Redox Reactions of Ligands

17 Redox Reactions of Ligands I. Oxidation of Oxalate Complexes of Chromium(III) by Cerium (IV) in Aqueous Sulfuric Acid JOHN E. TEGGINS, MARTHA T. WANG, and RONALD M. MILBURN Downloaded by CALIFORNIA INST OF TECHNOLOGY on December 11, 2017 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1963-0037.ch017

Boston University, Boston, Mass.

The cerium(IV) oxidations of Cr(C O )3-3, cisCr(OH ) (C O ) -, and Cr(OH ) C O + have been investigated in aqueous acidic-sulfate media. The observations are consistent with the reactions proceeding, at least initially, according to the stoicis-Cr(OH ) (C O ) chiometrics: Cr(C O ) 2

2

2

2

4

2

2

2

4

2

2

2

2

2

4

2

4

2

2

2

4

2

Cr(OH ) C O + + 2

2

4

2

4

Cr(OH )6+3 + 2CO .

2CO ; and Cr(OH ) C O + 2

4

4

3

+ 2CO ; cis-Cr(OH ) (C O ) 2

4

2

4

2

In moderately strong sulfuric acid each redox reaction appears to follow the simple rate law -d[Ce(IV)]/dt = k[Cr(lll)complex] [Ce(IV)]. The values of k in 1.83M sulfuric acid at 2 5 ° for Cr(C O ) -3 cis-Cr(OH ) (S O ) -, and Cr(OH ) C O + are, respectively, 1.27(±0.14) x 10- , 1.06 (±0.10) x 1 0 -1, and 7.5(±0.9) x 10-3 liter mole-1 sec-1. The cerium(IV) oxidation of cisCr(OH2)2(C2O4)2- has been examined in further detail. As the acidity is diminished at constant sulfate ion concentration, deviations from simple second-order behavior develop, primarily because of an increasing inhibition of the reaction by cerium(III). For each of several acidic-sulfate media studied, the results for a range of conditions may be summarized by the more general rate law -d[Ce(IV)]/dt = k'[cis-] [Ce(IV)]2/ "[Ce(lll)] + k'''[Ce(IV)], where values of the constants are characteristic for the particular medium. This result is interpreted on the basis of the simple mecha2

4

3

,

2

2

2

4

2

2

4

1

2

4

K

nism: cis- + Ce(IV)

cis° + Ce(III); cis° +

products where cis° is an intermediate Ce(IV) formed by the one-equivalent oxidation of cisCr(OH ) (C O ) -. Detailed interpretations are discussed with reference to the rates and mechanisms of related reactions. 2

2

2

4

2

226 Busch; Reactions of Coordinated Ligands Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

TEGGINS ET AL.

Oxidation

227

of Oxofoto Complexes

• η r e d o x r e a c t i o n s b e t w e e n l a b i l e m e t a l ions ( h y d r a t e d o r o t h e r w i s e c o m p l e x e d ) a n d • a n i o n i c o r n e u t r a l reagents,

the f o r m a t i o n o f intermediate

metal-ligand

complexes

has b e e n d e m o n s t r a t e d o r p r o p o s e d i n a n u m b e r o f cases (4, 73, 24, 46), y e t m u c h r e m a i n s to b e l e a r n e d a b o u t

the importance,

decomposition products o f s u c h intermediates.

p r o p e r t i e s , reactivities,

a n d primary

C o n s i d e r a b l y less a t t e n t i o n h a s b e e n

d e v o t e d to r e a c t i o n s o f n o n l a b i l e m e t a l - l i g a n d c o m p l e x e s i n w h i c h l i g a n d s a r e o x i d i z e d o r r e d u c e d b y e x t e r n a l reagents, b y o t h e r p a r t s o f the c o m p l e x , o r b y b o t h . e x a m p l e s o f these r e a c t i o n s a r e r e c o r d e d b y W e r n e r (45) ; o f s u c h reactions have been reported

S o m e classic

m o r e recent investigations

(70,26,28,37,37,38,48).

