................52

Summary ......................................................................... PART 1 7

mSCELLANEODS ASPECTS OF THE CHEMISTRY OF RHaTIDM. . .

60

I n t r o d u c t i o n ........................................................................ E x p e rim e n ta l D is c u s s io n . . . . .

60

. . . . .............................................................

60 61

Rhenide E x t r a c t i o n ................................................................................... S y n th e s is o f Lower Rhenium H a l i d e s ................................................ A tte m p te d S y n th esis o f R h e n id e .............................................................° S o l u b i l i t y o f T lia llo u s... Rhenide......................................................... ........................................... 64 Summary .................................................................................. BIBLIOGRAPHY............................................................................... VITA

. . . . . . . .

66

IIST OF TABLES AT® FIGURES Page L i s t o f T a b le s 1

R e d u c tio n o f KReO^ i n 4 N HCIO4 a t

......................... 1'^

2

R e d u c tio n o f Rhenium IIV ) an d Rhenium ( V I I ) .

3

R e d u c tio n o f RReO^, i n 2 N KOI a t 2o

4

P e r r h e n a te R e d u c tio n s i n 0 .0 5 M LaG lg a t 25°

. . .

17

......................... . . .

20

5

H alf-W ave P o t e n t i a l s o f A nodic R h en id e Waves i n HgSO^ ...........................................................................

6

S lo p e o f 0 .4 luM R henide Waves i n 2 .0 D H3 3 O4 a t 15°“ ........................ .......................................................

^

7

E f f e c t o f T em p eratu re and Hg-Head. on C u r r e n t

40

8

C hem ical E s tim a te o f E f o r t h e Re ( - I ) / ( D C o u p l e ...................................................................... ....

9

O x id a tio n P o t e n t i a l s of Rhenium C o u p le s ................... 42

10

P e r r h e n a te R e d u c tio n i n V a r io u s M e d ia........................54

11

R henide Waves i n H y d ro c h lo r ic A c i d ............................ 56

12

H alf-W ave P o t e n t i a l s o f C a th o d ie "R h en id e" W a v e s ................................................................................................

13

. . .

S o l u b i l i t i e s o f T h a llo u s S a l t s i n W ater a t O'

65

L i s t o f F ig u r e s 1

P e r r h e n a te R e d u c tio n i n P e r c h l o r i c A c id .............................. 18

2

P e r r h e n a te and rihenium (IV ) R e d u c tio n s i n HGl. . .

3

M o d ifie d Jo n e s R e d a c to r ............................................................

4

R henide C urves in S u l f u r i c A c id ................................................. 57

5

R henide C urves i n P h o s p h o ric A c id .............................................58

6

A m perom etric O x id a tio n o f R h en id e in S u l f u r i c A c i d .............................................................................

7

P o la ro g ra p h y o f R henide i n H y d ro h a lid e A c i d s ................................................................................................

8

19

^ o f ReCl v s . Concn. o f P y r i d i n e ........................................ 57

0 .5

——

Page 9

10

R a te o f F o rm a tio n o f R e C l ...................................... R henide A b s o r p tio n i n TvjoM e d i a ...............................

58

POIAROŒÎAPHIC INVESTIGATION OF THE OXIDATION USVEIS OF RHENIÜl^

(1 )

(BUS m a teria l la baaed on a p o rtion o f th e d o cto r a l th e a ia

presented by G, L. R u lfs, A ugust, 1949.

By Charles L. R u lfs , P h ilip J . l i v i n g and 2 . S t . C la ir Gantz AN ABSTRACT In 1937 LundeU and Knowles^ obserred t » t th e m e ta llic

(2)

L im d ell. S.

? . . and Knowles. H. B .. £ . Besearch N at. Bur.

Standards, IS , 629 (1 9 3 7 ). element rhenium appears to be capable o f e x is t in g in a u n in eg a tlre sta te ,

(3)

su b seq u en tly , se v e r a l workers»'^ have confirmed the gen eral

Tomlcek. 0 . . and Tomlcek. ? . . C o ll. Onech. Ohem.

U . 626

(1 9 5 9 ). (4) Lingane, J . J .» £ • A®* Ghem. S o c .,

2182 (1 9 4 2 ).

v a lid it y o f t h is o b servation w ith o u t, however, co n trib u tin g ap p reciably to a knowledge o f th e nature o f th e rhenide s t a t e .

The

remaining low er o x id a tio n s t a t e s o f rhenium are but l i t t l e b e tte r known, and the o x id a tio n p o te n tia ls o f th e variou s couples rep resen t

11

o n ly th e crudest

approilm atlons® .

Llngana^’® co n trib u ted an e a r ly

(5) Latim er, W. M., -O rid a tio n P o t e n t ia ls ” , P r e n tic e -H a U , I n c .. New York, N. Y ., 1938. (6 ) Lingane, J . J . , J . m . C ^ .

1001 (1 9 4 2 ).

survey o f th e polarographic behavior o f th e rhenide and perrhenate io n s and, r e c e n tly , Maun and Davidson"^ have reported a chem ical and

(7) Maun, E. K ., Ph.D. T h esis, C a lifo rn ia I n s t it u t e o f Technology, B erk eley, C a lifo r n ia , 1949. spectrophotom etrie study embracing many o f th e rhenium o x id a tio n le v e ls . The p resen t in v e s tig a tio n was undertaken w ith the purpose o f fu rth er c h a ra cterizin g the polarographic behavior o f rhenium and in the hope o f e s ta b lis h in g b e tte r v a lu es fo r the o x id a tio n p o te n tia ls o f the rhenium co u p les.

This paper i s a summary o f the work which i s

l o g ic a lly concerned w ith th ree a sp e c ts o f rhenium chem istry: (1) The polaiw graphic red u ction o f th e perrhenate ion in n eu tra l and in a cid s o lu t io n s . (2) The polarographic o x id a tio n of rhenide io n a t th e dropping mercury e le c tr o d e . (3) The nature o f th e rhenide sp e c ie s p resen t in hydrohalogen a cid media. The con clu sion s obtained in each o f th e se s tu d ie s w i l l be b r ie f ly summarized.

iii

r e d u c t i o n o f PERRHENATE

ré d u c tio n o f perrhenate in 4 N p e r c h lo r ic a c id occurs a t -0 .3 8 v o l t v e . 8 . C. E . and appears to in v o lv e th ree e le c tr o n s a t 25° (T& ble 1 ) .

s u b s t it u t io n o f th e d iff u s io n c o e f f ic ie n t f o r t h e

p e r r h e n a te io n i n the I lk o v ic equation y i e l d s a n a v e r a g e " n " - v a lu e o f 3 .0 4 e l e c t r o n s .

S in c e a readable and reproducible l i m i t i n g c u r ­

r e n t p l a t e a u d e f i n e s a s u b s t a n t i a l l y c o n s ta n t v a lu e f o r i^ /C o v e r a w ide ra n g e o f p e r r h e n a te c o n c e n t r a t i o n s , t h e wave i s w e l l s u i t e d f o r a n a ly tic a l a p p lic a tio n s . The d i f f u s i o n c u r r e n t w as fo u n d t o v a r y w ith t h e h e i g h t o f t h e m e rc u ry colum n i n th e e x p e c te d m anner f o r a d i f f u s i o n - c o n t r o l l e d p ro ce ss.

The s lo p e o f t h e wave and t h e n e g a tiv e s h i f t i n h a lf-w a v e

p o t e n t i a l w ith i n c r e a s i n g perrhenate c o n c e n t r a tio n c h a r a c t e r i z e t h e c u r r e n t- d e t e r m in in g p r o c e s s a s i r r e v e r s i b l e .

I n s o l u t i o n s more

c o n c e n tra te d than 0 . 4 m i l l i m o l a r , th e wave i t s e l f e x h i b i t s p e c u lia ritie s .

A f u r t h e r c u r r e n t r i s e s t a r t s a t a b o u t - 0 .6 v o l t ; t h i s

i s fo llo w e d a t a b o u t - 0 . 8 v o l t by a m arked d e c r e a s e i n c u r r e n t .

S in c e

rhenium (IV ) was a l s o fo u n d to re d u c e a t - 0 .6 v o l t , t h i s w ould seem t o c o n firm th e i n t e r p r e t a t i o n o f t h e p rim a ry wave a s a t h r e e - e l e c t r o n , r e d u c tio n o f perrhenate to rh en iu m ( I V ) .

G o u lo m e tric s tu d y , h o w ev er,

d oes n o t c o n firm t h i s r e s u l t ; c o u lo m e try o f a p e r r h e n a te s o l u t i o n r e s u l t s i n c o n tin u o u s e l e c t r o l y s i s a s w i l l b e s u b s e q u e n tly d is c u s s e d . The te m p e r a tu re c o e f f i c i e n t o f t h e d i f f u s i o n c u r r e n t was fo u n d t o be v e r y sm a ll o r s l i g h t l y n e g a t i v e .

The co n seq u e n ce o f t h i s l a t t e r

o b s e r v a tio n i s , o f c o u r s e , t h a t "n " w ould n o t h a v e th e i n t e g r a l v a lu e o f t h r e e a t an y te m p e r a tu re o th e r th a n 25°C . P e r r h e n a te r e d u c tio n s i n 2 N p o ta s s iu m c h l o r i d e a p p ro x im a te

iv

an e ig h t - e le c t r o n p rocess (Table 2 ) .

The wave i s p o orly d efin ed and

i s c lo s e ly follo w ed by a hydrogen d ischarge wave.

Goulometric

re d u c tio n s, even when confined to p o te n tia l s e t t in g s corresponding o n ly to the beginning of th e wave, r e s u lt in a continuous p ro cess in v o lv in g the consumption o f hydrogen io n s .

The rhenium, meantime,

undergoes o n ly a fr a c tio n o f th e exp ected r e d u c tio n . Some o f th e apparent anom alies in th e ca ses j u s t c it e d might be r eso lv ed by a more c r i t i c a l exam ination o f a fundamental premise in the coulom etric te c h n ic .

The assum ption, th a t a g ross mercury

cathode w i l l behave in an id e n t ic a l fa sh io n w ith a sm a ll, f r e s h ly formed d ro p let of mercury, i s probably open to con sid erab le q u estio n w ith r e s p e c t, in p a r tic u la r , to such m atters as overvoltage e f f e c t s and hydrogen d isch a rg e. OXIDATION OF RHENIDE ' A p a r tic u la r ly f r u i t f u l p rocedw e fo r studying the lower o x id a tio n le v e ls o f rhenium i s th e polarographic o x id a tio n o f rhenide s o lu t io n s .

The m ateria l to be d isc u ssed in t h is s e c t io n w i l l apply to

rhenide s o lu tio n s in s u lf u r ic , m eth an esu lfon ic, p e r c h lo r ic , and phosphoric a c id s .

Such media might be termed non-complexing in

d is t in c t io n to the hydrohalogen a c id s .

Die b a sis fo r t h is d is t in c t io n

w i l l appear during th e d is c u s s io n . The g en eral pa ttern o f the anodic rhenide curves in the noncomplexing media in clu d es f i v e waves which s h a ll be d esign ated as A, A*, B, G, and D.

In an e a r lie r study^, th ese waves were in te rp r eted as

in v o lv in g s u c c e s s iv e ly 3 - , 1 - , 8 - , and 2 -e le c tr o n o x id a tio n s o f rhenium follo w ed by an apparent mercury d isch a rg e.

This in te r p r e ta tio n

could no longer be considered ten ab le when i t was found th a t n e ith e r

rhenium ( I T ) , nor even rhenium ( I I I ) , compounds gave any anodic p r io r to th e mercury d isch a rg e.

In a d d itio n , the slo p e o f the

r e v e r s ib le A-wave i s c o r r e c t f o r a 2 - , rath er than fo r a 3 - , e le c tr o n p r o c e ss.

Moreover, coulom etric o x id a tio n e s ta b lis h e d the valu e fo r

th e i n i t i a l ste p a t 1 .8 e le c t r o n s .

The discrepancy between t h is

fig u r e and 2 .0 e le c tr o n s i s e x a c tly compensated when th e p o t e n tia l i s s e t t o in clu d e the sm all A '-w ave.

At t h is p o te n t ia l a b la c k ,

amorphous product separated from th e s o lu t io n ,

t t i s m a te ria l was

^

id e n t if ie d as th e rhenium (1) o x id e, or suboxide, o f Young and Ir v in e .

(8)

Young. E. 0 . . and I r v in e . 1 . W.. £ . Am. Ohem. S o c .. 59. 2648

(1 9 3 7 ). On th e b a s is o f t h i s evidence i t appears th a t the A-wave i s e s s e n t i a l ly the tw o -e le c tro n o x id a tio n o f rhenide to rhenium (I ) w h ile the A’-wave in clu d es the e x tr a energy n ecessary fo r conversion to the suboxide.

The o v e r a ll in te r p r e ta tio n o f th e wave p attern i s then

changed to 2 - , ( 0 ) , 1 - , and 1 -e le c tr o n rhenium o x id a tio n s follow ed by a mercury d isch arge which may obscure a fu rth er o x id a tio n s te p .

The

p o te n tia ls fo r th e lower rhenium couples have been evaluated on the b a s is o f t h is in te r p r e ta tio n (Table 3 ) .

The valu e fo r the rhenium

(-1 ) / ( I ) coup le, in p a r tic u la r , i s considered to be q u ite r e lia b le and has been evalu ated fo r u n it a c t i v i t y of hydrogen ion a t 25°C. The remaining v a lu es apply s p e c i f i c a l l y to 0 .4 m illim o la r rhenium in 2 .0 N s u lf u r ic a c id a t 15°G.

These va lu es must be considered l e s s

r e lia b le in view o f th e i r r e v e r s i b i l i t y o f t h e ir polarographic w aves.

vi

TOE rhenide SPECIES IN HYDROHALIDE ACID perhaps th e most in t e r e s t in g o b serv a tio n s made in the course o f th e p resen t study are th ose concerned w ith the nature o f the rhenide ' s p e c ie s in th e presence o f th e hydrohalide a c id s .

I t was found th a t

higher con cen tration s o f rhenide could be a tta in e d in h yd roch loric a c id than was the case in s u lf u r ic a c id .

S t i l l higher con cen tration s could

be reached in th e presence o f pyridinium c h lo r id e .

These r e s u lt s sug­

gested s t a b iliz a t io n o f th e rhenide s t a t e through com plex-forim tion. I t was found th a t polarograms o f rhenide in hydrochloric a c id s o lu tio n showed a d i s t i n c t cathodic wave; t h is was e s p e c ia lly so i f the s o lu tio n s had aged fo r a sh o rt tim e.

The i n i t i a l an od ic, or A -, wave

i s sim ila r in every way to th a t shown in s u lf u r ic a c id .

The presence

o f both a cath odic and an anodic wave im p lies the e x iste n c e in hydro­ c h lo r ic a c id medium o f a s p e c ie s which can be e ith e r o x id ize d or reduced.

The analogy o f th e rhenide io n i t s e l f w ith the h a lid e s i s

a t once su g g e stiv e o f a p la u sib le exp lan ation ; namely, th e e x iste n c e o f a rhenium m onochloride, ReCl, analogous w ith iod in e m onochloride. Hxe e s s e n t ia lly co v a len t ch aracter o f such a compound ex p la in s i t s a b i l i t y to be e ith e r o x id iz e d or reduced, w h ile the observed i r ­ r e v e r s i b il i t y o f the red u ctio n wave accounts fo r i t s displacem ent along the p o te n tia l a x i s . Comparable in terh a lo g en -ty p e compounds would be a n tic ip a te d , and were found to e x i s t , in each o f the remaining th ree hydrohalide a c id s .

The o n ly d e v ia tio n was in h y d roflu oric a c id s o lu t io n , in which

q u a n tita tiv e red uction could not be o b tain ed . The r a te o f form ation o f ReCl follow ed f i r s t order k in e t ic s in the presence o f ex cess hydrochloric a c id .

A s p e c if ic r a te constant

v il

o f 0 .0 1 1 m ln.'^ o b ta lM a t 15"0.

Maasurements a t two tem peratures

perm it e stim a tin g th e energy o f a o tiv a tio n as 1 0 .2 k ilo o a ls . per m ole. m e fu r th e r oomplexing w ith p yrid in e i s e v id e n t polarographio a lly hy th e p resence o f a second, more n e g a tiv e cath od ic w ave. S ystem atic stu dy o f th e s h i f t in half-w ave p o te n t ia l o f t h i s wave as a fu n c tio n o f th e p yrid in e co n cen tra tio n le a d s to the co n clu sion th a t th ree m olecules o f p y rid in e are coordinated per rhenium atom. formula o f the complex may be

R eC l(% 0)(p yr)s

or

The

E e c la tp y r ); " .

Such compounds could in v o lv e planar fou r-co o rd in a tio n in a 5d i“ . 6s^, 6p^ s t r u c t u r e as p red icted by P auling

(9) P a u lin g , L . , Ohem. E ng. News,

fo r rhenide.

2970 ( 1 9 4 7 ) .

A f u r t h e r c o n f ii m a ti o n f o r th e e x i s t e n c e o f a u n iq u e r h e n id e s p e c ie s i n h y d r o c h lo r ic a c i d a s com pared w ith " f r e e " r h e n id e i n s u l f u r i c a c id r e s u l t s from an e x a m in a tio n o f th e n e a r u l t r a - v i o l e t a b s o r p ti o n s p e c tr a i n t h e s e two m e d ia .

Q u ite s i g n i f i c a n t d i f f e r e n c e s

a r e o b s e rv e d . S u r p r i s i n g l y , i t was fou n d t h a t t h e s o l u b i l i t y o f t h a l l o u s r h e n id e m ust be much g r e a t e r th a n t h a t o f t h a l l o u s i o d id e and i s p r o b a b ly a t l e a s t tw ic e t h a t o f th e b ro m id e . A cknow ledgm ent.

One o f t h e a u th o r s (C .L .R .) w is h e s t o th a n k

th e A tom ic E n erg y Com mission f o r a p r e d o c t o r a l f e llo w s h ip upon w h ich th e w ork d e s c r ib e d was d o n e .

The a u t h o r s a l s o w ant t o th a n k th e

R e s e a rc h C o r p o r a tio n f o r t h e u s e o f eq u ip m en t p u rc h a s e d u n d e r a F r e d e r ic k G ard n er C o t t r e l l g r a n t . W est L a f a y e t t e , I n d ia n a

viii

T&ble 1 Reduction o f KReO^ in 4 N HCIO4 a t 25° C. KR604

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Reduction o f KRe0 4 in 2 N KOI a t 25°C. KReO.

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ix

T a b le 3 O x id a tio n P o t e n t i a l s f o r Rhenium C ouples

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POLAROGRAPHIC INVESTIGATION OF THE OXIDATION L3VSIS OF RHENIUM

INTRODUCTION S in c e i t s i s o l a t i o n by Tacke an d Noddack i n 1 9 25, rh en iu m h a s b e e n t h e s u b j e c t o f some 400 s c i e n t i f i c p a p e r s .

D e s p ite t h i s

volume o f w o rk , many f e a t u r e s o f rh en iu m c h e m is try re m a in o b s c u r e . E a r ly s t u d i e s by t h e N oddacks (27) in c lu d e a number o f i n e x a c t m e a su re ­ m ents o f p h y s i c a l c o n s ta n t s and th e q u a l i t a t i v e o b s e r v a tio n o f many p o o r ly d e f in e d compounds w hich l a t e r w o rk s rs h a v e b e e n u n a b le to c o n firm .

Move r e c e n t s u rv e y s o f t h e c h e m is tr y o f rh en iu m (5 ,2 5 )

le a v e th e iitg )re s sio n t h a t much new 'vrork rem a in s t o be done an d much o l d e r w ork r e q u i r e s f u r t h e r c o n f ir m a tio n . E v id e n ce h a s b e e n g iv e n f o r rh en iu m compounds e x h i b i t i n g a l l o x id a tio n s t a t e s from e i ^ t to m inus one.

The s e p t i v a l e n t s t a t e

i s t h e h ig h e s t l e v e l f o r w hich a c c e p ta b le e v id e n c e e x i s t s .

Rhenium

h e p to x id e , p e r r h e n ic a c i d , an d t h e p e r r h e n a te s a l t s a r e p e rh a p s t h e b e s t c h a r a c t e r iz e d o f a l l rh e n iu m compounds.

V a rio u s a l k a l i r h e n a te s

and rhenium t r i o x i d e a r e s u f f i c i e n t l y w e l l e s t a b l i s h e d t o v a l i d a t e th e s e x i v a l e n t s t a t e .