Downloaded by CALIFORNIA INST OF TECHNOLOGY on December 11, 2017 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1963-0037.ch017

F o r reactions o f these types, t h e i d e n t i t y o f t h e l i g a n d , its e n v i r o n m e n t w i t h i n t h e c o m p l e x — e . g . , the nature o f the central m e t a l a n d the properties a n d positions o f the other l i g a n d s — a n d the character o f a n y external redox reagent are i m p o r t a n t variables w h i c h w i l l i n f l u e n c e t h e t h e r m o d y n a m i c feasibility o f processes, t h e p r o d u c t s f o r m e d u n d e r g i v e n e x p e r i m e n t a l c o n d i t i o n s , a n d t h e rates a n d m e c h a n i s m s o f r e d o x processes. W i t h i n this f r a m e w o r k s o m e s i g n i f i c a n t p r o b l e m s a w a i t s o l u t i o n . I n t h e p r e s e n t i n v e s t i g a t i o n , w h i c h represents t h e first o f a series o f studies b e i n g u n d e r t a k e n i n this l a b o r a t o r y o n r e d o x r e a c t i o n s o f l i g a n d s , w e h a v e e x a m i n e d t h e r e a c t i o n s o f C r ( C 0 ) 3 " , cis-Gr(OH )2(0204)2" 3

2

2

4

i n aqueous acidic m e d i a .

a n d0Γ(ΟΗ )40 Ο 2

2

4

+

with cerium(IV)

E a c h reactant system involves a n o n l a b i l e c o m p l e x i o n

c o n t a i n i n g a n o x i d i z a b l e l i g a n d , a n d a n o x i d a n t k n o w n to b e c a p a b l e o f a t t a c k i n g b o t h the i s o l a t e d l i g a n d (3, 8, 14, 36, 46) a n d t h e p a r e n t a q u o c a t i o n (43, 47).

A l l experi

m e n t s w e r e p e r f o r m e d i n t h e presence o f a n excess o f s u l f u r i c a c i d , i n w h i c h m e d i u m t h e c e r i u m ( I V ) exists v e r y p r e d o m i n a n t l y as m o n o n u c l e a r s u l f a t e ( o r b i s u l f a t o ) c o m p l e x e s

(79, 43). U n d e r the chosen experimental conditions the reaction between C r ( O H 2 ) é C 0 4 2

+

a n d c e r i u m ( I V ) is s u f f i c i e n t l y s l o w to b e n e g l e c t e d i n k i n e t i c studies o f t h e e a r l y stages o f t h e r e a c t i o n b e t w e e n cw-Cr(OH2) (C 04)2~ a n d c e r i u m ( I V ) . 2

2

m-Cr(OH2)2(C204)2"* a n d C r ( C 2 0 ) ~ 4

a n d the reaction o f

3

3

O n the other h a n d ,

r e a c t w i t h c e r i u m ( I V ) at c o m p a r a b l e rates,

m-Cr(OH2) 2(0204) 2" m u s t b e t a k e n i n t o c o n s i d e r a t i o n i n e s t a b -

l i s h i n g t h e r a t e a t w h i c h Cr(C204)3~

3

reacts.

I t h a s t h u s b e e n a p p r o p r i a t e to d e -

v o t e m a j o r a t t e n t i o n a t this t i m e to the c e r i u m ( I V ) o x i d a t i o n o f