S u p p o rt f o r t h e o x i d a t i o n l e v e l o f f i v e i s

c laim ed i n th e e x i s te n c e o f h y p o r h e m te s i n a d d i t i o n to th e p e n t a h a lo g e n compounds # i i c h a re o b ta in a b le o n ly i n t h e d r y s t a t e . p e n to x id e i s known.

No

Rhenium d io x id e , a l k a l i r h e n i t e s , and a num ber

o f com plex h e x a h a lo g e n rhen iu m (IV ) s a l t s c h a r a c t e r i z e th e f o u r s ta te .

The t h r e e s t a t e i s e x e m p lifie d in t h e d im e r ic t r i h a l i d e s and

an u n s ta b le s e s g u io x id e .

The fo rm e r c a n o n ly be p re p a re d i n t h e d r y

w ay, an d th e p o t e n tio m e tr ic t i t r a t i o n o f lo w e r rh en iu m s t a t e s g iv e s

no e v id e n c e f o r a t r i v a l e n t rh en iu m s p e c i e s i n s o l u t i o n .

Thus th e

t h r e e s t a t e i s co m parable i n i t s b e h a v io r to t h a t o f m a n g an e se.

The

s t a b i l i t y and im p o rta n c e o f t h e b i v a l e n t s t a t e f o r m anganese i s n o t r e f l e c t e d i n i t s c o n g e n e r, rh en iu m .

I n d e e d , t h e h y d r a te d m onoxide

o b ta in e d i n an im pure s t a t e by Young a n d I r v i n e (3 7 ) i s t h e lo n e - re p re s e n ta tiv e f o r t h i s o x id a tio n le v e l .

The same a u t h o r s r e p o r t a n

im pure su b o x id e d ih y d r a t e , t h e o n ly unam biguous exam ple o f u n i p o s i t i v e rh en iu m .

The t h r e e rh en iu m p e n ta c a r b o n y l h a l i d e s p r e p a r e d b y

S c h u lte n (32) c o n s t i t u t e th e re m a in in g p o s s i b l e ex am p les o f t h i s

c la s s .

Compounds v h ic h m ight be c o n s tr u e d a s z e r o v a le n t in c lu d e th e t r i a l k y l s and t r i a l k o x i d e s r e p o r t e d b y D ruce (5) and t h e d im e r ic rh en iu m p e n ta c a r b o n y l ( 3 2 ) . L u n d e ll an d Knowles (22) r e p o r t e d i n 1937 t h a t q u a n t i t a t i v e t i t r a t i o n d a ta i n d i c a t e d t h a t u n d e r p r o p e r c o n d itio n s p e r r h e n a te in d i l u t e aqueous m edia u n d erw en t a r e d u c tio n e q u i v a le n t t o an e i g h t e le c tro n tr a n s i tio n .

The o b v io u s i m p l i c a t i o n o f t h i s r e s u l t i s t h a t

th e h ig h ly m e t a l l i c e le m e n t rh en iu m m ust be c a p a b le o f e x i s t i n g i n a u n in e g a tiv e s t a t e a n a lo g o u s , f o r e x a m p le , w ith io d id e .

T h is s u r p r i s i n g

o b s e r v a tio n g iv e s an u n e x p e c te d c o n f ir m a tio n o f t h e o ft-m a d e g e n e r a l i ­ z a t i o n t h a t th e d i f f e r e n c e s betw een t h e members o f t h e A a n d th e B f a m i l i e s w i th in a p e r i o d i c group become s m a lle r a s t h e a to m ic num ber in c re a s e s .

The u n e x p e c te d ly h ig h m e t a l l i c c h a r a c t e r o f t h e io a in e

co n g e n er a s t a t i n e a s r e p o r t e d by S eg re e t ^

(4 ,1 1 ) may b e c i t e d a s a

com parable and d i r e c t l y r e l a t e d in s t a n c e o f t h e im p o rta n c e o f t h i s p rin c ip le . The o r i g i n a l s tu d y by L u n d e ll an d lüiow les i n d i c a t e s t h a t o n ly v e r y d i l u t e , c o ld , an d a i r - f r e e r h e n id e s o l u t i o n s a r e s t a b l e ,

.-farming

in d i l u t e s u l f u r i c a c i d c a u se s o x i d a t i o n o f t h e rh en iu m w i t h lib e r a t io n o f a v o l a t i l e s u lfu r compound, p o s s ib ly su lfu r d io x id e . Rhenide i s a powerful reducing a g e n t, r e a c tin g w ith copper or s i l v e r s a l t s to g iv e th e e l e r s n t s , and red ucing even the t i t a n i c io n .

No

evidence o f a v o l a t i l e rh en iu m compound w as fo u n d . T itr a tio n data by th e Tomiceks (3 6 ), two papers on th e polarography o f rhenium by lin g a n e (1 7 ,1 8 ), and a recen t chem ical and spectrophotom etric study by Maun and Davidson (23) g iv e adequate o o n fim a tio n fo r th e e x is te n c e o f the rhenide s t a t e .

The Tomiceks'

paper d e fin e s the degree o f rhenium red u ctio n a tta in e d u sin g variou s reducing a gen ts and media.

They found hydrochloric a cid s u ita b le in

p lace o f th e s u lf u r ic a c id o f lu n d e ll and Knowles,

lin gan e ch a ra cterized

th e polarographic r e d u c t i o n o f p e r r h e n a te in s e v e r a l m e d ia .

H is s tu d y

o f th e o x id a tio n o f rhenide a t th e d ro p p in g m e rc u ry e l e c t r o d e i s o f

high i n t e r e s t , but h is in te r p r e ta tio n o f th e waves i s open to se r io u s q u estion on se v e r a l counts.

Maun and Davidson have d e f in it e ly

e s ta b lis h e d the fa c t th a t rhenide in hydrochloric a c id i s not v o la t ile and t h a t such a s o lu tio n cannot be concentrated w ith ou t c a u s in g

o x id a tio n o f th e r h en id e.

They in d ic a te th a t a rhenide s o lu tio n in

h yd rochloric a c id seems to grad u ally form su b stan ces which show strong absorption in th e u l t r a v i o l e t .

Chemical o x id a tio n o f rhenide appears

t o proceed thru t h e sta g e s (-1 ) t o (I) t o (IV) t o (V II).

I t i s evid en t th a t r e l a t i v e l y l i t t l e o f th e rhenide s t a t e .

i s known o f t h e n a t u r e

T his i s th e on ly known in sta n ce o f a m etal w ith

n eg a tiv e o x id a tio n number, and th e s t a b i l i t y of even d ilu t e s o lu tio n s o f such a powerful reducing agent in a cid media i s su r p r isin g . P auling (29) has sp ecu la ted th a t rhenide may be s t a b iliz e d by fou r

c o v a le n t b o n d s, p o s s i b l y w ith w a te r m o le c u le s , i n a c o p la n a r sq u a re ^ c o n f ig u ra tio n .

T h is p r e d i c t i o n i s b a s e d on an a n a lo g y w i t h b i p o s i t i v e

P la tin u m w ith w h ich r h e n id e i s i s o e l e c t r o n i c .

The rh e n iu m ( I ) and

I I I ) s t a t e s a r e l i t t l e b e t t e r known, an d n o th in g i s known o f th e ( H I ) s ta te in s o lu tio n .

The c u r r e n t l y r e p o r t e d o x i d a t i o n p o t e n t i a l s f o r th e

v a r io u s rh en iu m c o u p le s (16) r e p r e s e n t o n ly t h e c r u d e s t o f a p p ro x im a tio n s , The p r e s e n t s tu d y was u n d e r ta k e n w ith t h e p u rp o s e o f b e t t e r C h a r a c t e r is in g t h e lo w e r o x i d a t i o n l e v e l s o f rh e n iu m , e s p e c i a l l y th e r h e n id e s t a t e .

An e f f o r t was m d e t o e s t a b l i s h b e t t e r v a l u e s f o r th e

o x id a tio n p o t e n t i a l s o f t h e rh en iu m c o u p le s .

The f e a s i b i l i t y o f

i s o l a t i n g a r h e n id e compound by p r e c i p i t a t i o n o r b y s o lv e n t e x t r a c t i o n was i n v e s t i g a t e d .

The p o s s i b i l i t y f o r t h e o c c u r re n c e o f s e v e r a l n o v e l

r e a c t i o n s o f rh en iu m was i n v e s t i g a t e d . p o la r o g r a p h ic te c h n iq u e s w ere l a r g e l y r e l i e d upon a s b e in g th e m ost l i k e l y t o y i e l d u s e f u l in f o r m a tio n r e g a r d in g th e s t a b i l i t i e s nnd p o t e n t i a l r e l a t i o n s o f t h e v a r i o u s o x id a tio n s t a t e s .

C hem ical

t e s t s , q u a n t i t a t i v e t i t r a t i o n s , s p e c tr o p h o to m e tr y , and o t h e r te c h n iq u e s w ere a p p l ie d w here t h e i r u se seemed t o be i n d i c a t e d . m ie

th e c o u rse o f t h e i n v e s t i g a t i o n was u n if o r m ly c o n c ern ed

w ith rh en iu m c h e m is try , i t w i l l be c o n v e n ie n t t o s u b d iv id e t h e s u b j e c t u n d e r th e f o l l o m n g f o u r h e a d in g s : 1.

The p o la r o g r a p h ic r e d u c t i o n o f p e r r h e n a t e .

2.

The o x i d a t i o n o f r h e n id e a t t h e d ro p p in g m e rc u ry e le c tro d e .

3

.

4.

The n a t u r e o f t h e r h e n id e s p e c i e s i n h y d ro h a lo g e n a c i d s . M is c e lla n e o u s a s p e c t s o f t h e c h e m is tr y o f rh e n iu m .

PART I the

POLAROGRAPHIC REDUCTION OF lERRHSNATE

In tro d u c tio n

A p r e v io u s s u r v e y o f t h e r e d u c t i o n o f p e r r h e n a te io n b y lin g a n e (17) l e d t o t h e t e n t a t i v e a s s ig n m e n t o f th e re d u c e d s t a t e s i n s e v e r a l m e d ia .

R e d u c tio n s i n 4 N a c i d m e d ia a p p e a r e d t o p r o c e e d t o

t h e rh en iu m (17) s t a t e .

I t w as f u r t h e r i n d i c a t e d t h a t r e d u c t i o n s i n

2 N p o ta s s iu m c h l o r i d e may p ro c e e d a s f a r a s t h e r h e n id e s t a t e .

The

m arked i r r e v e r s i b i l i t y o f a l l o f t h e w aves an d c e r t a i n o t h e r p e c u l i a r i t i e s , how ever, p r e c lu d e t h e p o s s i b i l i t y Ox a r e l i a b l e q u a n tita tiv e in te r p r e ta tio n . I9 h ile t h e p r e s e n t i n v e s t i g a t i o n w as p r i m a r i l y c o n c e rn e d w ith t h e lo w er o x id a tio n l e v e l s o f rh en iu m , i t a l s o seem ed d e s i r a b l e t o s tu d y th e n a t u r e o f p e r r h e n a te r e d u c t i o n .

T h is s t u d y w as p r i m a r i l y

c o n c e rn e d w ith a c r i t i c a l e x a m in a tio n o f t h e r e d u c t i o n s r e p o r t e d b y lin g a n e i n 4 N p e r c h l o r i c a c i d a n d i n 2 N p o ta s s iu m c h l o r i d e .

A

number o f o th e r m edia w ere b r i e f l y exam in ed , and t h e r e d u c t i o n o f rh en iu m (17) and o f rh en iu m "am algam " w ere t r i e d . E x p e r im e n ta l P o ta s s iu m p e r r h e m t e s u p p li e d b y t h e ü u i v e r s i t y o f T en n e sse e was u se d i n m ost o f th e s e e x p e rim e n ts w ith o u t f u r t h e r p u r i f i c a t i o n . The s t a t e d p u r i t y o f t h i s m a t e r i a l i s 9 9 .8 ÿ .

An a s s a y o f th e m a t e r i a l

Showed 64.2 f. Ee and 13.55^ K i n e x c e l l e n t a g re e m e n t w ith t h e t h e o r e t i c a l v a lu e s o f 6 4 .4 /. Ee and 13.5/4 K.

F o r o o n d u c t lv ity m easu rem en ts a sam ple

w as em ployed w h ic h had b ee n tw ic e r e c r y s t a l l i z e d from c o n d u c t i v i t y -

g ra d e d i s t i l l e d w a te r and d r i e d f o r tw e lv e h o u rs a t 1 0 5 -0 . The p otassiu m h ex a eh lo ro rh en a te ( I V ) , employed a t s e v e r a l s t a g e s i n t h i s s tu d y , was p re p a re d b y io d id e r e d u c t i o n i n e x c e s s h y d ro o h lo ric a c id .

The s ta n d a r d s y n t h e s i s (1 ) was u s e d e x c e p t f o r an a d ­

d i t i o n a l r e c r y s t a l l i z a t i o n from c o n c e n tr a te d h y d r o c h lo r ic a c i d .

The

o x i d a t i o n l e v e l o f t h e rh e n iu m i n t h e p r o d u c t was v e r i f i e d by t i t r a t i on. L a b o r a to ry g ra d e d i s t i l l e d w a te r was r e d i s t i l l e d fro m p erm an g an ate f o r m ost o f t h e e x p e r im e n ts .

F o r c o n d u c t i v i t y e x p e rim e n ts

t h i s same w a te r w as r e d i s t i l l e d once more i n an a l l - g l a s s a p p a r a t u s . M allin ck ro d t r e a g en t grade p e r c h lo r ic a c id and p o ta ssiu m c h lo r id e were em ployed. The m ajo r p a r t o f th e p o l a r o g r a p h i c w o rk was done b y m anual m easurem ents w ith th e F is h e r E le c d ro p o d e .

S e v e r a l c u rv e s w ere p h o to ­

g r a p h i c a l l y re c o rd e d w ith t h e S a r g e n t Model X I I , a n d a p e n - r e c o r d in g S a rg e n t M odel XXI was u s e d i n a few i n s t a n c e s .

V o lta g e s e t t i n g s and

s l i d e w i r e s o f a l l in s t r u m e n ts w ere ch e ck e d w ith a c o n v e n tio n a l p o te n tio m e te r a s se ro b ly .

A l l p o t e n t i a l m easu rem en ts r e p o r t e d h e r e in

have b een c o r r e c te d t o w ith in - 1 mv. An H -ty p e c e l l s i m i l a r t o t h a t d e s c r ib e d b y E lv in g and T e lte lb a u m (6 ) was u se d a t 25- ± 0 . 1 - 0 .

A s a t u r a t e d c a lo m e l anode

w as u s u a l l y em ployed, an d a l l p o t e n t i a l s w i l l be s t a t e d r e l a t i v e t o th is c e ll.

The r e s i s t a n c e o f th e e n t i r e a s s e m b ly , from th e anode

c o n ta c t t o t h e m e rc u ry r e s e r v o i r , w as ch e c k e d w ith a p o r t a b l e IC ohlrausch b r id g e a s s e m b ly . r e c t l o n s w ere n o t a p p l i e d .

S in c e t h i s w as u s u a l l y c a . 80 ohms, P u r i f i e d n i t r o g e n , w h ich was u se d t o rem ove

a i r fro m t h e t e s t s o l u t i o n s , was p r e s a t u r a t e d i n a b u b b le r c o n ta in in g

e i t h e r w a te r o r a p o r t i o n o f t h e t e a t s o l u t i o n i n o r d e r t o m in im ize c o n c e n t r a tio n changes b y e v a p o r a tio n . The c o u lo m e tr ic assem b ly u s e d d id n o t d i f f e r i n p r i n c i p l e fro m t h e o r i g i n a l d e s c r i p t i o n s o f L in g a n e ( 1 9 .2 0 ) .

A

w a te r - J a c k e te d

c e l l d e s ig n e d f o r m e rc u ry c a th o d e a n a ly s e s w as em ployed b e c a u s e o f th e d e s i r a b i l i t y o f m a in ta in in g lo w e r te m p e r a t u r e s i n some o f t h e l a t e r w ork.

S in c e a n e x t e r n a l anode was u s u a l l y r e q u i r e d , a l a r g e s a l t

b r id g e was i n s e r t e d th r o u g h t h e t o p o f t h e c e l l .

The c e l l - e n d o f t h e

b r id g e was s to p p e r e d w ith a n a g a r p lu g s e a t e d on a f r i t t e d g l a s s d i s c .

D is c u s s io n D if f u s i o n C o e f f i c i e n t f o r t h e P e r r h e n a te I o n . d a t a f o r th e p e r r h e n a te io n have h ad a d e v io u s h i s t o r y .

C o n d u c tiv ity The e q u i v a le n t

co n d u c ta n ce o f p e r r h e n a te io n a t 25“C. and I n f i n i t e d i l u t i o n may b e c a l c u l a t e d from t h e d a t a o f a number o f a u t h o r s .

The r e s u l t s e x p r e s s e d

i n ohm- 1 cm.^ w ould show v a l u e s o f 6 0 .0 from th e w o rk o f t h e Noddacks ( 2 8 ) , 5 1 .5 from th e w ork o f P u s c h in a n d lu t u n d z i c ( 3 0 ) , e i t h e r 4 9 .2 (b a se d on p e r r h e n i c a c i d ) o r 5 3 .2 (b a s e d on p o ta s s iu m p e r r h e n a te ) fro m th e w o rk o f R o th a n d B e ck e r ( 3 1 ) , 5 4 .6 8 from th e p r e c i s i o n w ork o f 1 . H . Jo n e s ( 1 2 ) , o r 5 4 .9 5 ± 0 .0 2 from a r e t r e a t ^ n t o f the l e t t e r ' s d a ta by itonlt ( 2 6 ) .

m i l e th e r e c e n t v a lu e s o f Jo n e s

o r o f ISonlc m ig h t be a c c e p te d w ith o u t q u e s tio n f o r p o la r o g r a p h ic p u r p o s e s , a ro u g h c h e ck o f th e s e d a ta seemed d e s i r a b l e .

T h is w as

e s p e c i a l l y th e c a s e s in c e some o f t h e c o n c lu s io n s o f L in g an e depended on th e e a r l i e r v a l u e s o f P u s c h in an d T u tu n d z ic .

F u rth e rm .o re, th e o n ly

a v a i l a b l e v a lu e f o r t h e te m p e r a tu r e c o e f f i c i e n t o f c o n d u c tiv i ty i s t h a t g iv e n by t h e s e e a r l i e r w o rk e rs .

A t y p i c a l K ohlrauseU b r id g e was a s s e m b k d fro m p r e c i s i o n oom poaante em p lo y ia g th e u s u a l d e te c tio a . 10-^

1.000

c y c le o s c i l l a t o r w ith h ea d p h o ae s

" E d u llib r iu m w a te r" w ith a S ] » o i f i c c o a a u c ta a c e o f 1 .8 z

o m - ^ cm .-^ was u s e d .

S la c e r e s u l t s o f h i ^ p r e c i s i o a w e re a o t

s o u g h t, d e t a i l s o f t h e d e t a r m i a a t l o a w i l l a o t be g i v e a .

U s la g o a ly

t h e y a l u e s o b ta in e d fro m t h e t h r e e m ost d i l u t e s o l u t i o n s ( 0 . 0 0 2 .

0 .0 0 1 . and 0.0005 H ). c a lc u la tio n by th e method o f Monk gave a value o f 5 5 .4 i 0 .3 Ohm-1 ^ . 2 s t 2 5 .0 “G.

This s u f f ic e s t o su b sta n tia te the

v alu es o f Jones and Monk w ith in th e l im it s reduired fo r our purposes. Based on 0 .0 1 U s o lu tio n s measured a t 2 0 -0 °. 2 5 .0 " . and bO.O»C.. a temperature c o e f f ic ie n t o f c o n d u c tiv ity o f 0-0250 per degree was found. S ince t h i s fig u r e i s in good accord w ith the v alu e 0-0247 per degree reported by Puschin and Tutundzic. t h e ir d ata was ap p aren tly su b ject to a system atic err o r . I t w i l l be dUite s u f f ic ie n t for p o la r o g r a p h ic purposes to average th e v a lu e s o f Jones and Monk, ta k in g 54.8 as th e perrhenate eonductance a t 25".

The d iffu s io n c o e f f ic ie n t may th en be c a lc u la te d

from th e r e la tio n (.14) at 25 D® = 2.67 X 10-7 X 54.8 cm.^ sec."^ g iv in g 1 4 .6 X 1 0 -6 .

The s q u a re r o o t o f t h i s , 3 .8 3 x 10

th e i L o v i c e q u a tio n tim e s th e c o n s ta n t 605.

, a p p e a rs i n

l : u l t i p l i c a t i o n g iv e s

2 .3 2 an& th e I l k o v i c e q u a tio n may be if jr itte n i n t h e fo rm i /G

=

2 .3 2 n k

^ ^ « 2 /3 .1 /6 where k rep resen ts th e c a p illa r y co n sta n t, m t . R e d u c tio n Of P e r r h e n a te Io n i n A cid a d l a .