m - C r ( O H 2 ) ( C 204)2". 2

Experimental Materials. U n l e s s otherwise specified, chemicals were o f reagent grade. F o r the majority o f experiments d o u b l y distilled water was used, the second distillation b e i n g m a d e i n a b o r o s i l i c a t e glass a p p a r a t u s after t h e w a t e r h a d b e e n r e f l u x e d w i t h potassium d i c h r o m a t e a n d sulfuric a c i d for several hours. T h e kinetic d a t a for the singly a n d d o u b l y distilled water were identical within the usual limits o f error. Most stock c e r i u m ( I V ) sulfate solutions w e r e p r e p a r e d f r o m G . F . S m i t h C h e m i c a l C o . s t a n d a r d eerie sulfate s o l u t i o n [ a p p r o x i m a t e l y 0.1 Ν c e r i u m ( I V ) i n \N s u l f u r i c a c i d ] . I d e n t i c a l k i n e t i c results w e r e o b t a i n e d i n e x p e r i m e n t s w h e r e c e r i u m ( I V ) sulfate s o l u tions w e r e p r e p a r e d b y c o n v e r t i n g A n a c h e m i a C h e m i c a l s L t d . a m m o n i u m h e x a n i t r a t o c e r a t e ( I V ) to c e r i u m ( I V ) h y d r o x i d e w i t h aqueous a m m o n i a solution a n d dissolving the w e l l w a s h e d p r e c i p i t a t e i n s u l f u r i c a c i d . A l l c e r i u m ( I V ) stock s o l u t i o n s w e r e s t a n d a r d i z e d b y t i t r a t i o n a g a i n s t arsenious o x i d e u s i n g e s t a b l i s h e d p r o c e d u r e s (44). C o n c e n t r a tions o f c e r i u m ( I I I ) i n t h e s t a n d a r d c e r i u m ( I V ) s o l u t i o n s , w h i c h a m o u n t e d to ~ 4 % o f the t o t a l c e r i u m , w e r e d e t e r m i n e d f r o m t h e t o t a l a b s o r b a n c e a t 3 2 0 m/tt after o x i d a t i o n o f t h e c e r i u m ( I I I ) w i t h p o t a s s i u m p e r s u l f a t e (77, 30\ Stoichiometric concentrations of a c i d i n the s t a n d a r d c e r i u m ( I V ) solutions were m e a s u r e d b y direct alkaline titration, taking into account the h y d r o x i d e c o n s u m e d b y precipitation o f the c e r i u m . Standard c e r i u m ( I I I ) sulfate s o l u t i o n s w e r e p r e p a r e d b y t h e r e a c t i o n o f e q u i v a l e n t a m o u n t s o f s t a n d a r d c e r i u m ( I V ) sulfate a n d s t a n d a r d o x a l i c a c i d . S t a n d a r d solutions o f s u l f u r i c a c i d a n d s o d i u m b i s u l f a t e w e r e u s e d t o a d j u s t t h e c o n c e n t r a t i o n s o f h y d r o g e n i o n , bisulfate i o n , a n d sulfate i o n . [ T h e concentrations of

Busch; Reactions of Coordinated Ligands Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