L in g an e o b s e rv e d

w e ll - d e f i n e d r e d u c tio n w aves f o r p e r r h e n a te i n e i t h e r h y d r o c h lo r ic o r p e rc h lo ric a c id s .

S in c e t h e r e d u c t i o n s p ro c e e d » r e r e v e r s i b l e ( i . e . .

a t l e s s n e g a tiv e p o t e n t i a l s ) and f a r t h e r ( i . e . , l a r g e r d i f f u s i o n c u r r e n t s a r e o b ta in e d ) a s t h e c o n c e n t r a tio n o f a c i d i s i n c r e a s e d , i t i a n e c e s s a r y t o u s e r a t h e r c o n c e n tr a te d s o l u t i o n s o f a c i d .

A p p ro x im a te ly

c o n s ta n t v a l u e s o f d i f f u s i o n c u r r e n t f o r a g iv e n p e r r h e n a te c o n c e n tr a ­ t i o n a r e o b se rv e d f o r a c i d i t i e s g r e a t e r th a n

2 .5 N; i n o r d e r t o

p e r m it d i r e c t com pariso n w ith L in g a n e 's r e s u l t s , 4 N c o n c e n t r a tio n s o f a c id w ere u s e d .

I n 4 N h y d r o c h lo r ic a c id t h e e a r l y i n c e p ti o n o i t h e

c a lo m e l d is c h a r g e l e s s e n s t h e a c c u r a c y iv ith w hich t h e w ave h e i g h t can be d e te rm in e d .

F o r t h i s r e a s o n 4 N p e r c h l o r i c a c i d was th e p r e f e r a b l e

medium. A r e a d a b le c u r r e n t p l a t e a u i s n o r m a lly found i n p e r c h l o r i c a c i d , th o u g h t h i s i s somewhat d i s t o r t e d a t h ig h p e r r h e n a te c o n c e n tr a tio n s .

V a lu e s f o r i^ /C show good c o n s ta n c y o v e r a w ide ra n g e o f

p e r r h e n a te c o n c e n t r a tio n s .

C a l c u l a t i o n from t h e I l k o v i c e x p r e s s io n

i s a d s to a r e p r o d u c ib l e n - v a lu e o f 3 .0 a t 25» a s r e p o r t e d b y l i n g a n e . Some t y p i c a l r e s u l t s a r e shovm i n T ab le 1 . h ig h ly i r r e v e r s i b l e , an a v e ra g e v a lu e o f 0 .0 8

V.

The s lo p e o f t h e w av es i s E q .VS" ^ 0 .2 5

The i r r e v e r s i b i l i t y i s a l s o e v id e n t fro m th e c o n s i s t e n t s h i f t

o b se rv e d i n t h e h a lf-w a v e p o t e n t i a l s a s th e p e r r h e n a te c o n c e n t r a t i o n i s in c r e a s e d .

D e s p ite c e r t a i n o t h e r p e c u l i a r i t i e s , t h e w av es seem t o

m eet a l l o f t h e e s s e n t i a l r e q u ir e m e n ts to fo rm th e b a s i s of an a n a l y t i c a l m ethod. The w aves in s o l u t i o n s more concentrated th a n a b o u t 0 .4 mm p e r r h e n a te w ere alw ay s i i s t o r t e d .

An a d a i t i o n a l c u r r e n t r i s e s t a r t e d

a t a b o u t - 0 . 5 5 v . and t h i s w as fo llo w e d b y a marked c u r r e n t d e c r e a s e w hich r e a c h e d a minimum a t a b o u t - 0 .8 4 y .

T hese phenom ena w ere n o t

a l t e r e d wiien th e c u rv e vras r e c o r d e d a u t o m a t i c a l l y , w h e th e r a t slow o r

10

a t ra p id r a t e s o f p o la r iz a tio n . gave t h e same r e s u l t s .

The s u b s t i t u t i o n o f a n i n t e r n a l m e rc u ry p o o l

anode was w ith o u t e f f e c t . feren c e.

P e r c h l o r i c a c i d fro m s e v e r a l s o u r c e s

L onger p e r io d s o f d e g a s s in g made no d i f ­

A t y p i c a l e r ^ p l e o f s u c h a cu rv e a s r e c o r d e d p h o to g r a p h i c a l ly

i s Shown i n F ig u re 1 .

I t w as fou n d t h a t p o ta s s iu m h e x a c h lo r o r h e n a te

(IV ) re d u c e d a t - 0 .5 6 v .

T h is f a c t w o u ld seem t o c o r r e l a t e n i c e l y

w ith th e i n t e r p r e t a t i o n o f t h e p rim a ry p r o c e s s a s th e t h r e e - e l e c t r o n r e d u c tio n o f p e r r h e n a te t o rh e n iu m ( I V ) .

On t h e o t h e r h a n d , t h e r e a r e

s e v e ra l s e rio u s o b je c tio n s to t h i s in te r p r e ta t io n . The v a r i a t i o n o f t h e i n i t i a l c u r r e n t - s t e p w i t h t h e h e i g h t o f th e m r c u r y r e s e r v o i r f o llo w e d t h e e x p e c te d b e h a v io r f o r a t r u e d i f f u s i o n - c o n t r o l l e d p r o c e s s , b u t th e te m p e r a tu r e c o e f f i c i e n t w as a b n o rm a l,

m i l e i t p ro v ed t o be v e r y d i f f i c u l t t o o b t a i n r e p r o d u c ib le

v a lu e s f o r t h e te m p e r a tu re c o e f f i c i e n t o f th e d i f f u s i o n c u r r e n t , a l l v a lu e s o b ta in e d w ere i n t h e r a n g e o f m inus

1#

t o p lu s

0

. 2$ p er d e g re e.

One m ust co n clu d e t h a t t h e t r u e v a lu e i s v e r y s m a ll o r ev e n s l i g h t l y n e g a tiv e .

S in c e t h e te m p e r a tu r e c o e f f i c i e n t o f c o n d u c tiv i ty f o r t h e

p e r r h e n a te io n h a s a n o rm al v a lu e o f 2 .5 $ p e r d e g . . i t i s e v id e n t t h a t n - v a lu e s o f t h r e e w ould n o t be o b ta in e d f o r a n y te m p e r a tu r e o th e r tn a n 25".

The o b se rv e d te m p e r a tu re c o e f f i c i e n t i s c h a r a c t e r i s t i c o f an

a d s o r p ti o n wave ( 2 ) . a c o n c lu s io n w h ic h a ls o c o r r e l a t e s w i t h t h e u n u s u a lly s tr o n g d e p r e s s a n t e f f e c t o f g e l a t i n on t h i s w av e.

L in g an e

(17) o b s e rv e d t h a t a s l i t t l e a s 0 .0 1 3 $ g e l a t i n l e d t o co m p lete

O b lite r a tio n o f t h e w ave. The c o u lo m e tric r e d u c t i o n o f p e r r h e n a te i n a c id m ed ia a ls o

in d icates t h a t the simple in t e r p r ^ t lo n o f the vmve as a th reealectron process i s not v a lid .

Reductions in 4 K p erchloric acid at

11

p o t e n t ia l s e t t in g s o f -0 .4 5 to -0 .5 5 v . always gave r i s e t o a con­ tin u ou s p r o c e ss.

I n a t y p ic a l run conducted a t -0 .5 2 y . the red u ction

was continued u n t i l th e gas-coulojneter showed a volume o f gas eq u iv a len t to a current consumption o f seven tim es th e t h e o r e t ic a l valu e f o r th ree e le c tr o n s .

There v» s no In d ic a tio n o f any dim inution in the r a te of

th e p rocess a t th e end o f t h i s tim e .

T itr a tio n o f th e c e l l s o lu t io n

in d ic a te d th a t the o r ig in a l perrhenate had o n ly s u ffe r e d a reduction eq u iv a len t to 0 .2 e le c tr o n .

Gas e v o lu tio n a t th e m ercu ry-solu tion

in te r fa c e in d ic a te d th a t th e r e d u c tio n o f hydrogen io n s played a major r o l e i n th e c u r r e n t c o n su m p tio n . T here a r e i n t e r e s t i n g p o i n t s o f s i m i l a r i t y b e tw e e n t h e b e h a v io r o f p e r r h e n a te and t h a t o f p e r i o d a t e .

Ooe a n d R o g e rs

(s)

o b se rv e d an i n c r e a s e i n t h e n o r m a lly t w o - e le c tr o n p e r i o d a t e r e d u c t i o n wave a s th e a c i d c o n c e n t r a tio n was in c r e a s e d beyond t h a t c o r r e s p o n d in g t o pH 0 . 8 . w ave.

They a l s o n o te d a p ro n o u n ce d e f f e c t o f g e l a t i n on t h e

S ouchay (34) o b se rv e d a c u r r e n t d e c r e a s e b ey o n d t h e p e r i o d a t e

wave w h ic h , how ever, a p p e a r e d t o be f a v o r e d b y more a l k a l i n e I t i s p o s s i b l e , o f c o u r s e , t h a t t h e r e s u l t s o f t h e c o u l­ o m e tr ic r u n s sh o u ld n o t b e a c c e p te d a s d e f i n i t i v e w i t h r e s p e c t t o t h e p o la r o g r a p h ic p r o c e s s . O f

One m ig h t w e ll q u e s tio n th e g e n e r a l v a l i d i t y

t h e assum ed a n a lo g y b etw e en th e g r o s s m e rc u ry c a th o d e o f t h e

c o u lo m e tric asse m b ly w ith th e s m a lle r , f r c s h l y - f o m e d m e rc u ry d ro p ­ l e t s w h ich c h a r a c t e r i s e a p o la r o g r a p h ic r u n , i n p a r t i c u l a r , w here o v e r v o lta g e e f f e c t s an d h y d ro g en d i s d i a r g e a r e c o n c e r n e d , rem ark s h o u ld n o t be c o n s tr u e d a s s e r i o u s l y d e t r a c t i n g f ro m t h e w iae u s e f u ln e s s an d d e s i r a b i l i t y o f c o u lo m e tric t e c h n i q u e s . M ost o f t h e phenom ena o b se rv e d i n th e above medium a p p ly

12

e q u a l l y w e ll i n com p arab le c o n c e n t r a t i o n s o f h y d r o c h lo r ic a c i d , o n ly d i s t i n c t i o n i s t h a t t h e h a lf-w a v e p o t e n t i a l s u n d er, s i m i l a r

c o n d itio n s a re

0 .1 v . more p o s it iv e in hydrochloric a c id than in

p e rc h lo ric a c id . S e v e r a l curves o b ta in e d i n 2 .4 N h y d r o c h lo r ic a c i d a t 1 5 “ a r e o f in t e r e s t .

F ig u re 2 shows th e w av es fbund in s o lu tio n s o f p e r -

rhenate, hexachlororhenate (IV ), and a mjjcture o f th ese two s t a t e s . T ab le 2 sum m arizes th e data frd m th ese w av e s.

I t m i l be seen th a t

th e d i f f u s i o n c u r r e n t for th e mixed s o l u t i o n i s i n ag reem en t w i t h w hat one would p r e d i c t from t h e o t h e r tw o ru n s.

The f a c t t h a t a

rhenium (IV ) s a l t re d u c e s w i t h i n t h e bounds o f th e perrhenate r e d u c tio n

does n o t, o f cou rse, e it h e r prove nor disprove th e perrhenate red u ction a s a th r e e -e le c tr o n p r o c e s s inasm uch a s t h e p e r r h e n a te w ave i s d e c id e d ly i r r e v e r s i b l e . The s lo p e f o r t h e rhenium (IV) wave i s n o t i n co m p lete a c ­ co rd w ith th e t h e o r e t i c a l v a lu e o f 0 .0 5 5 f o r a o n e - e l e c t r o n p r o c e s s . S u b se q u e n t work on t h e anodic o x id a tio n o f r h e n id e so lu tio n s in d ic a te s

th a t th e r e v e r s ib le p o te n tia l o f th e ( I V ) /( I I I ) oouple must be much / more p o s it iv e than - 0 .5 v .

I t may be p ertin en t to note th a t th e

' observed s lo p e f o r t h e perrhenate red uction i s i n a c c o r d w i t h t h a t c a l c u l a t e d by T ish koff (35) fo r a o n e - e l e c t r o n red u ction in v o lv in g d im e r iz a tio n and a d s o r p ti o n a t t h e e l e c t r o d e . R e d u c tio n o f P e r r h e n a te I o n i n ^ j e u t r ^ o b s e rv e a a p e r r i e n a t e wave i n

8

L in g an e (17)

H p o ta s s iu m o b lo r ia e s o l u t i o n w bioli

a p p ro x im a te d to th e e x p e c t a t i o n f o r an e i g h t - e l e c t r o n p r o c e s s o v e r a l i m i t e d ra n g e o f c o n c e n t r a t i o n .

T h is i r r e v e r s i b l e wave a t - 1 .4 1 v .

w as fo llo w e d by a c a t a l y t i c h y d ro g en d is c h a r g e wave a t - 1 .7 v .

13

The p r e s e n t s t u d i e s i n t h i s medium a r e e s s e n t i a l l y i n a o o e rd w ith t h e e a r l i e r w o rk .

As s h o rn i n T a b le 3 . t h e p e r r h e n a te wave was

fo u n d a t - 1 .4 3 V. v s . t h e S .O .E . and c a l c u l a t i o n fro m th e I l k o v i c e q u a tio n i n d i c a t e s an 8 . 4 e l e c t r o n r e d u c t i o n . 8

The a g re e m e n t f o r an

- e l e c t r o n t r a n s i t i o n shows g r e a t e r c o n s ta n c y o v e r a w id e r

c o n c e n t r a tio n ra n g e th a n i n th e p r i o r s t u d y .

T h is f a c t c o u ld be

a s c r ib e d e i t h e r t o t h e u se o f a c a p i l l a r y o f w id e ly d i f f e r e n t c h a r a c t e r o r t o t h e em ployment o f m anual m easurem ent i n th e p r e s e n t s tu d y .

I t i s f e l t t h a t p o i n t s ta k e n m a n u a lly a p p ro a c h more n e a r l y

t h e t r u e e q u i lib r iu m c u r r e n t v a l u e s a t e a c h p o t e n t i a l s e t t i n g .

The

a tta in m e n t o f s t a b l e g a lv a n o m e te r r e a d in g s a t p o i n t s on th e w a v e -s te p r e q u ir e d a s much a s a f u l l m in u te in m ost c a s e s . The s l o p e o f c u r v e s o b ta in e d m a n u a lly o r a u t o m a t i c a l l y shows E g , , g . Eq.26

an a v e rag e v a lu e f o r irre v e r s ib le p ro cess.

“

The a p p e a ra n c e o f th e m a n u a lly p l o t t e d c u rv e s

t e n d s t o em p h asize t h i s f a c t .

'They h av e t h e a s p e c t o f tw o o r more

c l o s e l y f o llo v d n g w aves and o f t e n p r e s e n t ja g g e d i r r e g u l a r i t i e s . C o u lo m e tric s t u d i e s i n 2 H KOI r e v e a le d a c o n tin u o u s e l e c t r o d e p r o c e s s c o tp a r a b le t o t h a t fo u n d in a c id m e d ia.

T h is w as t r u e ev en

when th e p o t e n t i a l s e t t i n g c o rre sp o n d e d t o o n ly - 1 . 5 v . ( t o e n s u re e x c lu s io n o f th e f o llo v d n g c a t a l y t i c h y d ro g en w a v e ).

I n a ty p ic a l ru n

f i v e tim e s th e a n t i c i p a t e d g as c o u lo m e te r r e a d in g ( f o r a n e i g h t e l e c t r o n r e d u c tio n ) w as re a c h e d w ith o u t an y e v id e n c e t h a t t h e p r o c e s s w as Slow ing down. o a.

6

The pH o f t h e s o l u t i o n was fo u n d t o have r i s e n fro m

t o ç a . 9 d u r in g t h i s r u n .

b e e n re d u c e d by 1 .1 e l e c t r o n . d u r in g t h e r u n .

The p e r r h e n a t e , m ean tim e, h ad o n ly T here was c o n s id e r a b l e g a s e v o l u t i o n

14

i m l l e t h e r e re m a in s some q u e s tio n a s t o w h e th e r o r n o t rh en iu m form s a t r u e amalgam w i t h m e rc u ry , i t i s known t h a t rh e n iu m s a l t s can h e e l e c t r o l y z e l i n t o a m e rc u ry c a th o d e ( 7 , 8 , 2 1 ) .

I t was o f

i n t e r e s t t o e s t a b l i s h w h e th e r o r n o t s u c h a n "amalgam" em ployed a s t h e d ro p p in g e l e c t r o d e w o u ld g iv e t h e rh e n iu m ( 0 ) t o r h e n id e ( - 1 ) p o r t i o n o f th e r e d u c t i o n wave i n p o ta s s iu m c h l o r i d e .

A 4 .0 iM rh en iu m i n

m ercu ry "amalgam" was p r e p a r e d b y e l e c t r o l y s i s , and t h e p r e s e n c e o f t h e rh en iu m i n t h e m e rc u ry w as e s t a b l i s h e d b y p r o v in g i t s a b s e n c e i n th e s p e n t aqu eo u s p h a s e .

T h is m a t e r i a l a s a d r o p p in g e l e c t r o d e gave

no e v id e n c e o f any c a th o d ic wave from c h lo rid e .

0.0

to - 1 . 8 v . in

2

K p o ta s s iu m

I n view o f t h e q u e s tio n a b le s t a t u s o f t h i s m a t e r i a l a s a

t r u e am algam, t h i s n e g a tiv e r e s u l t i s n o t c o n c lu s iv e p r o o f t h a t t h e p e r r h e n a te r e d u c t i o n co u ld n o t p ro c e e d t o th e r h e n id e s t a t e . R e d u c tio n o f P e r r h e n a te I o n i n Lanthanum Chlo,rid_e.

The

p u r p o r te d e i g h t - e l e c t r o n r e d u c t i o n o f th e n i t r a t e a n io n i n t h i s medium s u g g e s te d i t s a p p l i c a t i o n i n t h e p r e s e n t c a s e .

P e r r h e n a te c o n c e n tra ­

t i o n s fro m 0 .0 8 t o 1 .0 sM w ere exam ined i n 0 .0 5 M la n th an u m c h l o r i d e s o lu tio n a t 25 ".

A v e r y p o o r ly d e f in e d d o u b le wave u s u a l l y r e s u l t e d .

The f i r s t wave o c c u rs a t ç a . - 1 .1 5 v . , and t h e se co n d a t c a .

-.

I n a few c a s e s a s m a ll t h i r d wave a t ç a . - 1 .5 7 v . was o b ta in e d .

The

c u rv e s a r e n o t h i g h ly r e p r o d u c i b l e , and t h e d e f i n i t i o n b e tw e e n th e w aves i s p o o r.

As shown i n T able 4 th e c a l c u l a t i o n o f n - v a lu e s fro m

th e I l k o v i c e q u a tio n le a d s t o v a l u e s o f 5 .4 t o 5 .5 f o r th e f i r s t w ave, an d 4 . 3 t o 6 .5 f o r t h e se c o n d .

W ith th e c o n tin u o u s p o l a r i z a t i o n

o f a u to m a tic r e c o r d in g ( S a rg e n t M odel .Q d ) s p o r a d ic , s h a r p c u r r i n c r e a s e s w ere o f t e n o b ta in e d .

T hese w e re so m etim es f o llo w e d by

m arked c u r r e n t d e c r e a s e s r e m in is c e n t o f t h e phenomena o b se rv e d i n a c i d

25

m e d ia .

I t sh o u ld he n o te d t h a t n - v a lu e s f o r t h e se c o n d wave w ere

c a l c u l a t e d on th e a s s u m p tio n t h a t w ould ap p ro x im a te

b" /"

f o r t h e s u h s te n c e b e in g re d u c e d

f o r p e rrh e n a te .

F r e s h l y p r e p a r e d s o l u t i o n s o f h e x a c h lo r o r h e n a te (IV) e x h i b ite d o n ly a m in u te p o r t i o n o f t h e f i r s t wave and a b o u t o n e - h a lf o f t h e a n t i c i p a t e d seco n d w av e.

A f t e r s ta n d in g f o r tw e n ty t o t h i r t y

m in u te s , a s i z e a b l e f i r s t w ave d e v e lo p e d a n d t h e s e c o n d wave ap p ro a c h e d i t s n o rm al h e i g h t .

A f t e r s ta n d in g o v e r n ig h t , t h e s o l u t i o n showed th e

t y p i c a l d o u b le wave o f p r o p e r h e i g h t f o r an e q u i v a le n t c o n c e n t r a tio n o f p e rrh e n a te .

The i n i t i a l r e s u l t im p l i e s t h a t rh en iu m r e d u c t i o n s a r e

p r o b a b ly in v o lv e d i n b o th w a v e s, th e se c o n d wave b e in g c o n fin e d t o o x id a tio n l e v e l s lo w e r th a n ( 1 7 ) .

The e f f e c t s o b se rv e d on s ta n d in g

p ro b a b ly r e f l e c t th e p r o g r e s s o f t h e d i s p r o p o r t i o n a t i o n r e a c t i o n 5

Re (IV )

3 Re ( I I ) +

2

Re ( V tl)

concurrent lA th a gradual approach to com plete o x id a tio n . The attainm ent o f c a lc u la te d t o t a l n -v a lu es ae high a e 1 1 .5 , th e g r e a t l y d e c r e a s e d s i z e o f th e se c o n d wave f o r a n o n -o x y g e n a te d (17) s t a t e , th e e r r a t i c s te e p c u r r e n t s te p s som etim es o b ta in e d , and a n a lo g ie s w ith t h e c a s e s p r e v io u s l y d is c u s s e d seem t o i n d i c a t e

(l) th a t

c a t a l y t i c h y d rogen d is c h a r g e c o n t r i b u t e s t o one o r b o th w av es, and ( 2 ) t h a t e l e c t r o l y t i c p e r r h e n a te r e d u c t i o n p r o b a b ly p r o c e e d s t h r u an o x y g e n a te d (17) s t a t e com parable w ith t h e r h e n l t e i o n , HeOs

Perrhenate R eductions in Other Media.

Of numerous other

media examined, o n ly th ree are worth n o tin g a s being p o t e n t ia lly u s e fu l, i n 2 M potassium thiocyan ate a sm all wave o f m oderately good d e f in it io n i s found at ç a . -1 .2 0 v .

In 0 .1 H barium ch lorid e a wave at -1 .3 5 v .

i s fo llo w ed by a la rg e wave beginning at about - 1 .6 v .

In sodium

16

s u l f i t e , with o r without potassium chloride p resent, two wov Obtained; the p o ten tia ls o f both f a l l between - 1 . 4 and - 2 . 3 v o l t s VS, a mercury pool anode. Summary I t does not seem p ossib le to reco n cile a l l o f the observed

behavior o f perrhenate reduction waves w ith explanations invoking any Simple combinations o f d iffu sio n , k in e tic , or adsorption p rocesses. There are strong in d ica tio n s, however, th at adsoiption e f f e c t s play a p ro m in e n t r o l e . The c o n c u r re n t e v o l u tio n o f h y d ro g en a p p e a r s t o b e a m a jo r c u r r e n t - c o n t r i b u t i n g f a c t o r i n m ost c a s e s , and i t seem s l i k e l y t h a t

the perrhenate i t s e l f may have a c a ta ly tic e ffe c t on th is a c tio n . Such an e ffe c t has already been postulated by Heyrovsky (9 ) m d by (17) t o a c c o u n t f o r t h e a b n o rm a lly l a r g e w aves o b ta in e d i n c e r t a i n b u f f e r m e d ia .

The e x p l a n a tio n o f L in g a n e , w h ic h p o s t u l a t e s a

m echanism b a s e d on co m p lete r e d u c t i o n t o th e r h e n id e s t a t e , d o es n o t , how ever, seem t o b e n e c e s s i t a t e d . I t i s q u i t e e v i d e n t t h a t a l l o f t h e o b s e rv e d p e r r h e n a te r e d u c tio n s a r e t o b e r e g a r d e d a s h i # l y i r r e v e r s i b l e .

T h u s, t h e m ost

p o s i t i v e p o t e n t i a l a t w hich any r e d u c tio n c o u ld be o b ta in e d , ç a . - 0 .3 5 V. v s . th e S .C .E ., i s s t i l l e n t i r e l y to o n e g a tiv e t o b e r e g a r d e d a s a r e v e r s i b l e p o t e n t i a l in v o lv i n g a p e r r h e n a te c o u p le .

The

su b se q u e n t i n t e r p r e t a t i o n o f r h e n id e o x id a tio n s v d l l em p h asize t h i s same p o i n t .

17

T a b le 1 R e d u c tio n o f KReO^ i n 4 N HCIO4 a t 25“C. h = 1 ,6 7 a t —0 ,6 0 V, E R 8 O4

ia

^ d /^

^

^ 0 ,5

V,

mM

8.,

0 .0 8

0 .9 2

1 1 .5

2-S7

0 .2 0

2 .3 7

1 1 .8

5 -0 5

0 .4 0

4 .8 0

1 2 .0

5 .0 9

0 .8 0

9 .2 5

1 1 .7

3 -0 2

1 .0 0

1 2 .0 4

1 2 .0

3 .0 9

A verage

- 0 .3 9 4

3 ,0 4

T a b le 2 R e d u c tio n o f Rhenium (V II) and Rhenium (IV ) i n 2 ,4 N HCl a t 1 5 “

Composition,

^ 0*5’

^d*

^0.75 ” ^ 0 .2 5 ’

mM

V•

a.

V.

KReO*

- 0 .3 9 8

3 ,7 4

0 .1 0 9

0 ,4 RgReClg

- 0 .5 2 8

1 ,1 3

0 ,0 6 1

0,2 in each

- 0 .4 5 2

2 ,4 2

0 ,1 1 9

0 ,4

18

F ig . 1

P e r r h e n a te R e d u c tio n i n P e r c h l o r i c A c id ;

0 .8 0 X 10” ^ M p e r r h e n a te io n i n 4 N p e r c h l o r i c a c i d a t 2 5 ° ; k = 1 .6 7 a t - 0 .6 0 v .

^ " -

0.25

■’

-

0,50

-

0.75

E, volts

1.00

19

fig . 2 c h l o r i c A cid:

P e r r h e n a te and R h en itm (IV) R e d u c tio n s in H ydro­

2 .4 N h y d r o c h lo r ic a c i d a t 15“ w hich i s (A)

0 . 4 I # i n p e r r h e n a te , (B) 0 . 2 itM e a c h , i n p e r r h e n a te an d i n H e ia o h lo r o r t a n a te ( I T ) , (0 ) k = 1 .6 6 a t -0 .6 0 v .

0.4

i n H e x a c h lo r o r ie n a ta (IV ) ;

r

4

3

mlcroamps. 2

1

■0.3

0.5

0 .4 E , V,

0.6

20

T a b le 3 R e d u c tio n o f EReO^ i n 2 H KCl a t 25*0. ^ k = 1 ,4 2 a t —1 ,5 5 v ,) EEeO*

id

^ V,

tM

0 .0 4

1 .0 6

2 6 .4

8 .0 0

- 1 .4 4

0 .0 8

8 .1 6

2 7 .0

8 .1 8

- 1 .4 2

0 .2 0

5 .7 0

2 8 .5

8

.6S

- 1 .4 0

0 .4 0

1 1 .6 0

2 9 .0

8 .7 8

- 1 .4 2

0 .8 0

22.8

2 8 .5

8 .6 3

- 1 .4 6

1 .2 0

3 3 .8

2 8 .2

8 .5 4

- 1 .4 3

2.0 0

4 5 .0

2 7 .5

8 .3 3

- 1 .4 5

A verage

8 ,4 4

- 1 .4 3

T ab le 4 P e r r h e n a te R e d u c tio n s i n 0 .0 5 M L aC ls a t 2 5 ‘ (k = 1 .4 7 a t - 1 .2 5 v . ) Jo n c n ., I#

Wave 1

d /" Wave 2

Vfave 1

Wave (.1 + 1 0 .3

2

)

Wave 2 4 .8

1.0

1 9 .5

3 6 .3

5 .5

—

2 8 .3

-

8.0

-

0.8 0.6

"

2 7 .2

-

7 .7

-

0 .4

1 6 .8

3 7 .8

4 .8

1 0 .7

5 .9

1 7 .8

4 0 .6

5 .0

1 1 .5

6 .5

0.2

0 .0 8

2 7 .0

3 .4

7 .7

4 .3

12.1

21

PART I I THE OXIDATION OF RHENIDE AT THE DROPPING MERCURY ELECTRODE

In tro d u ctio n L in g a n e 's p o la r o g r a p h ic s tu d y o f t h e o x i d a t i o n o f r h e n id e (18) i s o f h ig h i n t e r e s t from th e s ta n d p o i n t o f i t s p o t e n b i a l c o n t r i b u t i o n t o a b e t t e r u n d e r s ta n d in g o f t h e lo w e r o x i d a t i o n l e v e l s .

Even on tb s

b a s i s o f t h e o r i g i n a l d a t a , ho w ev er, one im y r a i s e a t l o a s t t h r e e s e r io u s o b je c ti o n s t o th e i n t e r p r e t a t i o n p la c e d on th e w av e s.

The

c o n c lu s io n s t h a t t h e f i r s t o x i d a t i o n s t e p i s a t h r e e - e l e c t r o n p r o c e s s and t h a t th e o v e r a l l cu rv e em braces co m p lete o x id a tio n t o th e p e r r h e n a te l e v e l a r e i n c o n s i s t e n t w ith t h e f o llo w in g o b s e rv e d f a c t s : (1)

The s lo p e o f t h e i n i t i a l w ave h a s th e c o r r e c t r e ­

v e r s i b l e v a lu e f o r a t w o - e l e c t r o n p r o c e s s . (2)

The a s su m p tio n i s made t h a t a l l s p e c ie s g iv in g r i s e t o

a n o d ic waves have a p p r o x im a te ly t h e same v a lu e f o r t h e s q u a re r o o t th e d i f f u s i o n c o e f f i c i e n t . 1 5 .3

Based on t h i s , a t o t a l i^ /C v a lu e o f

a t 0* f o r t h e a n o d ic waves i s e q u i v a l e n t to a v a lu e o f o n ly c a . 21

a t 25* le s t i m a t i n g a te m p e r a tu re c o e f f i c i e n t o f 1 .5 ^ p e r d e g . ) .

T h is

com pares -with a c a l c u l a t e d 1 ^ /0 v a lu e o f 4 0 .5 a t 25* f o r a n e i ^ t e l e c t r o n r e d u c tio n o f p e r r h e n a te em plo y in g a s i m i l a r c a p i l l a r y . (3)

I n t h e am p sro m e tric o x id a tio n o f r h e n i d e , c o n s id e r a b ly

more th a n h a l f o f th e t o t a l a n o d ic wave v i s i b l e in t h e a v a i l a b l e p o t e n t i a l ra n g e h a s d is a p p e a r e d a f t e r th e a d d i t i o n o f o n ly f o u r e q u i v a le n ts o f o x id a n t.

T h is s u g g e s ts t h a t i h r t b e r o x id a tio n s te p s

may be in v o lv e d beyond t h e a v a i l a b l e ra n g e o f p o t e n t i a l . I n v ie w o f t h e s e c o n s i d e r a t i o n s p lu s t h e d e s i r e t o g a i n a

22

more d e ta ile d knowledge o f rhenide behavior in variou s media, i t was decided to rep eat and extend th e e a r l i e r work.

Anodic o x id a tio n s o f

rhenide in s u lf u r ic , m eth anesu lib nic, p e r c h lo r ic , and phosphoric a c id s w ere found t o be q u i t e sim ila r i n n a tu re.

I n c o n t r a s t , hydrohalide

acid m d ia were found to o ff e r q u ite a d iffe r e n t b eh a v io r, as w i l l be d is c u s s e d i n a l a t e r s e c t i o n . E x p e rim e n ta l

The apparatus employed was id e n t ic a l in most r e s p e c ts w ith th a t described in th e preceding s e c t io n .

The curves were manually

p lo tte d from data obtained w ith a F ish er Elecdiopode which had been ca lib ra ted w ith r e sp e c t to both current and p o t e n t ia l rea d in gs.

A ll

p o te n tia ls are given w ith r eferen ce to the sa tu ra ted calom el e le c tr o d e u n le s s o th e rw is e s t a t e d .

The temperature in the s tu d ie s o f t h is s e c tio n was cont r o l l e d a t 15® - 0 . 2 ° . I n a d d i t i o n to t h e reagents u se d p r e v i o u s l y , a sanrole o f

rhenium p en tach loride su p p lied by the Fairmount Chemical Company was eiQ)loyed.

A p o rtio n o f t h i s was used to prepare rhenium tr ic h lo r id e

by thermal decom position in a n itro g en atmosphere ( l ) .

This m a teria l

was p u r i f i e d by resu b lim a tio n u n d e r r e d u c e d p ressu re.

Figure 3 shows th e s m a ll, w a ter-cooled Jo n e s reductor em ployed t o Obtain rhenide s o lu tio n s .

through th e jacket a t a rapid r a t e .

C old ta p w ater was c ir c u la te d

The red u ctor tube contained a

column, 1 cm. diem, by 16 cm. lo n g , o f amalgamated 20 to 30-mesh zinc granules.

A bout 25 m l. o f perrhenate s o l u t i o n i n t h e d e s ir e d medium

was placed in th e f la s k .

A ir -fr e e n itro g en ,ms flu sh ed thru the e n tir e

23

a sse m b ly f o r at le a s t tw e n ty m in u te s p r i o r t o th e r e d u c t i o n .

C a re fu l

r o ta tio n o f th e f la s k about th e a x is o f th e ground g la s s j o in t enabled th e p r e s s u r e o f n itr o g e n t o f o r c e s o l u t i o n t h r u th e r e d u c t o r .

The

amount o f flo w was c o n t r o l l e d by the d e g re e o f r o t a t i o n o f t h e f l a s k , and t h e r a te by t h e n itr o g e n p r e s s u r e .

A f r e s h z i n c f i l l i n g w as u se d

a f t e r e v e ry tw o o r , a t m o st, t h r e e r u n s.

The f i r s t

5

t o 10 m l. o f

s o l u t i o n was u se d f o r f l u s h i n g and a c tiv a tin g th e red u ctor, a f t e r w h ich

the desired volume, u s u a lly o n ly 10 m l . , was passed.

I f t h i s was to

be p o la ro g ra p h e d a t once, i t was c o lle c t e d d i r e c t l y i n a n i t r o g e n sw ept c e l l .

I n th e ampsrometric r u n s , th e rhenide s o l u t i o n o f knovm

concentration was adm itted near th e top o f a narrow graduated tube which contained a Bunsen v a lv e at th e top and a stopcock at th e bottom. A d e fin ite volume o f th e rhenide s o l u t i o n c o u ld th en be r u n i n t o th e p o la ro g ra p h c e l l and mixed m t h a f ix e d volume o f dearated w a te r

containing th e d esired q u a n t i t y o f c h e m ic a l o x id a n t. D is c u s s io n G e n e ra l C h a ra c te r o f A nodic Rhenide Waye_s_.

Rhenide oxid ation

c u rv e s o b ta in e d i n a s u l f u r i c a c i d medium alw ay s h ad the gen eral f e a t u r e s shown i n F ig u re 4 when p l o t t e d from d a ta obtained m a n u a lly a t 1 5 °.

F iv e d i s t i n c t a n o d ic w aves w ere alw a y s o b se rv e d and t h e s e

w i l l be design ated a s A, A ', B, G, a n d D , r e s p e c tiv e ly .

These fe a tu r e s

co n c u r w ith th ose fbund b y L in g a n e e x c e p t f o r th e f a c t t h a t t h e seco n d

wave, w hich i s here c a lle d A ', apparently was not a s c le a r ly demarcated when a u to m a tica lly recorded a t 0 ° .

The p resen t in te r p r e ta tio n o f th e

p r o c e s s e s in v o lv ed i n th e v a r i o u s waves, however, w i l l d i f f e r c o n s id e r a b ly from t h a t p r e v i o u s l y g iv e n .

24

R hen id e o u rv ee i n 2 N m e th a n e s u lf o n io a c i d w ere fo u n d t o h e i d e n t i c a l i n e v e r y r e s p e c t w i t h th o s e o b ta in e d i n 2 N s u l f u r i c a c i d . The same p a t t e r n a ls o r e s u l t e d i n

2

N p e rc h lo ric a c id , b u t g re a t d if -

f i c u l t y was e x p e r ie n c e d i n a t t a i n i n g an d m a in ta in in g lOOf. r e d u c tio n o f t h e rh e n iu m ,

m i l e L in g an e was a p p a r e n tly a b le t o m a in ta in r h e n id e

s o l u t i o n s i n d i l u t e p e r c h l o r i c a c i d m edia a t 0 ° , i t was n o t p o s s i b le to do so a t 1 5 ° .

T h is i n s t a b i l i t y m ig h t be a n t i c i p a t e d fro m th e f a c t s

t h a t r h e n id e h a s b een shown t o re d u c e t i t a n i c i o n (22) w h ile t i t a n o u s io n h a s b e e n u se d to re d u c e p e r c h l o r a t e ( 1 0 ) . The r h e n id e o x id a tio n s i n a 1 M ( o r 2 N, b a s e d on t r o io n iz e a b le h y d rogen i o n s ) p h o s p h o ric a c i d s o l u t i o n f o llo w an i d e n t i c a l p a t t e r n a s shown i n F ig u re 5 .

The G-w ave, h ow ever, h a s b ee n d is p l a c e d

fro m i t s n o rm a l p o s i t i o n o f - 0 . 0 2 v . i n s u l f u r i c a c i d t o th e more n e g a tiv e p o s i t i o n o f - 0 .2 1 v .

A p o s s i b le r e a s o n f o r t h i s d is p la c e m e n t

m i l be d is c u s s e d l a t e r . A com plete s tu d y o f t h e w aves in a s u l f u r i c a c id medium w as

undertaken w ith r e sp e c t t o a l l v a r i a b l e s o f probable s i g n i f i c a n c e . The p r in c ip a l fe a tu r e s o f t h is stu d y w i l l be ta b u la ted and b r i e f l y d iscu ssed . Most of th e c h a r a c t e r is t ic s described should apply eq u a lly w e ll to rhenide s o l u t i o n s i n m eth an esulfon ic, p e r c h lo r ic , o r phosphoric a c i d s . The h a lf- w a v e p o t e n t i a l s o f t h e v a r i o u s w aves a r e g iv e n i n T ab le 5 .

The A-wave was found t o e x h i b i t o n ly m o d e ra te s h i f t i n g w i t h

te m p e r a tu re o r a c i d i t y c h a n g e s and no s h i f t w ith ch a n g e s i n th e con­ c e n t r a t i o n o f r h e n id e .

T h is e v id e n c e a c c o r d s w ith th e e x p e c te d b e h a v io r

f o r a r e v e r s i b l e w ave.

The A '-w ave i s to o s m a ll t o be r e a d w ith an y

s ig n ific a n t p re c is io n .

The B- and G -waves show s i ,g n i f i c a n t s h i f t s

w ith te m p e r a tu re an d w i t h a c i d i t y , r e s p e c t i v e l y .

25

B o th w aves d ia p la y p o s i t i v e s h i f t s , o o r re s p o n d ih g t o more d i f f i c u l t o x i d a t i o n , w ith in c r e a s i n g r h e n id e c o n c e n t r a tio n . th e B- and 0 - s t e p s a r e i r r e v e r s i b l e .

A p p a r e n tly , b o th

Tbs p o t e n t i a l o f t h e D-wave i s

somewhat v a r i a b l e , t h e t e m i n a l end o f t h e wave i s o f t e n d i s t o r t e d , and b o th e n d s o f t h e w ave d i s t o r t w ith h ig h c o n c e n t r a tio n s o f r h e n id e o r a t e l e v a te d te m p e r a t u r e s . The s lo p e s o f t h e v a r i o u s w av es a r e g iv e n i n T a b le

6

.

A

v a lu e o f 0 .0 2 8 6 io -0 0 2 2 v . was o b ta in e d f o r t h e A-wave a s t h e a v e ra g e from e le v e n r u n s .

T h is com pares f a v o r a b ly w ith th e t h e o r e t i c a l v a lu e

o f 0*0275 V. f o r a r e v e r s i b l e t w o - e le c tr o n p r o c e s s a t 15 .

The

v a lu e s f o r t h e A’ - and D -w aves a r e n o t r e a d a b le w ith s u f f i c i e n t a c c u ra c y t o w a r ra n t comment.