ADVANCES IN CHEMISTRY SERIES


228

h y d r o g e n i o n , b i s u l f a t e i o n , a n d sulfate i o n w e r e c o m p u t e d b y u s i n g t h e c a l c u l a t i o n s o f B a e s (2) o n t h e bisulfate i o n d i s s o c i a t i o n i n s u l f u r i c a c i d - s o d i u m sulfate s o l u t i o n s . ] T h e concentrations o f s o d i u m i o n a n d available h y d r o g e n i o n i n the standard s o d i u m bisulfate s o l u t i o n w e r e f o u n d to b e e q u i v a l e n t . T h e s o d i u m i o n c o n t e n t was d e t e r m i n e d b y t i t r a t i o n o f the a c i d i t y after w a s h i n g s a m p l e s o f solutions t h r o u g h c a r e f u l l y p r e w a s h e d c a t i o n exchange resin i n the h y d r o g e n f o r m . T h e c a t i o n e x c h a n g e resin u s e d f o r these a n d o t h e r p u r p o s e s w a s D o w e x 5 0 - X 8 ; the a n i o n e x c h a n g e r e s i n u s e d t h r o u g h o u t t h e study was D o w e x 1 - X 8 . Solutions c o n t a i n i n g the m - b i s o x a l a t o d i a q u o c h r o m a t e ( I I I ) i o n were m a d e b y first p r e p a r i n g t h e r e a d i l y c r y s t a l l i z ^ b l e p o t a s s i u m / r a n i - b i s o x a l a t o d i a q u o c h r o m a t e ( I I I ) f r o m o x a l i c a c i d a n d p o t a s s i u m d i c h r o m a t e b y t h e m e t h o d o f W e r n e r (39). Portions o f the r e c r y s t a l l i z e d trans salt w e r e d i s s o l v e d i n w a t e r , a n d r e s u l t i n g solutions w e r e passed t h r o u g h c a t i o n e x c h a n g e r e s i n i n t h e s o d i u m f o r m , to r e p l a c e t h e potas s i u m i o n b y s o d i u m i o n a n d to r e m o v e a n y c a t i o n i c c h r o m i u m (111) species, a n d t h r o u g h a n i o n e x c h a n g e r e s i n i n t h e p e r c h l o r a t e f o r m to r e m o v e a n y h i g h l y c h a r g e d a n i o n i c c h r o m i u m ( I I I ) species. T h e a n a l y s i s o f effluent solutions f o r c h r o m i u m b y t h e m e t h o d o f H a u p t (20) as d e s c r i b e d b y A l t m a n a n d K i n g (7), a n d f o r o x a l a t e b y a m e t h o d s i m i l a r to t h a t d e s c r i b e d b y H a m m (77), g a v e c h r o m i u m - o x a l a t e r a t i o s o f the o r d e r o f 1.00 to 2.02. T o ensure essentially c o m p l e t e i s o m e r i z a t i o n o f t h e trans to the cis i s o m e r , t h e solutions w e r e a l l o w e d to s t a n d o v e r n i g h t before b e i n g u s e d i n k i n e t i c e x p e r i m e n t s (7, 77). Solutions c o n t a i n i n g the m o n o - o x a l a t o t e t r a a q u o c h r o m i u m ( I I I ) i o n were p r e p a r e d b y reaction o f the h e x a a q u o c h r o m i u m ( I I I ) i o n w i t h oxalic a c i d . A solution w h i c h was 0 . 1 0 0 0 Λ / i n h e x a a q u o c h r o m i u m ( I I I ) p e r c h l o r a t e a n d 0.100M i n p e r c h l o r i c a c i d was first p r e p a r e d b y t h e r e a c t i o n o f h y d r o g e n p e r o x i d e w i t h a s o l u t i o n c o n t a i n i n g calculated amounts of potassium dichromate a n d perchloric acid. A f t e r t h e excess p e r o x i d e h a d b e e n d e s t r o y e d b y p r o l o n g e d b o i l i n g , the s o l u t i o n w a s stored o v e r n i g h t at 0 ° , the p o t a s s i u m p e r c h l o r a t e w h i c h c r y s t a l l i z e d w a s filtered o f f a n d w a s h e d w i t h i c e c o l d w a t e r , a n d the filtrate w i t h w a s h i n g s w a s d i l u t e d to t h e r e q u i r e d v o l u m e . Solu tions w h i c h c o n t a i n e d e q u i m o l a r a m o u n t s o f t h e h e x a a q u o c h r o m i u m ( I I I ) p e r c h l o r a t e a n d o x a l i c a c i d w e r e h e a t e d to i n c i p i e n t b o i l i n g f o r a b o u t a n h o u r . T h e cooled solu tions w e r e passed t h r o u g h a n i o n e x c h a n g e resin i n the p e r c h l o r a t e f o r m to r e m o v e a n i o n i c i m p u r i t i e s , a n d t h r o u g h c a t i o n e x c h a n g e resin i n t h e h y d r o g e n f o r m to r e m o v e u n r e a c t e d h e x a a q u o c h r o m i u m ( I I I ) i o n . E f f l u e n t solutions w e r e p l a c e d o n a n o t h e r c o l u m n c o n t a i n i n g c a t i o n e x c h a n g e resin i n t h e h y d r o g e n f o r m , a n d t h e c o l u m n w a s eluted with 0 . 3 A Î perchloric acid. M i d d l e fractions o f the c h r o m i u m ( I I I ) - c o n t a i n i n g effluent solutions w e r e r e t a i n e d a n d p o r t i o n s w e r e a n a l y z e d f o r c h r o m i u m a n d o x a l a t e as before (7, 77, 20) a n d f o r h y d r o g e n i o n b y m e a s u r i n g t h e total a c i d i t y o f a n a l i q u o t after it w a s passed t h r o u g h a c o l u m n c o n t a i n i n g c a t i o n e x c h a n g e resin i n t h e h y d r o g e n form. T h e h y d r o g e n i o n g e n e r a t e d b y the m o n o - o x a l a t o t e t r a a q u o c h r o m i u m ( I I I ) i o n was t a k e n i n t o c o n s i d e r a t i o n . W i t h i n ~ 2 % , c h r o m i u m - o x a l a t e ratios o f 1 to 1 w e r e obtained. Perchlorate was r e m o v e d f r o m the m o n o - o x a l a t o t e t r a a q u o c h r o m i u m ( I I I ) s o l u t i o n b y a d d i n g a n e q u i v a l e n t q u a n t i t y o f p o t a s s i u m sulfate, s t o r i n g t h e s o l u t i o n overnight at 0 ° , a n d quantitatively filtering o f f the potassium perchlorate. Further analysis for c h r o m i u m , oxalate, a n d h y d r o g e n i o n c o n f i r m e d a c h r o m i u m - o x a l a t e ratio o f v e r y close to 1 to 1, established t h e h y d r o g e n i o n c o n c e n t r a t i o n , a n d d e m o n s t r a t e d that negligible quantities of potassium perchlorate remained i n solution. A p o r t i o n o f t h e s o l u t i o n w a s subjected to a q u a l i t a t i v e e l e c t r i c a l m i g r a t i o n e x p e r i m e n t u s i n g a t h r e e - c o m p o n e n t c e l l s i m i l a r to t h a t d e s c r i b e d b y H a r d w i c k a n d R o b e r t s o n (79). T h e solution u n d e r investigation was placed i n the center c o m p a r t m e n t , a n d i n the o u t e r c o m p a r t m e n t s w e r e p l a c e d s u l f u r i c a c i d solutions o f c o n c e n t r a t i o n s s i m i l a r to t h a t o f t h e test s o l u t i o n . N o d i f f u s i o n o c c u r r e d o v e r a p e r i o d o f 24 h o u r s . A p p l i c a t i o n o f a b o u t 50 volts d . c . across the electrodes c a u s e d m i g r a t i o n o f v i o l e t c o l o r to t h e c a t h o d e a n d n o v i o l e t c o l o r to t h e a n o d e . T h e results i n d i c a t e a p o s i t i v e l y c h a r g e d c h r o m i u m species. P o t a s s i u m t r i s o x a l a t o c h r o m a t e ( I I I ) w a s p r e p a r e d b y G r a h a m ' s m e t h o d (75), as d e s c r i b e d b y P a l m e r (32). S o l u t i o n s o f the r e c r y s t a l l i z e d salt, u s e d f o r studies o n t h e t r i s o x a l a t o c h r o m a t e ( I I I ) i o n , w e r e s t a n d a r d i z e d b y analysis f o r c h r o m i u m as before (7, 20). Stoichiometrics.