The s l o p e s o f t h e B- and G-waves o f t e n

ap p ro x im a te t h e s i n g l e - e l e c t r o n v a lu e o f 0 -0 5 5 v . , b u t one can n o t a t t a c h much im p o rta n c e t o t h i s f a c t i n v iew o f t h e i r v a r i a b i l i t y an d th e p ro b a b le i r r e v e r s i b i l i t y o f b o th w av es. The v a l u e s o f i^ /C w ere fo u n d t o be e s s e n t i a l l y c o n s ta n t f o r a l l o f t h e w aves w ith changes i n a c i d c o n c e n t r a tio n o v e r th e ra n g e 0 .8 to 4 N. The ia /C v a lu e s f o r t h e B- and G-waves a p p e a re d t o be c o n s ta n t w ith in t h e l i m i t s o f e x p e r im e n t a l e r r o r f o r c h a n g e s i n ih e n id e c o n c e n tr a tio n o v e r t h e ra n g e 0 .8 t o 0 . 8 rM.

B oth t h e A- and t h e A '-

w aves a p p e a re d t o e v id e n c e a d e f i n i t e a n d com plem entary v a r i a b i l i t y . T hus, i j / O f o r th e A-wave

a p p r o a c h e d

a

m axim al v a lu e a t a 0 .5 5 iffl

c o n c e n t r a tio n , w h ile 1 ^ /0 f o r t h e A '-w av e a p p ro a c h e d a m in im al v a lu e a t t h e same c o n c e n t r a t i o n .

I f one com bines th e c u r r e n t s o f th e A-

and A '-w a v e s, p l o t t i n g i ^ / 0 o f th e com bined w aves v s . 0 , a s t r a i g h t

26

une

r e la tio n r e s u lt s t t i o h i s p a r a l l e l to th e O -a U s w ith in

experim ental e r r o r . I t i s worth n o tin g , in a d d itio n , th e approximate r e la t iv e magnitudes o f such ij /O v a lu e s f o r a t y p ic a l case.

As an averaged

in stan ce a t 0.5 S t # rhenide co n cen tra tion , i j / 0 v a lu e s were found o f 5 .5

fo r t h e A -w ave, 0 .7 fo r t h e A ' -w av e, 6 .2 for t h e com bined A -w aves,

3 .1 f o r

the

B rw ave, and

2.5

f o r t h e G-wave.

The v a r i a t i o n i n t h e o b se rv e d c u r r e n t s o f fO ur o f th e w aves

w ith temperature and w ith the h eig h t o f th e mercury r e se r v o ir i s presented in Table 7 .

While none o f t h e observed temperature

c o e f f ic ie n t s are n o ta b ly abnormal f o r a d iffu s io n p r o c e ss, i t i s tr u e th a t th e A-wave value o f 3 .2 $ i s somewhat high w hile th e A'-wave f i g i r e Of 1.2% i s s l i g h t l y lo w .

The A -, B -, and G-waves seem t o behave w ith r e sp ec t t o th e h e ig h t o f t h e m e rc u ry r e s e r v o i r i n a c c o r d m t h th e e x p e c t a t i o n f o r

d iffu s io n -c o n tr o lle d p r o c e sse s.

ru

The A'-wave, however, seems t o fo llo w

an in ten m d ia t^ ca se between d iffu s io n and ad so rp tio n c o n tr o l ( 2 ) . To summarize th e r e s u l t s of t h i s se c tio n , one may say th a t the fo llo w in g con clu sio n s are a t l e a s t t e n t a t iv e ly e sta b lish e d : (1 )

The A-wave i s a r e v e r sib le , tw o -e le c tr o n , d i f f u s i o n -

c o n tro lle d p ro c e s s . (2)

The A’ -wave d oes n o t a p p e a r to in v o lv e a p u re

d iffu s io n p ro cess; an (3 )

adsorption e f f e c t maybe in v o lv e d .

T here a r e s e v e r a l i n d i c a t i o n s t h a t th e A - and A

w aves may have some d i r e c t i n t e r r e l a t i o n . (4)

Both the B- and G-waves may in volve o n e-electro n

p r o c e s s e s , b u t b o th a p p e a r t o be i r r e v e r s i b l e .

27

(5 )

The m a g n itu d e o f th e A '-w ave i s a lm o s t to o s m a ll t o

j u s t i f y i t s i n t e r p r e t a t i o n a s e v e n a f u l l o n e - e le c tr o n p r o c e s s . (6 )

The B- and G-waves a r e 'e a c h r o u g h ly o n e - h a l f t h e s i z e

o f t h e com bined A -w aves. "A m nerom etric" O x id a tio n o f R h e n id e .

F o r b r e v i t y , th e u s e

o f t h e te r m am perom etry w i l l h e r e b e p e r v e r te d t o a p p ly t o t h e p r o c e s s o f r e c o r d in g t h e c o m p le te p o la ro g ra m o f a r h e n id e s o l u t i o n a f t e r t h e a d d i t i o n o f e a c h s u c c e s s iv e , o n e - e q u iv a le n t in c re m e n t of o x id a n t u n t i l

e ig h t e q u iv a le n ts have been in c lu d e d .

The apparatus and technique

f o r a c c o m p lish in g t h i s r e s u lt have a l r e a d y been o u tlin e d . s p e c i f i c ex am p le, a

0.5

nM p e r r h e n a te s o l u t i o n i n

was reduced i n t h e s m a ll r ed u cto r.

2

N s u l f u r i c a c id

E x a c tly 1 0 -0 m l. o f t h e r e s u l t i n g

rh e n id e s o l u t i o n was d is p e n s e d i n t o t h e p o la ro g r a p h c e l l . 5.0

I n a more

E x a c tly

m l. o f a d e a r a te d s o l u t i o n co n ta in in g , e . g . , th r e e chem ical

e q u iv a len ts o f dichromate (based on 0-005 m illim o le o f rhenium) was added fro m a p i p e t and m ix ed by means o f a nitrogen -b u b b ler.

The

com position o f th e r e s u lt in g s o lu tio n would th e n be reckoned a s 1.53 N in s u lfu r ic a c i d a n d 0 .3 3 mM i n rhenium o f "equivalent oxid ation le v e l"

(II).

I t i s perhaps worth n o tin g s e v e r a l n e c e s s a r y precautions

w hich w ere o b s e rv e d .

A z i n c r e d u c to r f i l l i n g c o u ld b e u s e d f o r

more than two runs to ensure q u a n tita tiv e reducing a c t io n .

A p o s itiv e

pressure o f n itro g en was alw ays used for moving t h e so lu tio n sin c e th e en^loyment o f vacuum e n t a ils th e danger of con cen tration changes due to evaporation o f w a ter, lo s s o f perrh en ic a cid or other v o la t ile rhenium intem nediates ( 2 3 ) , e t c .

Only one polarogrsm for a p a r tic u la r

"equivalent o x id a tio n le v e l" was attem pted per m n; i . e . , the tim e req u irem n t was too great to s a f e l y perm it th e d eteim ination o f a

28

co m p lete cu rv e a f t e r e a c h o f s e v e r a l s u c c e s s iv e a d d i t i o n s o f o x i d a n t . D i l u t e o x id a n t s o l u t i o n \m s a lw a y s added g r a d u a l l y t o t h e r h e n id e s o l u t i o n w ith s t i r r i n g , th u s m in im iz in g a n y p o s s i b le e f f e c t o f l a r g e lo c a l e x c e sse s o f o x id a n t. The c u r v e s shown i n F ig u re

6

r e p r e s e n t t h e s u c c e s s iv e

" e q u iv a le n t l e v e l s " from t h e r h e n id e t o th e p e r r h e n a te s t a t e s a s o b ta in e d w ith d i(h ro rn a te o x i d a t i o n .

The o b se rv e d b e h a v io r o f th e w aves

on o x id a tio n t h r u s u c c e s s iv e s t a g e s i s r a t h e r u n u s u a l an d i s v e r y d i f ­ f i c u l t t o i n t e r p r e t w i t h a b s o lu te c e r t a i n t y . I t i s a p p a r e n t t h a t some o f t h e lo w e r s t a t e s m ust p e r s i s t t h r u s e v e r a l s t a g e s , b u t t h i s i s t r u e r o f t h e r h e n id e i t s e l f t h a n o f an y o f t h e o th e r in te r m e d ia te s t a t e s .

I n t h i s c o n n e c tio n , th e a u th o r s

co n c u r w i t h th e c o n c lu s io n o f lin g a n e (18) t h a t th e p lu s one s t a t e o b ta in e d by w arm ing r h e n id e s o l u t i o n s a c c o r d in g t o L u n d e ll and Knowles (2 2 ) i s a c t u a l l y o n ly an " e q u iv a le n t p l u s o ne” s t a t e c o n s i s t i n g o f a m ix tu re o f lo w er rh en iu m s p e c i e s .

The c o m p o s itio n o f s u c h a

s o l u t i o n seems to in c lu d e some b o n a f i d e p lu s o n e , a l a r g e r am ount o f r h e n id e , and e i t h e r rh en iu m ( I I I ) ,

( IV ) , o r b o th s u f f i c i e n t t o b a la n c e

th e (- I ) . I t s h o u ld b e n o te d t h a t t h e c a th o d ic - a n o d ic t r a n s i t i o n o f th e A-wave i n p a r t i a l l y o x id iz e d s o l u t i o n s p r e s e n t s a sm ooth tra n s itio n .

T h is f a c t m a y b e c i t e d i n f u r t h e r e v id e n c e o f th e

r e v e r s i b i l i t y o f th e i n i t i a l s te p . I n l i g h t o f t h e i n t e r p r e t a t i o n w h ich i s u l t i m a t e l y r e q u ir e d f o r th e c u r v e s , i t c a n a l s o be o b se rv e d t h a t t h e ( I ) s t a t e h a s l i t t l e te n d e n c y f o r in d e p e n d e n t e x i s te n c e a s a s p e c i e s i n s o l u t i o n i n

29

s u lf u r ic a c id .

T h is rem ark i s p r e d i c a t e d on t h e f a c t t h a t th e B-wave

re m a in s o n ly t o th e e x t e n t t h a t t h e A -w aves a r e p r e s e n t .

For re a s o n s

t o be shown l a t e r one m ust em p h asize t h e p r o v is o " i n s o l u t i o n in s u l f u r i c a c id " s in c e a s t a b l e b u t in s o lu b le (I) compound i s t h e u lti m a t e p r o d u c t o f t h e A '-w ave a n d s in c e i t i s p r o b a b le fro m t h e p r e s e n t w ork and t h e w ork o f o t h e r s (2 3 ,2 4 ) t h a t a s t a b i l i z e d (I) s p e c ie s d o es e x i s t i n s o l u t i o n i n h y d r o c h lo r ic a d d m e d ia . The l a r g e s h i f t o f t h e G-wave on t h e a d d i t i o n o f o n e e q u i v a le n t o f o x id a n t and i t s disappearance a f t e r t h e a d d itio n o f tw o e q u i v a le n ts a r e r a t h e r s p e c t a c u l a r . in te r p r e te d a s th e ( I I ) to

I f th e G-wave i s c o r r e c t l y

( I I I ) o x i d a t i o n , th e c o n c lu s io n may be

j u s t i f i e d t h a t th e ( I I I ) s t a t e a p p e a r s a s a s e p a r a te s p e c i e s i n s o l u t i o n o n ly when l a r g e am ounts o f lo w e r o x id a tio n fo rm s a r e p r e s e n t . A p p a r e n tly , t h e little

( I I I ) s t a t e i s r e l a t i v e l y u n s ta b le i n s o l u t i o n an d h a s

tendency t o e x i s t a s a s e p a r a te s p e c i e s .

Such b e h a v io r i s

an a lo g o u s w ith t h a t o f m anganese ( I I I ) f o r w h ich one f i n d s l i t t l e e v id e n c e by p o t e n t i om etr i c t i t r a t i o n s i n s o l u t i o n .

I t a ls o ac co rd s

w ith w hat i s known o f rh en iu m ( I I I ) s a l t s i n t h a t th e y a r e b e s t p r e -

pared in th e dry way and th e r e i s l i t t l e evidence fo r them in e le o t r o c h e m ic a l r e d u c t i o n s i n s o l u t i o n .

The n eg a tiv e s h i f t i n phosphate

m edia (F ig u re 5) im p l ie s a s t a b i l i z a t i o n o f t h e o x id a tio n p ro d u c t o f th e G-wave i n t h e p r e s e n c e o f p h o s p h a te . h e re w ith t h e i o d in e compound IPO4 . o f io d in e i n t h e

An a n a lo g y m ig h t be draw n

The s t a b i l i t y a n d v e ry e x i s te n c e

( I I I ) s t a t e seem s to be f a v o r e d by p h o sp h a te ( l o ) .

The appearance o f t h e E-wave a t l a t e r s ta g e s i n th e o x id a tio n may be a t t r i b u t a b l e t o t h e e x is te n c e o f a f r e e rh en iu m (7)

30

s p e c ie s i n s o l u t i o n a t t h i s p o i n t .

T h ere i s some e v id e n c e fro m o u r

w ork a n d fro m t h a t o f Maun ( 2 3 .2 4 ) t h a t t h e o x i d a t i o n p o t e n t i a l f o r t h e ( V ) / ( 7 I I ) c o u p le m a y b e more p o s i t i v e (L a tim e r c o n v e n tio n ) th a n th o s e o f t h e ( I 7 ) /( V ) o r (X 7 )/(V I1 ) c o u p le s .

T h e r e f o r e , w h ile t h e

v i s i b l e p o r t i o n o f t h e s te p w is e o x i d a t i o n o f r h e n id e d o e s n o t in c lu d e o x id a tio n s up t o o r beyond t h e (IV ) s t a t e , i t i s n e v e r t h e l e s s p o s s i b l e t h a t rh e n iu m (V) when p r e s e n t i n s i t u may be o x id is e d i n t h e a v a i l a b l e p o te n tia l ran g e . one i s te m p te d t o c o n c lu d e fro m t h e ch a n g es o f t h e D-wave i n am perom etry t h a t i t m ust h av e some s i g n i f i c a n c e w ith r e s p e c t t o rh en iu m p r o c e s s e s .

On th e o t h e r h a n d , i t s p r o x im ity w ith a n y calom el

d is c h a r g e v Jiich may o c c u r a s a r e s u l t o f c h l o r i d e i o n s d i f f u s e d fro m th e r e f e r e n c e a n o d e , th r o w s some d o u b t on t h i s i n t e r p r e t a t i o n .

I t w as

fo u n d , how ever, t h a t a s i m i l a r w ave e l s e e x i s t s a t a c o rre s p o n d in g p o t e n t i a l v s . a m e rc u ro u s s u l f a t e r e f e r e n c e e l e c t r o d e .

The h a lf- w a v e

p o t e n t i a l o f t h i s wave i s , u n f o r t u n a t e l y , o n ly r e p r o d u c ib le w ith in io .0 3

V.

The b e h a v io r o f rh en iu m (111) c h l o r i d e w i l l be m e n tio n e d

l a t e r i n t h i s c o n n e c tio n . I n t e r p r e t a t io n and R t e n t i a l s o f R h en lte C o u ^ .

numerous

. s u b s i d ia r y l i n e s o f e v id e n c e h av e a l r e a d y b e e n p r e s e n t e d i n su p p o rt o f the c o n c lu s io n s W hich w i l l now b e draw n fro m more d i r e c t e v id e n c e . The a g g r e g a te e v id e n c e i s q u ite c o n v in c in g ly i n f a v o r o f th e i n t e r p r e t s . t i o n s g iv e n . I t w as f i r s t o b se rv e d t h a t p o ta s s ln m h e x a c h lo r o r h e n a te (IV) i n s u l f u r i c a c i d m edia gave no a n o d ic w aves p r i o r to w here ca lo m e l d is c h a r g e m ig h t be a n t i c i p a t e d .

A g a in s t t h e o bj

0.

31

t t a t t h i e r e s u l t mlEjit m e re ly e v id e a o e t h e p o s i t i v e a i s p k o e m e u t o f th e t r u e

(IT ) wave due t o c o m p le tin g , th e more r e a d i l y o x i d i t e d (IT )

a i a e i e s n o te d hy I ^ u n (33) was a l s o t e s t e d .

U nder a n i t r o g e n atm o s­

p h e r e , h y d ro u s rh en iu m d io x id e w as p r e c i p i t a t e d fro m a h e x a o h lo r o r h e n a te s o l u t i o n w ith h a s e , d is s o l v e d i n th e minimum amount o f h y d ro ­ c h l o r i c a c i d , a n d a p p r o p r i a t e l y d i l u t e d i n a i r - f r e e s u l i U r i c a c id , im m ediate e x a m in a tio n o f s u c h a s o l u t i o n showed no a n o d ic w av e s p r i o r t o i n c e p t i o n o f a c a lo m e l wave a t . 0 .1 8 v .

E le c tr o ly s is of hexa-

c h lo r o r h e n a te (IT ) w ith a s t a t i o n a r y p te tin u m m ic r o e le c tr o d e d o es r e s u l t i n a n a n o d ic w ave, h u t o n ly a t . 0 . 3 v .

A p p a r e n tly , t h e p a t t e r n

o f a n o d ic r h e n id e w aves o b s e rv e d a t t h e d ro p p in g m e rcu ry e l e c t r o d e m ust in v o lv e o x id a tio n o f t h e r h e n id e t o a l e v e l o f (IT ) o r l e s s . I t was n e x t fo u n d t h a t rh e n iu m ( I I I ) c h lo r id e i n s u l f u r i c a c id gave no a n o d ic w aves p r i o r t o t h e r e g io n o f th e D -w ave.

The

h a lf-w a v e p o t e n t i a l v a r i e d b etw een . 0 .1 4 ,and .0 .1 7 v . . b u t t h e wave seemed to b e r e a s o n a b ly d e m a rc a te d fro m t h e i n c e p t i o n o f t h e o a lo .a e l d is c h a r g e w h ich f o llo w s a t ç a . . 0 . 2 v .

I t a p p e a rs l i k e l y t h a t t h e

B-wave may in v o lv e t h e ( I I I ) t o (IT ) s t e p .

I t seem s c e r t a i n t h a t none

Of th e p r e c e d in g a n o d ic w aves can in v o lv e rh e n iu m l e v e l s h ig h e r th a n

(III). U n f o r tu n a te l y , th e o o u lo m e tric (1 9 .2 0 ) o x id a tio n o f a rh e n id e s o l u t i o n a p p e a r s to in v o lv e a n o v e r v o lta g e phenomenon w h ich d o e s n o t p e rm it t h e e n t i r e l y unam biguous i d e n t i f i c a t i o n o f a g iv e n p o . e n t i a s e t t i n g ï à t h t h e p o t e n t i a l o f a s p e c i f i c w ave.

At p o t e n t i a l s

a p p r e c ia b ly more n e g a tiv e t M n - 0 . 3 v . no s i g n i f i c a n t flow o f c u r r e n t was o b ta in e d th ro u g h th e c o u lo m e ta r. Of 1 .8 1 e l e c t r o n s was o b ta in e d .

A t - 0 .2 v . a c u r r e n t- e q u i v a le n t

I f t h e p o t e n t i a l w as s e t a t - 0 .1 v . .

32

a f i g u r e o f 1 .9 7 e l e c t r o n s w as f o u n d .

W ith o u t, tm d u ly e l a b o r a t i n g o n

th e p o i n t , i t may sim p ly be s t a t e d t h a t f o r v a r i o u s r e a s o n s i t seem s r e a s o n a b le t o i d e n t i f y t h e f i r s t r e s u l t w ith th e i n i t i a l o x id a tio n s t e p , o r m t h t h e A-w ave. a n d t h e s e c o n d f i g u r e w i t h t h e same p r o c e s s p lu s t h e se c o n d s t e p (A *-w ave). A t t h e c o n c lu s io n o f r u n s o f t h e se c o n d t y p e , i t was fo u n d t h a t an in s o l u b l e p ro d u c t h ad s e p a r a t e d .

T h is m a t e r i a l w as a

b r o w n is h -b la c k , am orphous s o l i d , was i n s o l u b l e i n c o n c e r.tn > te d h y d ro ­ c h l o r i c a c i d , c o n ta in e d rh en iu m , c o n ta in e d no more th a n i n c i d e n t a l m e rc u ry , and r e s u l t e d from a tv io - e le c tr o n o x id a tio n o f r h e n i d e .

It

seems l o g i c a l to i d e n t i f y su c h a p r o d u c t w ith t h e s u b o x ld e d e s c r ib e d by Toung and I r v i n e

(3 7 ).

A s m a ll sam ple o f t h e m a t e r i a l w as c a r e f u l l y

s e p a r a te d from a d h e r e n t m e rc u ry and d r ie d f o r tw e lv e h o u r s o v e r c a lc iu m c h lo r id e a t s l i g h t l y re d u c e d p r e s s u r e .

D i s s o l u t i o n o f 5 mg. o f t h i s

m a t e r i a l i n e x c e s s a c i d i f i e d d ic h ro m a te s o l u t i o n r e q u ir e d n e a r l y 40 h o u rs.