From

preliminary

cerium(IV) a n d each of C r ( C 0 ) - , 2

4

3

3

observations

on

aj-Cr(OH ) (C 0 ) 2

2

2

4

2

the

reactions

between

a n d C r ( O H ) C 0 + it 2

4

2

4

w a s e v i d e n t t h a t c a r b o n d i o x i d e is e v o l v e d , as i d e n t i f i e d b y p r e c i p i t a t i o n o f b a r i u m


TiOOINS ET AL

Oxidation

of Oxalato

Complexes

229


carbonate from barium hydroxide solution. It was also evident that cerium(IV) is reduced to cerium(III), as demonstrated by disappearance of the characteristic cerium(IV) color and the ability of solutions to oxidize iodide ion. Spectral analyses and qualitative tests showed that no dichromate is formed until all the oxalate has been consumed. The stoichiometry of the cerium(IV) reaction with m-Cr(OH2)2(0204)2" was investigated by quantitative and additional qualitative examination of the reaction products. A stream of nitrogen, which had been freed from any traces of carbon dioxide by first passing through alkaline solution, was used to sweep the carbon dioxide from a reaction mixture (Initially carbon dioxide-free) prepared from equivalent amounts of potassium m-bisoxalatodiaquochromate(III) and cerium(IV) sulfate i n 1J0M sulfuric acid. T h e issuing nitrogen stream was allowed to pass through a measured volume of standard barium hydroxide solution. After the reaction had proceeded essentially to completion, the barium hydroxide solution was filtered to remove barium carbonate and was then titrated with standard sulfuric acid, using methyl orange as indicator. (Five hours at 35° were allowed, this condition being estimated to be adequate from a consideration of relevant kinetic data.) The amount of carbon dioxide evolved during the oxidation was calculated to be 9 6 % of the theoretical amount which would correspond to the oxidation of one oxalate to carbon dioxide per chromium. The reaction vessel was observed to contain a violet solution, and white precipitate which had begun to appear only after the reaction had been proceeding for some time. The solution and precipitate were separated by filtration. Qualitative analysis of the precipitate showed that it contained c e r i u m ( l l l ) and sulfate ion. [No precipitate appeared when aV-bisoxalatodiaquochromate(III) ion and cerium(IV) were allowed to react at this acidity under conditions where the sulfate and/or cerium concentrations were somewhat lower.] T h e filtrate, including washings, was diluted to a measured volume, and two measured aliquots of this solution were withdrawn for study. T h e first aliquot was passed onto a cation exchange column i n the hydrogen form. Elution with \M sulfuric acid left a small dark violet band of hexaaquochromium(III) ion at the top of the column, while the rest of the chromium passed through the column. [In separate experiments it was confirmed that the G r ( O H ) f e ion is retained as a dark violet band at the top of the resin when the resin is eluted with \M sulfuric acid. This is i n agreement with the observations of K i n g and Dismukes (23).] Analysis of the effluent solution showed that 4 to 5 % of the chromium had been retained by the column. The second aliquot was passed onto another cation exchange resin and eluted as before, the effluent solution was passed onto a second cation exchange column, and the elution process was repeated. N o dark band was left on the second column, and analysis of a portion of the new effluent solution showed that there had been no further loss of chromium on passage through the second column. T w o additional portions of the last-mentioned solution were withdrawn for examination. T h e first portion was analyzed for oxalate, as before, and chromium-oxalate ratios of 1.00 to 1.02 were ob tained. T h e spectrum of the second portion was measured through the visible range and found to agree well with the spectrum of the mono-oxalatotetraaquochromium (III) ion at the same chromium concentration. +3

2

Some critical experiments of the type described above were repeated, and similar results were obtained. T h e observations are consistent with the following stoichiom etry for the initial reaction: m-Cr(OH2)2(C 0 )2- + 2Ce(IV) + 2 H 0 2

2

4

CrCOH^aO^

+ 2Ce(III) +

2C0

2

(1)

The production of the small amounts of hexaaquochromium(III) ion described above suggests that the further Reaction 2 occurs C r ( O H ) 4 G 0 4 - + 2Ce(IV) + 2 H 0 — C r ( O H ) 2

2

i

2

2

6

+ 3

+ 2Ce(III) + 2 G 0


2

(2)

230

ADVANCES IN CHEMISTRY SERIES

but that the specific rate for this reaction is less than that for Reaction 1. This indica tion of the relative reactivities of Cr(OH )4C20 and m-Cr(OH )2(0204)2" toward cerium(IV) is confirmed by kinetic experiments described below. I n addition, support for the stoichiometry specified by Reaction 2 is provided by agreement between the rates of cerium(IV) consumption by Cr(OH2)4C204 as measured directly by titrimetry and as measured by spectrophotometry on the basis that C r ( O H ) 6 is the only absorbing chromium product. For the initial reaction between Cr(C204)3~ and cerium(IV) the most obvious stoichiometry is: 2

4

+

2

+

2

+3

3

Cr(C 04)3" + 2Ge(IV) + 2 H 0 — m - C r ( 0 H ) ( C 0 ) -

+ 2Ce(Ill) + 2 C 0


3

2

2

2

2

2

4

2

(3)

2

This view receives support, i n a similar way, from the observations that the initial rates of cerium(IV) consumption by Cr(Ci0 )3~ , as measured directly by titrimetry, agree well with those obtained by spectrophotometry on the assumption that cisC r ( O H ) 2(0204)2" is the only absorbing chromium product. T h e composition of a reactant system initially containing Cr(C204)3~ and cerium(IV) soon becomes com plicated, however, since Cr(C204)3~ and m - C r ( O H 2 ) 2(0204)2" react with cerium(IV) at comparable rates. 3

4

2

3

3

Measurement of Reaction Rates by Spectrophotometry. T h e rates of reaction were measured with a Beckman M o d e l D U spectrophotometer. T h e observations were made at 480 τημ, at which wave length the molar absorptivity coefficients for the cerium(IV) i n the chosen media are significantly greater than the coefficients for the oxalatochromium(III) complexes or for the C r ( O H ) ion. During the course of reaction solutions were kept i n 1-cm. quartz cells which were held at the required temperature to within ± 0 . 1 ° . F o r a given rate determination two solutions were prepared, one containing the chromium salt and the other containing the cerium(IV) sulfate, sulfuric acid, and any required sodium bisulfate. T h e two constituent solutions were brought to the required temperature i n a thermostat, rapidly mixed, and quickly introduced into a clean, dry thermostatéd cell i n the spectrophotometer. (For the experiments i n 1.83Af sulfuric acid, part of the acid was added to the solution of the chromium salt, shortly before final thermostating, i n order to avoid excessive heating on mixing the reactant solutions. F o r the other acidities studied this precaution was not necessary. O n mixing equal volumes of 1 M sulfuric acid and water at 25° the temperature increased'by only 0.12 ± [ 0 . 0 2 ° . ) The absorbance at 480 ταμ was measured 1 minute after mixing the solutions, and thereafter at appropriate intervals. 2