The o x id a n t con su m p tio n w as found by b ac k t i t r a t i o n w itn

f e r r o u s io n t o have b e e n

1 1 .2

é q u i v a l a n t s p e r m ole o f (assum ed)

rhenium s u b o x id e d ih y d r a te a s com pared to a c a l c u l a t e d v a lu e o f

12

.

A p p a re n tly , t h e A-wave i s t o be i n t e r p r e t e d a s a n e s s e n t i a l l y tw o - e le c tr o n o x id a tio n o f r h e n id e t o rh en iu m ( I ) , a n a t h e A '-w ave a s in c lu d in g t h e e x t r a e n e rg y f o r t h e c o n v e rs io n o f rh en iu m ( I ) t o rhen iu m Cl) o x id e . The rh e n iu m "amalgam" m e n tio n e d e a r l i e r w as t r i e d a s a d ro p p in g e l e c t r o d e i n t o a 2 N s u l f u r i c a c id m edium .

The f a c t t h a t no

an o d ic w aves r e s u l t e d i s . a g a i n , p r o b a b ly o n ly a r e f l e c t i o n on th e d u b io u s s t a t u s Of t h i s m a t e r i a l a s a b o n a f i d e am algam. The u s e o f am algam ated cadmium a s a r e d u c t a n t f o r p e r r h e n a te

33

in

6

to

8

N s u l f u r i c a c i d was fo u n d t o g iv e a n " e q u iv a le n t (Y) s t a t e

o f rh e n iu m Ib y t i t r a t i o n ) h a v in g a c h a r a c t e r i s t i c ^ ^ llo w -b ro w n o o lo ra tio n .

INhen d i l u t e d t o ç a .

a d j u s te d t o + 0 .0 9 V.

2

i # rh e n iu m c o n c e n t r a t i o n and

0 .4

N a c i d i t y , su c h s o l u t i o n s show ed an a n o d ic w ave a t ç a .

One i s te m p te d to v e n t u r e an i d e n t i f i c a t i o n o f t h i s wave

w ith t h e E-w ave n o te d i n a m p ero m etry , a n d t o d e s c r ib e i t a s a n o x id a tio n o f t h e (V) s t a t e .

No c o n c lu s iv e p r o o f f o r t h i s h y p o th e s is

can b e o f f e r e d . I t w ould b e d e s i r a b l e t o o b ta in some in d e p e n d e n t v e r i f i c a t i o n o f t h e p o t e n t i a l s c o r r e s p o n d in g t o th e rh en iu m c o u p le s In v o lv e d i n th e v a r io u s p o la r o g r a p h lc w av es.

C l a s s i c a l p o t e n t i a l m easu rem en ts a t

a b r i g h t p la tin u m e l e c tr o d e c a n n o t v e r y w e l l be a t t a i n e d s in c e b o M f U e s a n p le s o f p u re rh en iu m compounds I n t h e v a r i o u s lo w er o x id a tio n s t a t e s a r e n o t o b ta in a b l e a n d , m o re o v e r, th e c o u p le s more p o s i t i v e (L a tim e r c o n v e n tio n ) th a n h y d rogen w ould g iv e m ixed p o t e n t i a l s I n a c id m e d ia .

I t I s o f I n t e r e s t , how ever, t o n o te t h a t t h e p o t e n t i a l o f t h e

rhenium ( - D / t D te s ts .

c o u p le may be a p p r o x im a te ly c o n firm e d b y ch e m ic a l

To t h i s e n d . 0 .5

r h e n id e s o l u t i o n s i n 2 N s u l f u r i c a c id

w ere m ixed w ith e q u a l volum es o f d i l u t e s o l u t i o n s o f t h e o x id iz e d form s Of v a r io u s c o u p le s .

By f i n d i n g t h e m ost p o s i t i v e c o u p le w h ich

i s j u s t r e d u c i b l e b y r h e n id e an d th e m ost n e g a tiv e c o u p le

w h ich I s

j u s t n o t r e d u c i b l e , one may e s t a b l i s h a p p ro x im a te l i m i t s b e tw e e n Which th e r h e n id e co u p le m ust l i e .

P ro c e e d in g I n t h i s m anner th e

r e s u l t s r e c o r d e d I n T a b le 8 w ere o b ta in e d ,

m us. i t was fo u n d t h a t

r h e n id e I n 1 K s u l f u r i c a c i d a t 15" w i l l n o t re d u c e c o b a lto u s I o n b u t w i l l re d u c e cu p ro u s I o d i d e .

A c c o rd in g ly , one may e s tlm a t

r h e n id e c o u p le a s ly in g I n te r m e d ia t e b etw een . 0 .2 8 v . a n d .0 .1 9 v . .

34

o r a t ç a . . 0 .2 3 ± 0 .0 5 T . « . t h e H .H .B .

T h is a g r e e s q u ite w e l l w ith

th e p o l a r o g r ^ h l o h a lf-w a v e p o t e n t i a l f o r t h e A-wave o f - 0 .5 0 5 v . v s . t h e S .O .E . ( p o la r o g r a p h lc c o n v e n tio n ) o r . 0 .2 6 v . v s . th e N .H .E . ( o x i d a t i o n p o t e n t i a l h y L a tim e r c o n v e n tio n ) . Summary E vidence has been giv en t o in d ic a t e t h a t t h e p o la ro g r a p h ic w aves o b ta in ed f o r rh en id e s o lu t io n s in d i l u t e s u l f u r i c a c id m ust in v o lv e th e ( - 1 ) to ( I ) o x id a tio n a t - 0 .5 0 v . , th e (-1 ) t o ( I ) - o x ld e o x id a tio n a t - 0 .3 8 v . , th e ( I ) t o ( I I ) o x id a tio n a t - 0 .2 7 v . . and th e ( I I ) t o ( I I I ) o x id a tio n a t - 0 .0 2 v . when e x p r e sse d by th e p o la r o ­ g ra p h lc co n v en tio n and v s . th e S.O .E . ' There I s some e v id e n c e t h a t th e wave a t ç a . . 0 .1 7 v . may In v o lv e o x id a tio n o f th e ( I I I ) s t a t e and t h a t th e p resen ce o f a fr e e (V) s p e c ie s r e s u l t s In a wave a t ç a . +0*09

V.

The p o t e n t i a l s a p p ly in g t o th e v a r i o u s c o u p le s b a s e d on th e p r e c e d in g I n t e r p r e t a t i o n a r e s u m m rlz e d I n T ab le 9 . th e v a lu e f o r t h e ( - l ) / ( D

I t Is f e l t th a t

co u p le o f .0 .2 3 0 v . v s . th e N .H Æ . I s q u i t e

r e l i a b l y e s t a b l i s h e d ; t h i s f i g u r e h a s b e e n c o r r e c te d t o a p p ly a t u n i t a c t i v i t y o f h y d ro g e n Io n and a t 2 5 " . c o u p le s a p p ly s p e c i f i c a l l y t o 0 . 4 a c id a n d a t 1 5 " .

The f i g u r e s f o r t h e r e m a in in g r h e n id e s o l u t i o n s I n 2 K s u l f u r i c

L e s s therm odynam ic s i g n i f i c a n c e I s t o be a t ta c h e d

t o th e s e v a lu e s i n view o f th e p ro b a b le i r r e v e r s i b i l i t y o f t h e w aves on which t h e y a re p r e d ic a te d . The g e n e r a l c h a r a c t e r an d b e h a v io r on o x id a tio n o f r h e n id e s o l u t i o n s i n s u l f u r i c . m e th a n e s u lA n lc . p e r c h l o r i c , o r p h o s p h o ric a c i d s i s f irid a m e n ta lly th e sam e.

The s t a b i l i t y o f r h e n id e I s v e r y

35

lim it e d i n p e r c h lo r ic a c id , how ever, and th e e x is t e n c e o f th e rhenium ( H I ) s t a t e a s an in te r m e d ia te s p e c i e s seems t o be fa v o re d in p h osp h oric a c i d . I t i s c l e a r ly in d ic a t e d t h a t rhenium shows l i t t l e te n d e n c y in g e n e r a l t o e x i s t in th e in te r m e d ia te ( I ) , independent s p e c ie s in aqueous m edia.

HD,

or ( I I I ) s t a t e s a s

36

F ig . 3

M o d ifie d J o n e s R e d u c to r:

r e d u c t o r tu b e

i s 12 mm. diem. O.D. x 20 cm. lo n g ; W, w a te r ja c l® t; C, s to p c o c k ; J , gro u n d g l a s s j o i n t ; F , sam ple f l a s k ; T, n itr o g e n i n l e t tu b e .

37

F ig , 4

R henide C u rv es i n S u l f u r i c A cid ;

g e n e r a l a p p e a ra n c e and n o m e n c la tu re o f t h e w av es; th e number o f e l e c t r o n s in v o lv e d p e r s te p i s in d i c a t e d a c c o r d in g t o th e p r e s e n t s tu d y ( a t th e to p ) and a c ­ c o rd in g t o L in g a n e ( i n p a r e n th e s e s , a t th e b o tto m ).

i

(Hÿ)

E

38

î>ig. 5

RlieiiicLe C u rv es i n P h o s p h o ric A cid:

0 . 5 m r h e n id e i n

1

M p h o s p h o r ic a c i d a t 1 5 “ .

0

1

mlcroamps* 2

3

0.0

0.2 E , V.

0 .4

0.6

■

39

T ab le 5 H alf-W ave P o t e n t i a l s o f A nodic R h en id e Waves i n S u l f u r i c A cid S h i f t i n Eq

V. A

V.

- 0 .5 0 5

A'

5

m th

T.

+ 0 .0 2 1

- 0 .3 8

________

T.

n il

* 0 .0 1 9

( n o t a o o m a te ly r s a i a b l e )

B

- 0 .2 6 8

* 0 .0 6 0

* 0 .0 4 6

* 0 .0 1 1

Q

- 0 .0 2 0

* 0 .0 1

* 0 .0 4 0

* 0 .0 4 6

+ 0.17

( n o t a c c u r a t e l y r e a d a b le )

T ab le

6

S lo p e o f 0 , 4 irM R henide Waves i n 2 ,0 N E 3 SO4 a t 15* Remarks Wave

% . 7 5 " ^ 0 ,2 5 ^

A

0 .0 2 9

A*

0 .0 6

B

0 .0 5 0

G

0 .0 5 3

D

0 .0 4

Highly r e p r o d u c ib l e ( -0 * 0 0 2 ). Too s m a ll t o r e a d a c c u r a t e l y . V a r ia b le :

0*04 m i n . , 0*08 max.

V a r ia b le :

0 .0 3 m in ., 0*07 max.

U n sy ram etrical a b o u t m id p o in t.

40

T ab le 7 E f f e c t o f T em p eratu re a n d Hg-Head on C u r r e n t

Wave

Temp. C o e ff . o f Xa 1R® f n 2 *^® ra n g e ) ^/* C

id , ua. O b served a t 30 cm. 70 cm.

C a lc d . f o r 70 cm. K in . D i f f . Ads.

3 . 29 G

2 .7 7

4 .1 2

4 .2 3

6 .4 8

2 .7 7

A

l.BgG

0 .5 6

1 .1 1

0 .8 6

1 .3 1

0 .5 6

A*

1 . 6^

1 .6 2

2 .4 1

2 .4 8

3 .7 9

1 .6 2

B

2 .5 ^

1 .1 8

1 .7 8

1 .8 0

2 .7 6

1 .1 8

G

41

E ig .

6

AifÇ)erometric O x id a tio n o f R h en id e i n

S u l f u r i c A cid :

0 .3 3 i # r h e n id e i n 1 .3 3 N a c id (b o tto m

c u rv e ) o x id iz e d t h r u s u c c e s s iv e " e q u iv a le n t o x id a tio n l e v e l s " b y in c re m e n ts o f d ic h ro m a te t o th e p e r r h e n a te l e v e l (c u rv e 8 ) :

k - 1 .6 6 5 a t - 0 .5 2 v . , 1 .6 7 0 a t

- 0 .2 6 V ., and 1 .6 8 4 a t + 0 .0 5 v .

0

2 microamps o 4

0.2

■0.4

0 .0

E, volt

0, 6

■

42

T ab le

8

C hem ical E s tim a te o f E f o r th e Re ( - I ) / ( D S ystem

Couple R e s u lt

So

T l / T l ’*’

+ 0 ,3 4

neg.

Co/Oo’*"*’

+ 0 ,2 8

neg.

N i/N i’’"*’

+ 0,25

(? )

Cu + I "/O u i

+ 0,19

p o s.

'Ag + l " / Â ^

+ 0,15

p o s.

- 0 ,0 5

pos.

S b/S b’*’®

- 0 ,1 0

p o s.

Bi/Bi'*'®

—0 , 2 0

pos.

rh e n id e on th e o x id iz e d fo rm o f t h e sy stem , * i , e , , th e e f f e c t o f

T able 9 O x id a tio n P o t e n t i a l s o f Rhenium C o u p les E ( N ,H .E .) , T,

C ouple Re" 2Re" Re

+

Re'*’

•I- HgO -> RegO TN

Re

+2

+0,230

2 e"

+

+

Re’*’® -> Re’*’® + (Re’*'®

o x id a tio n

(Ee*®

o x id a tio n

+

2 H’*’

+

4e

+C,1 3

e

+0 , 0 2

e"

- 0 ,2 3 - 0 ,4 2 , o r more n e g .) - 0 ,3 4 , a p p r o x ,)

43

PART I I I TB2 m rU RE OF THE RHSmDE SBSCEES IN HYDR0HA10ŒN ACIDS

I n tro d u c tio n A c u r s o r y p e r u s a l o f t h e r h e n id e cu rv e i n h y d r o c h lo r ic a c id shows t h a t t h e v a r i o u s a n o d ic w aves a r e n o t n e a r l y a s w e l l d e f in e d i n t h i s medium a s i n t h e f o u r a c i d s p r e v io u s l y d is c u s s e d .

C lo s e r

e x a m in a tio n o f t h e c u r v e , how ever, r e v e a l s t h e i n t e r e s t i n g f a c t t h a t a d e f i n i t e c a th o d ic wave a l s o a p p e a r s i n h y d r o c h lo r ic a c i d , e s p e c i a l l y i f t h e s o l u t i o n h as ag e d f o r a s h o r t tim e .

I t so o n e v o lv e d t h a t th e

phenomenon was common t o r h e n id e s o l u t i o n s i n a l l o f t h e h y d ro h a lo g e n a c id s .

O th e r l i n e s o f e v id e n c e w ere fou n d t o s u p p o r t th e v iew t h a t

some s p e c i f i c ty p e o f c o m p le x a tio n o r compound fo rm a tio n m ust o b ta in f o r r h e n id e s o l u t i o n s i n th e s e m e d ia .

A p r e li m in a r y s tu d y o f th e

n a t u r e and b e h a v io r o f r h e n id e i n t h i s en v iro n m en t w as u n d e r ta k e n . T e n ta tiv e h y p o tlB s e s have b e e n a d v a n ce d i n e x p la n a tio n o f m ost o f t h e phenomena fo u n d .

A numbear o f h i g h l y i n t e r e s t i n g p o s s i b i l i t i e s a r e

i n d i c a t e d w hich r e q u i r e f u r t h e r i n v e s t i g a t i o n . E x p e r im e n ta l D e t a i l s o f t h e p o la r o g r a p h ic eq u ip m e n t, te m p e r a tu re c o n t r o l , e t c . , u s e d w ere th e same a s p r e v io u s l y d e s c r ib e d . w ere m a n u a lly p l o t t e d .

Most o f t h e c u r v e s

The te m p e r a tu r e was m a in ta in e d a t 1 5 “i o . 2 ° .

A l l p o t e n t i a l s w i l l be s t a t e d r e l a t i v e t o th e S .C .E .

R henide

s o l u t i o n s f o r p o la r o g r a p h ic w ork w ere o b ta in e d w i t h t h e s m a ll r e d u c to r em ployed e a r l i e r . For t h e t i t r a t i o n s t u d i e s r e p o r t e d i n t h i s p a r t , a la r g e

44

Z ones r e d u c t o r w as em ployed.

T h is w as a s se m b le d fro m a 100 m l. b u r e t

and a l a r g e c o n d e n s e r j a c k e t . c o d e n t,

Tap w a te r a t 15" was em ployed a s a

m e t e c b n l g i e o f u s i n g t h e a s s e m b ly t o r p e r r h e n a te

r e d u c t i o n s was e s s e n t i a l l y t h a t o f L u n d e ll an d K now les ( 2 2 ) .

Reduced

s o l u t i o n s w ere c o l l e c t e d u n d e r a n e r c e s s o f f e r r i c alu m s o l u t i o n , t r e a t e d w ith p h o s p h o ric a c i d , and t i t r a t e d w i t h 0 .0 5 N d ic h ro m a te to a d ip h e n y lam in e e n d p o in t. A Beckmann M odel DU s p e c tr o p h o to m e te r w as em ployed i n t h e a b s o r p ti o n s t u d i e s u s in g m atched p a i r s o f s i l i c a c e l l s . D isc u s Sion E v id e n ce and M ature o f R henide Complex I n H y d ro c h lo r ic A c id . The f i r s t e v id e n c e o f a n y d i s t i n c t i o n b etw een th e b e h a v io r o f r h e n id e s o l u t i o n s i n h y d r o c h lo r ic a c i d a s com pared t o t h a t i n s u l f u r i c a c id came from a q u a n t i t a t i v e s tu d y o f t h e r e d u c tio n o f p e r r h e n a te to r h e n id e .

I n s u l f u r i c a c i d c o n c e n t r a tio n s fro m $

t o 20% (0 .7 t o 7 N)

t h e maximum c o n c e n t r a tio n o f p o ta s s iu m p e r r h e n a te v h ic h c o u ld be q u a n t i t a t i v e l y re d u c e d t o th e r h e n id e s t a t e w as found t o be 0 .0 2 4 gms. p e r 100 m l. o r ç a . 0 .6 3 i g . tio n s o f a t le a s t a c id .

1 .3

As s h o rn i n T ab le 1 0 , r h e n id e c o n c e n tr a ­

nM a r e a t t a i n a b l e i n

10%

( 1 . 2 M) h y d r o c h lo r ic

The d i f f e r e n c e i s n o t s p e c t a c u l a r a n d c o u l d a s w e l l be a t t r i b u t e d

t o o x id a tio n i n s u l f u r i c a c i d a s t o c o m p le x a tio n i n h y d r o c h lo r ic a c i d . I t seem ed d e f i n i t e l y i n d i c a t e d . h o « v e r . t h a t t h e r h e n id e s t a t e was f a v o re d by c o m p le x a tio n a f t e r i t w as d is c o v e r e d t h a t r h e n id e c o n c e n t r a tio n s a s h i ^ a s

2 .6

mM w ere a t t a i n a b l e on th e a d d i t i o n o f

p y r id in iu m c h l o r i d e t o t h e h y d r o c h lo r ic a c i d s o l u t i o n s . A p r e c i s e s tu d y o f t h e v a r i o u s a n o d ic r h e n id e w av es o b ta in e d

45

i n h y d r o c h lo r ic a c id i s n o t p o s s i b l e f o r s e v e r a l r e a s o n s .

W hile a

w e l l d e f in e d A-wave i s u s u a l l y I b m d , th e d e f i n i t i o n o f t h e s u c c e e d in g B - a n d G-waves i s e x tre m e ly p o o r .

I n a d d i t i o n , th e e a r l y i n c e p t i o n o f

a l a r g e ca lo m e l d is c h a r g e i n t h i s medium p r e v e n ts t h e s t u d y o f a n y w aves more p o s i t i v e th a n ç a . - 0 . 1 v .

T h is m edium , n e v e r t h e l e s s ,

assum ed a h ig h im p o rta n c e when i t w as r e c o g n iz e d t h a t c o m p le te ly re d u c e d r h e n id e s o l u t i o n s y i e l d e d a c a th o d ic , a s w e l l a s a n o d i c , w ave. The g e n e r a l a p p e a ra n c e o f t h e w av es may be s e e n fro m t h e HOI c u rv e Of F ig u re 7 , D a ta from a t y p i c a l r u n a r e g iv e n i n T ab le 1 1 .

The h a l f ­

wave p o t e n t i a l o f t h e A -w ave, - 0 .4 7 4 v . , i s n o t m a rk e d ly d i f f e r e n t from t h a t o b s e rv e d i n s u l f u r i c a c i d , - 0 .8 0 5 v .

At u n i t a c t i v i t y o f

hydrogen io n i n e i t h e r a c i d , a p o t e n t i a l o f - 0 .4 8 6 v . a p p l i e s a t 1 5 » . As i n t h e m edia p r e v i o u s l y s t u d i e d , t h e slo p e o f t h e A -w ave, 0 .0 5 1 v . , a g r e e s w e ll w i t h th e t h e o r e t i c a l v a lu e o f 0 .0 2 8 v . f o r a r e v e r s i b l e tw o - e le c tr o n p r o c e s s .