6

+ 3

Interest was generally centered on the first stages of reaction. I n the times involved no turbidity appeared i n reactant solutions, even for the highest sulfate ion and cerium concentrations and for the lowest hydrogen ion concentrations. For each kinetic run the sulfate ion concentration was buffered by large and thus essentially constant con centrations of hydrogen ion and bisulfate i o n ; therefore the distribution of cerium(IV) among various sulfato (or bisulfato) complexes remained essentially constant throughout each run (79, 43). Under this condition of a constant medium each species absorbing at 48θ2ηιμ—cerium(IV), C r ( C 0 ) 3 - , a x - C K O H s M C g O * ^ " , C r ( O H ) C 0 4 , and Cr(OH )« —was shown to obey Beer's law throughout the relevant concentration range of about 2 Χ 1 0 " to 1 X 1 0 ~ W . I n 1.83Af sulfuric acid the molar absorptivity coefficients of C r ( C 0 ) 3 " ~ , m - C r ( O H ) ( C 0 ) - Cr(OH )4C 0 , and C r ( O H ) at 480 m/* and a slit width of 0.015 mm. were measured and found to be 16.1, 15.2, 11.0, and 3.1. While the absorptivity coefficients for these chromium ions changed only very slightly with changes i n the medium, the coefficient for the cerium(IV) was noticeably dependent on the medium and values were determined for each set of condi tions used. I n 1.83Λ/ sulfuric acid the absorptivity coefficient of the cerium(IV) at 480 τημ and a slit width of 0.015 m m . was found to be 24.0. Since the absorbances of 3

2

2

2

4

4

2

+

+s

2

s

2

4

2

2

2

4

2

2

2

4

+


2

6

+ 3

Oxidation

TEGGINS ET AL.

of Oxofofo

Complexes

231

the c e r i u m ( I V ) s o l u t i o n a r e s i g n i f i c a n t l y a l t e r e d b y s m a l l c h a n g e s i n w a v e l e n g t h i n t h e r e g i o n o f 4 8 0 τημ, i t w a s c o n v e n i e n t t o a d j u s t t h e w a v e l e n g t h setting before a n y series o f m e a s u r e m e n t s , so t h a t a stock s o l u t i o n w h i c h w a s 0 . 0 1 0 0 M i n c e r i u m ( I V ) a n d 1.83 M i n s u l f u r i c a c i d g a v e a n a b s o r b a n c e o f 0.240 a t a slit w i d t h o f 0.015 m m .

T h e same

slit w i d t h w a s m a i n t a i n e d t h r o u g h o u t t h e series o f m e a s u r e m e n t s . T h e m e t h o d o f e s t a b l i s h i n g t h e c o n c e n t r a t i o n s o f r e a c t a n t species f r o m the m e a s u r e d a b s o r b a n c e s a t v a r i o u s t i m e s d u r i n g t h e e a r l y stages o f r e a c t i o n w i l l b e i l l u s t r a t e d f o r t h e c e r i u m ( I V ) r e a c t i o n w i t h c w - C r ( O H ) 2(0204)2" 2

A t z e r o t i m e , t h e a b s o r b a n c e a t 4 8 0 m/t c o u l d b e a t t r i b u t e d to the c e r i u m ( I V ) a n d


the

a,r-Cr(OH2) 2(C 04) 2" 2

i o n alone.

Thus

= *ce[Ce(lV)J

Ao

+ *

cie

[cis-]

(4)

w h e r e A is t h e a b s o r b a n c e , s q u a r e b r a c k e t s i n d i c a t e c o n c e n t r a t i o n s i n m o l e s p e r l i t e r , zero subscripts indicate zero time, a n d e

Ce

ande

cis

a r e a b s o r p t i v i t y coefficients,

b e i n g a n a v e r a g e f o r t h e v a r i o u s c e r i u m ( I V ) species i n s o l u t i o n .

e

Ce

V a l u e s o f AQ, c a l

culated f r o m the absorbances o f the components using Beer's l a w , agreed w i t h the values o b t a i n e d b y extrapolating the m e a s u r e d absorbances b a c k to zero time, a result w h i c h indicates that any intermediate complexes w h i c h m a y be f o r m e d between c e r i u m (IV) and

a>-Cr(OH2) 2(0204) 2"

are n o t spectrally detectable a t 480 πιμ.

A t a n y t i m e , /, i t w a s a s s u m e d t h a t + * c i [ c i s ~ ] i +

Redox Reactions of Ligands

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