The s lo p e o f t h e B-wave a p p ro x im a te s t h a t o f a

o n e - e le c tr o n p r o c e s s , a n d th e G-wave slo p e i s d e f i n i t e l y i r r e v e r s i b l e . The s lo p e o f t h e c a th o d ic wave i s h i ^ l y i r r e v e r s i b l e .

The c a th o d ic

and t h e A-waves a r e o f a p p r o x im a te ly th e seme s i z e i n d i c a t i n g t h a t th e n - v a lu e s f o r t h e tw o w aves a r e t h e sam e, i . e . , 2 . t h e Q-waves a r e ro u g h ly o n e - h a lf t h i s s i z e .

B oth t h e B- end

An A --wave d o es n o t

a p p e a r i n hydro c i i l o r i c a c i d . I t seem s p ro b a b le t h a t th e A-wave i s a g a in c o n c e rn e d w ith t h e o x id a tio n s t e p ( - 1 ) t o

ID .

The a b s e n c e o f t h e A --wave may

r e a s o n a b ly be a s c r i b e d t o a com plexed ( I ) l e v e l su c h a s S e C la - , i n s t e a d o f h y d ro u s ReaO a s i n s u l f u r i c a c i d .

I n t h i s c o n n e c tio n

i t may be n o te d t h a t w hen r h e n id e s o l u t i o n s i n s u l f u r i c a c i d a r e

46

ex p o sed t o th e a i r , t h e y g r a d u a l l y t u r n y e llo w -b ro w n an d a d a rk s o l i d s e p a ra te s .

On th e o t h e r h an d , e x p o s e d s o l u t i o n s o f r h e n id e i n h y d ro ­

c h l o r i c a c i d become c l e a r y e llo w o n ly on lo n g e r s ta n d i n g a n d no s o l i d s e p a ra te s . I t vjas o f p a r t i c u l a r im p o rta n c e t o e s t a b l i s h t h e n a t u r e o f t h e c a th o d ic w ave, i . e . , w h e th e r i t i s c o n t r o l l e d b y t h e n o rm al p r o c e s s o f d i f f u s i o n , o r b y a d s o r p ti o n (2 ) o r k i n e t i c (15) l i m i t a t i o n s .

The

s i z e o f t h e c u r r e n t s te p was found t o r e l a t e d i r e c t l y t o th e s q u a re r o o t o f th e e f f e c t i v e h e i g h t o f t h e m e rc u ry r e s e r v o i r .

S in c e t h i s

r e l a t i o n w as v a l i d w i t h i n ±2 ^ f o r a ra n g e o f r e s e r v o i r h e i ^ t fro m 30 to 70 cm ., th e c a th o d ic wave a p p e a r s t o be t r u e d i f f u s i o n - c o n t r o 1 le d w i t h i n t h e l i m i t a t i o n s o f t h i s

c rite rio n .

A c tu a lly , a s w i l l

p r e s e n t l y be show n, a k i n e t i c f a c t o r may a l s o be in v o lv e d h e re b u t i t s e f f e c t i s n e g l i g i b l y s m a ll due t o th e low r a t e o f t h e p r o c e s s c o n c e rn e d .

The p r o c e s s i n q u e s tio n i s t h e f o m a t i o n o f th e s p e c ie s

i t s e l f w hich g iv e s r i s e t o th e c a th o d ic w ave.

From t h i s much d is c u s s io n

i t w i l l b e e v i d e n t t h a t t h e d e t e r m in a tio n o f t h e te m p e r a tu r e c o e f f i c i e n t o f t h e c u r r e n t dem ands some c a u t i o n , i . e . , i t i s im p o r ta n t t o be a s s u r e d o f t h e c o m p le te f o m a t i o n o f t h e r e d u c i b l e s p e c ie s a t a g iv e n te m p e r a tu r e , sa y 15% b e f o r e p r o c e e d in g to m easu rem en ts a t a new te m p e r a tu r e , sa y 25%

T ak in g s u i t a b l e p r e c a u t i o n s , th e te m p e r a tu r e

c o e f f i c i e n t o f t h e c u r r e n t o v e r th e ra n g e o f 5« t o 2 5 ° h a s b ee n d e te rm in e d a s 2 .2 ^ p e r d e g r e e .

T h is v a l u e i s a l s o c o n s i s t e n t w ith

p u re d i f f u s i o n c o n t r o l . At t h i s p o i a t i t w o u ld b e w e l l t o c o n s id e r t h e o rig L n o f th e c a th o d ic TOve.

The e x i s te n c e o f a c a th o d ic , i . e . , a r e d u c t i o n .

47

wave i n a c o m p le te ly r e d u c e d r h e n id e s o l u t i o n i s d e c id e d ly u n e x p e c te d . The i n i t i a l o x i d a t i o n s t e p (A-wave) i n h y d r o c h lo r ic a c i d seem s t o d i f f e r i n no s i g n i f i c a n t n n n n e r fro m th e co m p arab le wave i n s u l f u r i c a c id .

The a n a lo g y o f r h e n id e m t h th e h a l i d e s , e . g . , io d id e , h a s

a lre a d y been s tr e s s e d .

T h is a n a lo g y i s a t once s u g g e s tiv e o f a v e r y

p l a u s i b l e e x p l a n a tio n f o r t h e p r e s e n t c a s e , n am ely , th e e x i s te n c e o f an e s s e n t i a l l y c o v a le n t rh en iu m an d c h l o r i n e compound. R eC l, a n a lo g o u s w ith th e i n te r h a lo g e n compounds such a s io d in e m o n o c h lo iid e , I C I . S uch a compound s h o u ld be c a p a b le o f b e in g e i t h e r o x id iz e d o r r e d u c e d . The o b se rv e d h i g h - i r r e v e r s i b i l i t y o f t h e r e d u c t i o n p r o c e s s a c c o u n ts f o r t h e d is p la c e m e n t o f t h e c a th o d ic wave from th e p o t e n t i a l o f th e r e v e r s i b l e o x id a tio n w ave. F o r co n v e n ie n c e a n d b e c a u s e t h i s f o r m u la tio n seems q u ite l o g i c a l , th e r h e n id e s p e c i e s i n h y d r o c h lo r ic a c i d w i l l be r e f e r r e d t o a s ReCl th r o u g h o u t th e r e m a in d e r o f t h i s p a p e r .

At th e p re se n t w ritin g ,

t h i s p a r t i c u l a r fo rm u la c a n n o t be s a id t o b e i n c o n t r o v e r t i b l y e s ta b lis h e d .

C l e a r l y , more t h a n one c h lo r in e co u ld b e so b o u n d ; w a te r

m o le c u le s m ig ht a l s o be in v o lv e d . R henide i n th e F our H y d ro h a lid e A c id s .

The q u e s tio n a t once

a r i s e s a s to w h e th e r o r n o t co m parable " i n t e r h a l o g e n - l i k e ” compounds o f rh en iu m w i l l e x i s t i n s o l u t i o n s o f th e r e m a in in g h y d r o h a lid e a c i d s . E x p erim en t showed t h a t com parable exam ples o f c a th o d ic w aves do i n f a c t e x i s t i n a l l su c h m edia (F ig u re 7 ) .

The a n o d ic w aves i n h y d ro -

brom ic a c i d a r e q u i t e s i m i l a r to th o s e in h y d r o c h lo r ic a c i a .

In

h y d r o io d ic a c i d , o f c o u r s e , th e e a r l y in c e p ti o n o f t h e m ercu ro u s lo a id e d is c h a r g e m i l n o t p e rm it a n e x a m in a tio n o f t h e n o rm al a n o d ic r h e n id e w av e s.

I n h y d r o f lu o r ic a c id com plete r e d u c tio n t o t h e rh e n id e s t a t e

48

( a s fo u n d by t i t r a t i o n ) i s n o t a t t a i n a b l e ; a p p a r e n t l y , o n e can o n ly e x c e e d s i x - and a p p ro a c h a s e v e n - e le c t r o n r e d u c tio n o f p e r r h e n a te i n t h e 2 . 4 N a c id a t 1 5 ° .

I t i s n o ta b le t h a t su ch a n o d ic an d c a th o d ic

p o r t i o n s o f t h e wave a s o b ta in e d i n t h i s medium o c c u r a t a s i n g l e p o t e n t i a l , p r e s e n t i n g a sm ooth c a th o d ic - a n o d ic t r a n s i t i o n .

The h a l f ­

wave p o t e n t i a l s o f th e c a th o d ic ReX w aves i n th e f o u r h y d r o h a lid e a c id s a r e p r e s e n te d i n T ab le 1 2 . P r e s e n t e v id e n c e le a d s one t o a r a t h e r s u r p r i s i n g c o n c lu s io n c o n c e rn in g t h e p o l a r i t y o f th e ReX s t r u c t u r e s .

The s i m i l a r i t y o f th e

A-wave i n HX w ith t h a t i n s u l f u r i c a c id s u g g e s ts t h e o x id a tio n o f n e g a tiv e rh en iu m i n b o t h c a s e s .

The d i f f i c u l t y o f a c h ie v in g an ReF

compound i s s u g g e s tiv e o f t h e d i f f i c u l t y o f im p o s in g a p o s i t i v e c o n f ig u r a t io n on t h e h ig h ly e l e c t r o n e g a t i v e f l u o r i n e .

The h a lf-w a v e

p o t e n t i a l s o f t h e f o u r ReX compounds show t h a t R e l h a s th e m o st n e g a tiv e p o t e n t i a l o f th e f o u r and t h e r e f o r e i s t h e m ost d i f f i c u l t l y r e d u c i b l e . T h is im p l ie s t h a t R e l i s t h e m ost s tr o n g ly bonded o f t h e com pounds, w h ich s h o u ld b e t r u e i f t h e rhen iu m i s n e g a tiv e an d t h e h a lo g e n p o s i t i v e ; i . e . , o f t h e f o u r h a lo g e n s , io d in e c a n most r e a d i l y a c c e p t a p o s i t i v e c h a rg e .

On t h e b a s i s o f su c h r e a s o n in g one i s l e d t o

p o s t u l a t e t h a t t h e a s s ig n m e n t o f a n y r e s i d u a l c h a rg e in t h e e s s e n t i a l l y c o v a le n t ReX compounds seems to be i n t h e d i r e c t i o n o f a n e g a tiv e rhen iu m w i t h t h e more p o s i t i v e c h a r a c t e r on t h e h a lo g e n .

T h is , o f

c o u r s e , i s th e in v e r s e o f th e a s sig n m e n t f o r io d in e m o n o c h lo rid e , and a compound su c h a s ReBr w ould t h e r e f o r e be named b ro m in e m o n o rh en id e. I t h a s b e e n fo u n d t h a t an add ed co m p le x a tio n o c c u rs i n th e p r e s e n c e o f s u b s ta n c e s such a s p y r id in e , a s e v id e n c e d by th e ap p e a ra n c e Of a s e c o n d , more n e g a tiv e c a th o d ic w ave.

On t h e a s s u m p tio n t h a t t ^

49

e l e c t r o n s a r e in v o lv e d i n each o f th e c a th o d ic w aves, i t i s p o s s i b l e

to c a lc u la te from th e s h i f t in half-w ave p o te n tia l as a fu n c tio n o f th e p y r id in e c o n c e n t r a tio n (14) t h a t t h r e e m o le c u le s o f p y r i d i n e a r e c o o r d in a te d p e r rh en iu m ato m .

The data vJhich j u s t i f y t h i s c o n c lu s io n

are presen ted g r a p h ic a lly in Figure 8 .

I t must a ls o be assumed th a t

th e p re s e n c e o f l a r g e amounts o f z i n c io n ( r e s u l t i n g f ro m th e J o n e s r e d u c to r ) does n o t v i t i a t e t h e r e s u l t s , e i t h e r b e c a u s e th e y a r e p r e s e n t i n l a r g e a n d r e l a t i v e l y constant e x c e s s o r b e c a u se a n y z i n c -

p yridine complexing in a c id s o lu tio n i s com paratively w eaker.

A

s i m i l a r s o l u t i o n c o n t a in i n g o n ly z i n c a n d p y r i d i n e in a c i d g i v e s th e c a th o d ic z in c wave a t th e norm al p o s i t i o n o f ç a . one v o l t . C om parable r e s u l t s a r e fo u n d w ith th io c y a n a te o r w ith

eth ylen e diam ine.

With s m a ll amounts o f e t h y le n e d i am ine, h ow ever, some

ReCl wave rem ains a t -0 .6 6 v . w hile th e str o n g ly bound en-complex reduces a t -0 .7 8 v .; however, a l l anodic waves are a lso reduced in s iz e and remain o n ly t o th e e x t e n t t h a t t h e s im p le ReCl wave r e m a in s .

Apparently, the' complex r e s u lt in g from ethylenediam ine i s so str o n g ly bound a s to r e s i s t o x id a tio n ( i . e . , t o , a t l e a s t , - 0 . 1 v. ) . P o s s ib le fo rm u la e f o r t h e s e co m p lex e s and th e h a lf- w a v e

p o te n tia ls o f t h e ir red u ctio n waves h ave b een in c lu d e d in T ab le 1 2 . The p o s s i b i l i t y i s n o t e x c lu d e d , o f c o u r s e , t h a t c h l o r i d e i o n s o r

water m olecu les may a ls o be coordinated such, e . g . , as to a t t a in th e s t a b i l i z e d f o u r - c o o r d i n a t i o n p o s t u l a t e d b y P a u lin g (29) f o r t h i s c a s e . R a te o f Formation o f ReCl.

B r ie f a l l u s i o n h a s a l r e a d y b e e n

made to the f a c t th a t th e fo m a tio n o f EeCl req u ires a f i n i t e len g th o f tim e .

S ta r tin g w ith f r e s h ly reduced rhenide in hydrochloric a c id

on ly a v e r y modest cath od ic wave i s found i f the s o lu tio n i s examined

50

a t o a ta .

A g W th e s o l u t i o n u n d e r n i t r o g e n f o r 30 o r 40 m in u te s

p ro d u c e s a l a r g e r c a th o d ic » v e , co m parab le i n s i z e w i t h th e A -w ave. I t m ig h t be a n t i c i p a t e d , i f t h e f o r m u l a ti o n E eO l I s c o r ­ r e c t , t h a t S eO l fo rm a tio n c o u ld be b lm o le c u la r an d s e c o n d o r d e r . E x p e rim e n ta l c o n d i tio n s f o r t e s t i n g t h i s a r e d i f f i c u l t t o r e a l i z e . A 0 .4

hM

r h e n id e c o n c e n t r a tio n i n , s a y , 2 . 4 N h y d r o c h lo r ic a c i d

c o n s t i t u t e s a s m a ll q u a n t i t y o f one r e a c t a n t i n t h e p r e s e n c e o f a huge e x c e s s o f th e o t h e r .

A c c o rd in g ly , f i r s t o r d e r k i n e t i c s w o u ld b e e x ­

p e c te d t o a p p ly u n d e r th e a v a i l a b l e e x p e r im e n ta l c o n d i t i o n s . A s e r i e s o f t h r e e r u n s a t e a c h o f tw o te m p e r a tu r e s , 1 S .8 ” and 2 5 .0 » , w as co n d u c te d i n w h ic h t h e r a t e o f f o rm a tio n o f R e C l, a s m easured by th e grow th o f t h e c a th o d ic wave w ith t i m e , was d e t e m l n e d f o r 0 .4 m

r h e n id e i n 2 .4 N h y d r o c h lo r ic a c i d .

I n o n e s e t o f ru n s

f o r e a c h t e n p e r a t u r e t h e b e h a v io r o f t h e A-wave w ith tim e was a ls o fo llo w e d a s a c r i t e r i o n o f t h e e x t e n t t o w h ich c o n m r r e n t o x id a tio n o f r h e n id e was in v o lv e d .

I n t h i s c o n n e c tio n i t was Sound t h a t

o x id a tio n d id n o t become s i g n i f i c a n t u n t i l a f t e r 60 m in u te s a t 25» o r 110 m in u te s a t 1 5 .8 ° . P l o t s o f t h e d a t a a s 1^ o f t h e c a th o d ic wave v s . tim e d e f in e d r e a s o n a b ly sm ooth c u rv e s i n e v e ry c a s e .

W hile t h e a b s o lu te

v a lu e s o f th e c u r r e n t f o r a p a r t i c u l a r tim e v a r i e d a s much a s

10%

b etw een t h e t h r e e s e t s o f d a t a a t a g iv e n te m p e r a t u r e , t h e t i m e - r a t e o f change a s d e f in e d b y t h e sh a p e s o f th e t h r e e c u rv e s w as c l o s e l y S im ila r b etw een r u n s .

The f i r s t - n a m e d f a c t seemed t o b e an

u n a v o id a b le f u n c ti o n o f m in o r d i f f e r e n c e s i n e x p e r im e n ta l te c h n iq u e , b u t t h e se co n d f a c t o r i s t h e one o f d i r e c t k i n e t i c s i g n i f i c a n c e .

For

t h i s r e a s o n , th e i , v s . tim e p l o t s o f F ip x r e 9 r e p r e s e n t t h e a v e r a g e

51

s h a p e s fro m t h r e e r i m s ; I.e., a v e ra g e d a f t e r a d j u s t i n g t h e d a t a fro m t h r e e r im s to n e a r e s t o o in o id e n o e hy s l i d i n g a lo n g t h e c u r r e n t a x i s . On th e a 8 8 in q )tio n t h a t f i r s t o r d e r k i n e t i c s w i l l o b t a i n , a r e p l o t t i n g o f t h e d a t a a s lo g i ^ v s . tim e sh o u ld yd&d a s t r a i g h t l i n e r e l a t i o n . The v a l i d i t y o f t h i s r e l a t i o n i s e v i d e n t fro m th e se c o n d h a l f o f F ig u re 9 lA id h h a s b e e n p l o t t e d fro m t h e a v e ra g e d v a l u e s on th e p re c e d in g c u r v e s .

The r e l a t i o n i s fo u n d t o b e v a l i d u n t i l th e

a fo re m e n tio n e d t i m e - l i m i t a t i o n o f o x id a tio n i s e x c e e d e d . From th e s lo p e s o f t h e s t r a i g h t l i n e p o r t i o n s o f t h e l o g p l o t s , one ca n e v a lu a te f i r s t o r d e r s p e c i f i c r a t e c o n s ta n t s o f m in ." ^ a t 1 5 .8 ° and 0-0191 m in ." ^ a t 2 5 .0 ° .

0 .0 1 1 1

S u b s titu tio n o f th e s e

v a lu e s i n t h e A rr h e n iu s e q u a tio n p e r m i ts th e e s tim a tio n o f an a c t i v a t i o n e n e rg y o f

1 0 .2

k g . - c a l . p e r m o le .

A b s o rp tio n S p ectru m o f R e C l.

The fu n d a m e n ta l p re m ise in th e

p r e s e n t p a p e r o f a d i s t i n c t i o n b etw e en t h e r h e n id e s t a t e i n s u l f u r i c a c id an d t h a t i n h y d r o c h lo r ic a c i d would l e a d one t o e x p e c t a p o s s i b l e d i f f e r e n c e i n th e u l t r a v i o l e t a b s o r p ti o n i n th e tw o c a s e s . Maun and D av id so n (23) h ad a l r e a d y co m p ]^ ted t h e i r e x p e r im e n ta l s tu d y Of rh en iu m a b s o r p ti o n i n h y d r o c h lo r ic a c i d m ed ia b e f o r e t h e p r e s e n t a u th o r s s u g g e s te d t h a t t h e y compare t h e i r d a t a t o r h e n id e a b s o r p tio n in s u lf u r ic a c id .

We a r e in d e b te d t o th e m , h o w ev er, f o r t h e

i n f o m a t i o n (24) w ith r e s p e c t t o r h e n id e i n h y d r o c h lo r ic a c id t h a t th e y h ad o b s e rv e d t h e g r a d u a l f o m a t l o n o f s u b s ta n c e s w ith h ig h a b s o r p ti o n a t w a v e le n g th s lo w e r th a n 400 mu.

An e x p e r im e n ta l

co m p ariso n o f t h e r h e n id e a b s o r p ^ o n i n th e tw o m ed ia seemed w o rth w h ile . To t h i s e n d , 0 .5 mM rh e n id e s o l u t i o n s i n t h e 2 R r e s p e c tiv e a c id s were prepared b y r e d u c tio n w ith amalgamated z m c .

Comparison

52

s o lu tio n s o f

2

H s u l f u t i c and

2 2

. h y d r o d t l o r i c a c i d s c o n t a in i n g no

rh e n iu m w ere c o n t a c te d w ith t i n e u n d e r co m p arab le c o n d i t i o n s .

Tbs

s o l u t i o n s w ere a g e d i n th e c o v e re d a b s o r p ti o n c e l l s f o r ç a . 30 m in u te s p r i o r t o r e a d i n g .

to 1050 mu on t h e Beckmann DO s p e c tr o p h o to m e te r u s in g m a tch e d p a i r s

240

of

The a b s o r p ti o n d a t a w ere d e te rm in e d from

1 -cm .

p a th , re c ta n g u la r s i l i c a c e l l s .

in s tru m e n t was s e t t o r e a d

100$

A t each w a v e le n g th t h e

tr a n s m is s io n on th e p ro p e r s o lv e n t

b e f o r e r e a d in g th e t r a n s m i s s i o n o f t h e c o r r e s p o n d in g r h e n id e s o l u t i o n , c r o s s co m p ariso n o f t h e s o l v e n t s showed o n ly s m a ll d i f f e r e n c e s below 300 mu an d n e g l i g i b l e d i f f e r e n c e s above t h i s r e g i o n . w ere a t 25” when r e a d .

The s o l u t i o n s

R e a d in g s w ere t a k e n e v e ry 5 mu from 240 to

350 mu, e v e ry 10 mu from 350 t o 500 mu, e v e ry 20 mu from 500 t o 600 mu, e v e ry 25 mu from 600 t o 900 mu, an d a t 95 0 , 10 0 0 , mid 1050 mu. The

u s a b le s l i t v iid th was em ployed th r o u g h o u t. As th e a b s o r p ti o n c u r v e s o f F ig u re 10 dbow, q u ite

s i g n i f i c a n t d i f f e r e n c e s e r i s t b etw een th e n e a r u l t r a v i o l e t a b s o r p tio n s p e c t r a o f r h e n id e i n s u l f u r i c a c i d and i n h y d r o c h lo r ic a c i d .

Summary H henide s o l u t i o n s i n th e h y d r o h a lid e a c i d s e v id e n c e some u n iq u e p r o p e r t i e s as s u lfu ric .

i n d i s t i n c t i o n t o r h e n id e s o l u t i o n s i n a c id s ^ c h

The e r i s t a n c e o f a

u n iq u e a b s o r p ti o n sp e c tru m i n HK

c a th o d ic p o la r o g r a p h ic wave a n d th e m ed ia may b e a t t r i b u t e d

t o an

e s s e n t i a l l y c o v a le n t s p e c i e s , KeX, a n a lo g o u s w ith t y p i c a l i n te r h a lo g compounds su ch a s 1 0 1 .

P r e s e n t e v id e n c e seem s t o j u s t i f y th e h y p o th e s is

t h a t t h e ch a rg e a s s ig n m e n t i n su c h a s p e c ie s may b e i n th e d i r e c t i o n o f th e more p o s i t i v e c h a r a c t e r r e s i d i n g on th e h a lo g e n a to m .

53

L i t t l e i s known o f t h e p r o c e s s e s b y w h ic h a n e n t i t y s u c h a s E eC l c o u ld a r i s e fro m r h e n id e and c h l o r i d e io n s b u t th e k i n e t i c s o f i t s f o r m a tio n h a s b e e n s tu d ie d and t h e e n e r g y o f a c t i v a t i o n e s t i m a t e d . The f o rm a tio n o f f u r t h e r co m plexes w ith p y r i d i n e , th lo c y a n a te io n , and e th y le n e d ia m in e i s i n d i c a t e d .

I n t h e c a s e o f p y r i d i n e t h r e e m o le c u le s

a r e c o o r d in a te d p e r rh en iu m atom .

54

T a b le 10 P e r r h e n a te R e d u c tio n i n V a rio u s M edia

Medium

C oncn. gms. KRe0 4 p e r 1 0 0 m l. o f so I n .

No. o f S ta g e s Reduced

10^ HOI

0 .0 2 5 0

7 .9 6

10^ HCl

0 .0 3 7 5

7 .9 4

10$ HCl

0 .0 3 7 5

7 .8 8

HCl

0 .0 5 0 0

7 .3 6

10$ HCl

0 .0 7 5 0

7 .0 0

d o . + 2$ Py.H C l

0 .0 5 0 0

7 .8 5

do. + 5$ Py.H C l

0 .0 7 5 0

7 .9 5

d o . + 5$ P y.H C l

0 .0 7 5 0

7 .9 5

10$

55

F ig , 7 A c id s :

P o la r o g ra p h y o f R h en id e i n Hydro h a l id e

0 . 4 mM r h e n id e i n 2 .4 N s o l u t i o n s o f t h e f o u r

h y d r o h a lid e a c i d s a t 1 5 ° , k = 1 .6 6 a t - 0 .6 v .

2

HCî

HF

0

mlcroampa.

HBry

2

-4

0. 2

0.6

0 .4 E,

V.

0.8

56

T ab le 11 R henide Waves i n H y d ro c h lo r ic A cid 0 .4 mM Re" i n 2 .4 N HCl a t 15% k = 1 .6 6 a t - 0 .6 v .

Wave

^' 0 .. 5 ’' V,

^^0-75 " ^ 0 * 2 5 ^ ’ V.

C a th o d ic

- 0 ,6 5 7

0 .0 9 0

2 .3 2

A

- 0 .4 7 4

0 .0 3 1

2 .1 2

B

- 0 .5 4

O'OSS

0 .9 2

G

- 0 .1 7

0 .0 7 2

0 .9 5

T ab le 12 H alf-W ave P o t e n t i a l s o f C a th o d ic "R h en id e" Waves

^ o th e tic a l F orm ula

^ 0 .5 ’ V.

ReF

-0 .4 2 6

ReCl

- 0 .6 5 7

ReBr

-0 .6 9 9

R el

- 0 .7 3 5

R e (p y r ) 3

- 0 .7 0 0

Re(NCS ) 3

- 0 .7 1 8

R s g te n ) 3

-0 .7 8 0

57

F ig .

8

Eq

5

o f ReCl v s . Concn. o f P y r i d i n e :

s h i f t i n h a lf-w a v e p o t e n t i a l o f c a th o d ic wave f o r 0 ,4

inM r h e n id e i n

2 .4

H h y d r o d i l o r i c a c i d w i t h added

in c re m e n ts o f p y r id in in m c h lo r id e a t 15®.

0 .5

-

Log, M

-

1 .5

2.0

0.70

-

0.68

Go.5'

-

0.66

58

F ig . 9

R a te o f F o rm a tio n o f R eC l;

c a th o d ic wave w i t h tim e f o r 0 . 4

bM

g ro w th o f

r h e n id e i n 2 .4 N

h y d r o d i l o r i c a c i d a t two te m p e r a t u r e s .

12

! Log M»

micro­ amps.

75 Time, minutes

100

59

l i g . 10

R henide A b s o rp tio n i n T ro M edia:

co m p ariso n o f a b s o r p ti o n s p e c t r a o f 0 . 5 i # r h e n id e i n 2 N s u l f u r i c a c i d (d a sh e d l i n e ) and 2 N h y d r o c h lo r ic a c id ( s o l i d l i n e ) a t 25°C , 1 cm. p a t h .

80

T

60

40

300

500 400 Wavelength, mu.

600

60

PART IV MESCELLMEOUS ASPECTS OP THE CHEIvHS'TRY OF RHSNIUM

I n tro d u c tio n îJ h ile s t u d i e s o f t h e r h e n id e io n an d o f t h e re m a in in g lo w e r o x id a tio n s t a t e s o f rh en iu m i n s o l u t i o n a r e o f u n d is p u te d v a lu e and i n t e r e s t , th e i d e a l s i t u a t i o n , of c o u r s e , w u l d b e t o p r e p a r e o r i s o l a t e p u re compounds.

The s t u d i e s r e p o r t e d i n t h i s s e c t i o n a r e

l a r g e l y co n c e rn e d w ith e x p e rim e n ts g iv in g n e g a tiv e r e s u l t s , b u t a r e n e v e r t h e l e s s o f some i n t e r e s t i n e v a l u a t i n g th e c h e m ic a l b e h a v io r o f rh en iu m . A tte m p ts to p re p a re r h e n id e compounds w ere made by a l k a l i m e ta l f u s io n an d by l i t h i u m alum inum h y d rid e r e d u c t i o n .

A tte m p ts t o

i s o l a t e r h e n id e compounds fro m s o l u t i o n w ere made by s o lv e n t e x t r a c t i o n an d by p r e c i p i t a t i o n te c h n iq u e s .

S e v e r a l a t te m p ts w ere

made to p r e p a re an io d id e o f rh e n iu m . Expe r im e n ta l Ito st o f th e e x p e rim e n ts i n t h i s s e c t i o n em ployed sim p le ch e m ic al te c h n iq u e s w hich w i l l be d e s c r ib e d i n se q u e n c e .

The p o la r o ­

g ra p h ic t r i a l s u t i l i z e d th e same equipm en t a s p r e v io u s l y d e s c r ib e d . R eag en t g ra d e s o f th e common c h e m ic a ls w ere u se d th r o u g h o u t. E astm an Kodak Company " p r a c t i c a l g ra d e " s u l f u r y l c h l o r i d e w as u s e d . Rhenium m e ta l powder o f 99.5^ p u r i t y w as o b ta in e d from t h e U n i v e r s i t y o f T e n n e sse e .

The l i t h i u m aluminum h y d rid e u s e d was t h e p ro u u c t o f

M e ta l H y d rid e s I n c .

61

D is c u s s io n Hheniae E x t r a c t io n . R eC l,

% o p o s tu la t e d c o t a ie n t n a tu re o f

R e(p y r )e . e t c . , su g g e ste d th e p o s s ib le i s o l a t i o n o f su ch

compounds by s o lv e n t e x t r a c t io n .

Great d i f f i c u l t y w as e x p ex len ce d in

p u r ify in g e th e r s u f f i c i e n t l y f o r ' t h i s p u r p o s e .

P o la r o g ra p h ic exami­

n a tio n Of t h e sp en t aqueous phase u s u a ll y in d ic a t e d t h a t some e x t r a c t io n had occu rred , but o x id a tio n o f t h e rh en id e alw ays seemed t o have b een th e p r in c ip a l p r o c e ss in v o lv e d .

C hloroform seemed t o be more s u it a b le

fo r t h e purpose and l i t t l e o x id a tio n was in c u r r e d .

P o la r o g ra p h ic

exam in ation o f t h e aqueous phase a f t e r ch loroform e x t r a c t io n alw ays in d ic a t e d th a t an a p p r e c ia b le e x t r a c t io n o f rh en id e complex had occurred (a s evid en ced by a d ecr ea sed A -w ave).

The e x t r a c t io n o f

0 .4 mK rh en id e in 2 M h y d r o c h lo r ic a c id w ith an eq u a l volum e o f Chloroform in d ic a te d a d is t r ib u t io n c o e f f i c i e n t o f 5 .7 ( so lv e n tla q u e o u s phase) a t 15®. U n fo r tu n a te ly , th e p r e s e n c e o f t h e rh en id e in th e o r g a n ic la y e r has n ot b een c o n c lu s iv e ly e s t a b lis h e d .

E v a p o ra tio n o f th e

o rg a n ic la y e r in t h e p resen ce o f aqueous p h a ses o f b o th a c i d ic and a lk a lin e ch a ra c ter and c o n ta in in g e i t h e r o x id iz in g or red u cin g a g e n ts was t r i e d a s w e l l a s e v a p o r a tio n b y e v a c u a tio n th r u a s e r i e s o f t r a p s . The r e s id u a l m a t e r ia ls , a f t e r p ero x id e o x id a tio n , n ev er gave more than f a in t n itr o n t e s t s fo r p e r r h e n a te .

S u lf id e t e s t s on th e

a c i d i f i e d r e s id u e s were o f t e n p o s i t i v e but never to th e a n tic ip a te d d egree. S y n th e s is o f Lower Rhenium g l i d e s .

The sequence o f compounds

Which n orm ally r e s u lt from th e a c t io n o f t h e v a r io u s halogens on

62

rh e n iu m m e ta l, R eF a-R eC la-E eB ra, s u g g e s ts t h a t t h e p r e p a r a t i o n o f a n io d id e m ig h t y i e l d a compound o f low o x i d a t i o n l e v e l .

The N oddacks-

(87) r e p o r t o f a n R e l . r e s u l t i n g fro m t h e a c t i o n o f i o d in e v a p o r on rh en iu m m e ta l h a s n e v e r b ee n c o n firm e d , a n d , i n f a c t , no sim p le io d id e s o f rh en iu m a r e known. Rhenium pow der p lu s i o d in e h e a te d i n a s e a l e d tu b e f o r s i x h o u rs a t 1 7 0 -8 0 “ gave no e v id e n c e of an y r e a c t i o n . Rhenium m e ta l su sp e n d e d i n c a rb o n t e t r a c h l o r i d e c o n t a in i n g a n e x c e s s o f d is s o l v e d io d in e

[ c f . , s y n t h e s i s o f a n tim o n y t r i i o d i d e

( 1 )] gave no e v id e n c e o f r e a c t i o n a f t e r s e v e n ty -tw o h o u rs o f r e f l u x i n g . The s ta n d a r d R u ff s y n th e s is f o r io d in e m o n o o h lo n d e a n d t r i c h l o r i d e s u g g e s ts a p o s s i b le a n a lo g o u s r e a c t i o n f o r rh e n iu m . Rhenium pow der p lu s s u l f u r y l c h l o r i d e , w ith and w ith o u t t r a c e s o f aluminum c h lo r id e p r e s e n t , w ere t r e a t e d

(a ) b y b o i l i n g f o r f o r t y - e i g h t

h o u rs a t a tm o s p h e ric p r e s s u r e , (b ) by h e a tin g i n a s e a le d tu b e a t 150“ f o r s i x h o u r s , an d (c ) by r e f l u x i n g i n e x c e s s e t h e r f o r s i x t y h o u rs.

I n e v e r y c a s e a l i m i t e d am ount o f r e a c t i o n to o k p la c e , th e

amount o f w h ich d id n o t seem to b e n o ta b ly Im proved b y lo n g e r t r e a tm e n t.

E th e r e x t r a c t i o n o f t h e r e a c t i o n r e s i d u e s gave a r e d t o

v i o l e t s o l u t i o n w hich was f i l t e r e d t o rem ove u n r e a c te d rh en iu m . Vacuum e v a p o r a tio n o f t h e e t h e r l e f t a r e d r e s i d u e , t i o h h ad a h ig h te n d e n c y t o su b lim e o r d i s t i l l i n t o t h e t r a p a s a g r e e n is h v a p o r . A p p a r e n tly , rh en iu m t r i c h l o r i d e w as t h e p ro d u c t o f th e r e a c t i o n s b u t th e y i e l d w as p o o r u n d e r a l l c o n d i tio n s t h a t w ere t r i e d . A tte m p te d S y n th e s is o f R h e n id e .

The f u s io n o f a n a c t i v e

m e ta l w i t h rh en iu m pow der seemed an o b v io u s m eans o f a tte m p tin g a r h e n id e s y n t h e s i s .

F in e ly d iv id e d rh en iu m m e ta l w as fu s e d w i t h e x c e s s

63

sodium m e ta l i n an e v a c u a te d and s e a l e d tu b e a t 1 8 0 -2 0 0 “ f o r s i x h o u r s w ith o u t r e s u l t .

I t w as l a t e r fo u n d t h a t Maun (24) had made a

s i m i l a r a tte m p t em ploying p o ta s s iu m h e x a c h lo r o r h e n a te (IV ) w ith p o ta s s iu m f u s io n w h ich r e s u l t e d i n r e d u c t i o n t o t h e m e t a l . The r e d u c t i o n o f rh e n iu m t r i c h l o r i d e i n e t h e r s o l u t i o n was a ls o a tte m p te d u t i l i z i n g l i t h i u m alum inum h y d r id e a s th e r e d u c in g a g e n t.

R e d u c tio n p ro c e e d e d o n ly t o th e m e t a l . S o l u b i l i t y o f T h a llo u s R h e n id e .

The many a p p a r e n t a n a lo g ie s

o f rh e n id e io n w ith th e h a l i d e s s u g g e s ts th e p o s s i b l e i s o l a t i o n o f in s o l u b le s i l v e r , l e a d , o r m e rc u ro u s s a l t s b y p r e c i p i t a t i o n .

The

p o w e rfu l r e d u c in g a c t i o n o f r h e n id e i o n , h o w ev er, p r e c lu d e s t h e s e p a rtic u la r p o s s ib ilitie s .

T h a llo u s io n , on t h e o th e r h a n d , i s n o t

re d u c e d b y rh e n id e an d t h e t h a l l o u s h a l i d e s a r e r e l a t i v e l y i n s o l u b l e . One dM p e r r h e n a te i n 2 . 4 N h y d r o c h lo r ic a c i d was r e d u c e d i n th e w a te r - c o o le d Jo n e s r e d u c t o r .

The r e d u c e d s o l u t i o n w as

s a t u r a t e d w ith t h a l l o u s c h l o r i d e a t ç a . 2 5 “ and t h m a llo w e d to s ta n d u n d e r n i t r o g e n f o r n i n e t y m in u te s a t c a . 0 “ .

The w h ite c r y s t a l s w h ic h

s e p a r a te d from t h e t e s t s o l u t i o n w ere exam ined m ic r o s c o p ic a l ly and fo u n d to b e isom orphous w ith t h a l l o u s c h l o r i d e .

A p o r t i o n o f th e

w ashed p r e c i p i t a t e was warmed an d d is s o lv e d i n b ism u th t r i c h l o r i d e s o l u t i o n , g iv in g no e v id e n c e o f any d i s c o l o r a t i o n .

A d ro p Oi t h e

s u p e r n a ta n t t e s t s o l u t i o n gave an im m ed iate b la c k p r e c i p i t a t e w hen added to t h e same tu b e , i n d i c a t i n g t h e p r e s e n c e o f r h e n id e i n th e s u p e r n a ta n t l i q u i d . c o n t r a r y t o e x p e c t a t i o n , i t m u st be o o n o lu a e d t h a t t h a l l o u s r h e n id e i s c o n s id e r a b l y more s o lu b le th a n t h e io d id e an d a t l e a s t

64

tw ic e a s s o lu b l e a s th e b ro m id e .

More s p e c i f i c a l l y , th e s o l u b i l i t y

o f t h a l l o u s r h e n id e m ust be e q u a l t o o r g r e a t e r th a n 0*039 gms. p e r 100 m l. o f 2 .4 N h y d r o c h lo r ic a c i d a t 0 “ .

The s o l u b i l i t i e s

(3 3 ) of

th e v a r io u s s a l t s a r e com pared i n T a b le 1 3 . Summary I t i s p o s s i b l e t h a t c o v a le n t rh en iu m h a l id e co m p lex e s may be e x t r a c t a b le i n o r g a n ic m e d ia . No e v id e n c e c o u ld be fo u n d i n s u p p o r t o f r e p o r te d rh en iu m io d i d e s . The a c t i o n o f s u l f u r y l c h l o r i d e on rh en iu m pow der shows li m i t e d p ro m ise a s a s y n t h e s i s f o r rh en iu m t r i c h l o r i d e . Strong r e d u c in g a g e n ts under s p e c if i e d c o n d itio n s do not reduce rhenium compounds below th e m eta l (0) l e v e l . The i n s o l u b i l i t y o f t h a l l o u s h a l i d e s i s n o t r e f l e c t e d i n t h e rh e n id e i o n .

65

T ab le 13 S o l u b i l i t i e s o f T h a llo u s S a l t s i n VJater a t 0 “C. S o lu b ility Gffls./lOO m l.

S a lt T lC l

0 .1 6 1

T lB r

0 .0 2 2

TU

0 .0 0 2 g r e a t e r th a n

(* i n 2 .4 N h y d r o c h lo r ic a c i d . )

0*039*

66

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2648 (1937)

VITA.

Charles L e s lie H ulfs was born in S t . L ou is, M issouri, on October 2 1 , 1920.

He entered the U n iv e r sity o f I l l i n o i s , Urbana,

I l l i n o i s , in 1938 and r e c e iv e d the degree o f Bachelor o f S cien ce from t h i s i n s t it u t io n in 1942. Sigma P i Sigma, and Sigma X i.

He i s a member o f Phi Lambda U p silo n , He i s the author o f " P h o to electric

Balance In d icator" , A nal. Chem. ,

263 (1 9 4 8 );“Simple Demonstration

Polarograph", J . Chem, E ducation, 25, 224 (1948); and "Volumetric M icrodeterm ination o f Water in M inerals", Mikrochemie v e r . Mikrochim. A cta, 33, 338 (1 9 4 8 ).