thermally unstable brown Cl2O3 in the photodecomposi- tion of chlorine dioxide. ..... Chem., 54, 569 (1906). L. T. Bugaenko and B. M. Roshchektaev, Zh. Fiz.
Progress in Inorgunic Chemistry; Volume 15 Edited by Stephen J. Lippard Copyright © 1972 by John Wiley & Sons, Inc.
THE CHEMISTRY OF CHLORINE DIOXIDE By Gilbert Gordon
D e p a r t m e n t of C h e m i s t r y , U n i v e r s i t y of I o w a , Iowa C i t y , Iowa Robert G. Kieffer P e n n w a l t Corporation, P h a r m a c e u t i c a l Division, Rochester, New York David H. Rosenblatt Edgewood A r s e n a l Research Laboratories, E d g e w o o d Arsenal, M a r y l a n d I.
Decomposition of Chlorite Ion and Chlorous Acid Solutions A. Preparation of Sodium Chlorite B. Dissociation Constant for Chlorous Acid. C. Absorption Spectra of HC102 and NaC102 D. Decomposition of Alkaline NaC102 Solutions. E. Decomposition of NaC102 in Neutral Solutions. F. Decomposition of Acidic Solutions of Chlorine(II1). G. Photochemical Decomposition of Chlorine(II1) Solutions H. Thermal Decomposition of Sodium Chlorite.. 11. Reactions of Chlorine ( 0 ) and Chlorine (I) with Chlorine (111) A. Stoichiometry. B. Rate. 111. Reaction of Chlorine (0) and Chlorine (I) with Chlorine(1V) and the Reaction of Chlorate Ion with chloride Ion. 20 1
. . . . . . . . . . . . . . . 202 . . . . . 203 204 . 204 . . . . . . . . . . . . . . . . . 206 . . . . . . . . . . . . . . . 206 . . . . . . . . . . . . . 207 . . . . . . . . . . . 224 . . . . . . . . . . . . . . . . . 225 . . . . . . . . . . . . . 226 . . . . . . . . . . . . . 226 . . . . . . . . . . . . . . . . . 232 ...........
234
202
G. Gordon, R. G. Kieffer, and D. H. Rosenblatt
. . . . . . . . . . . . . 234 . . . . . . . . . . . . . . . . 237
A. Stoichiometry. B. R a t e . . IV. Chlorine Dioxide--Properties, Preparation, Disproportionation, and Decomposition Reactions. A. Absorption Spectrum. B. Physical Properties. C. Thermal and Photochemical Decomposition. D. Synthesis. E. Disproportionation Reactions F. Complex Ion Formation. G. Organic Chemistry. H. Chemical Analysis. References
. . . . . . . . . . . . . . . . . . . 244 . . . . . . . . . . 244 . . . . . . . . . . 245 246 . . . . . . . . . . . . . . . 248 . . . . . . 250 . . . . . . . . . 256 . . . . . . . . . . . 259 . . . . . . . . . . . 274 . . . . . . . . . . . . . . . . . . . . 276
Although the reactions of oxygen-containing halogen-
ates are of considerable interest to chemists, little
information is presently available in textbooks about the general properties of many of these species.
Chlorine dioxide is a powerful oxidizing agent used in
both organic and inorganic chemistry; the details of these reactions are the subject of numerous papers
presented in recent years.
In this chapter we set
forth the physical and chemical properties of chlorine
dioxide and discuss the rates and mechanisms of many reactions involving this species. I.
DECOMPOSITION OF CHLORITE ION AND CHLOROUS ACID SOLUTIONS
We only differentiate between chlorite ion and
chlorous acid when the mechanistic implications are
clear. when both species may be present, we use the
The Chemistry of C h l o r i n e Dioxide designation c h l o r i n e ( I I 1 ) .
20 3
I n t h e pH r e g i o n between
1 and 4 , a p p r e c i a b l e c o n c e n t r a t i o n s of both s p e c i e s
Above pH 4 , t h e predominant s p e c i e s i s
a r e present. c h l o r i t e ion.
P r e p a r a t i o n of Sodium C h l o r i t e
A.
I n g e n e r a l , sodium c h l o r i t e can be produced by t h e a c t i o n of a reducing a g e n t on c h l o r i n e d i o x i d e i n an a l k a l i n e medium ( 1 8 3 ) . have been used:
The f o l l o w i n g r e d u c i n g a g e n t s
hydrogen p e r o x i d e , plumbous o x i d e ,
s u l f u r and s u l f u r - c o n t a i n i n g compounds, carbon and carbonaceous m a t e r i a l s , z i n c , i o d i d e s , and sodium amalgams.
C u r t i and Montaldi (36) prepared " f r e e " c h l o r -
ous a c i d by t h e f o l l o w i n g r e a c t i o n :
H o l s t ( 97 ) summarized some of t h e methods t h a t have been used t o p r e p a r e sodium c h l o r i t e : 1.
Reychler (182) r e a c t e d c h l o r i n e d i o x i d e w i t h
hydrogen peroxide. 2.
Levi (139, 140) prepared sodium c h l o r i t e by t h e
following reaction: 2C102
3.
+
H202
+
2NaOH-2NaC102
+
2H20
+
02
(2)
Commercially, O l i n (225) h a s prepared sodium
c h l o r i t e a s follows : 4C102
+
4NaOH
+ C a (OH) 2 + C-4NaC102 + C a C 0 3 + 3H20
(3)
G. Gordon, R. G. K i e f f e r , and D. H. Rosenblatt
204
Commercially, I . G. Farbenindustrie (49, 1 0 7 )
4.
prepares c h l o r i t e s a s follows: 2C102
NaOH
+ Zn
+
d u s t i n H209Zn(C102) 2
+
Zn(C102)2-NaClO2
Zn(0H) 2
(4) (5)
Holst suggests t h e following method f o r preparing sodium c h l o r i t e commercially: 2NaC103
+
2 ~ ~ +1 S02-2C102 0 ~
2C1o2
B.
+
+
2H2S04-2HC103
+
PbO
+
NaOH-NaOPbOH
NaOPbOH
+
NaOH-2NaClO2
2NaHS04
H2SO4
(6)
(7) (8)
+ Pb02 + H20
(9)
Dissociation Constant f o r Chlorous Acid
Chlorous a c i d , a r e l a t i v e l y weak a c i d , d i s s o c i a t e s
as follows:
Table I summarizes t h e various values of t h e d i s s o c i a t i o n constant t h a t have been reported.
C.
Absorption Spectra of H C l 0 2 and NaC102
The absorption s p e c t r a of chlorous acid and sodium c h l o r i t e a r e given i n publications by Buser and HXnisch (281, Leonesi and Piantoni (1361, and S t i t t , e t a l . (211).
Konopik and co-workers
(125) r e p o r t t h e u l t r a -
v i o l e t spectra of highly p u r i f i e d sodium c h l o r i t e ,
The Chemistry o f C h l o r i n e Dioxide
205
TABLE I
I o n i z a t i o n C o n s t a n t of Chlorous Acid, K = a
K1.01 x
Conditions
Reference
23OC
147
1.07 x
39
1.10 x
D i l u t e s o l u t i o n a t 2OoC
98
1.1 x
25OC, Zero i o n i c s t r e n g t h
10
1.15 x
136
4.9 x
25OC, I o n i c s t r e n g t h o f O.OM
215
8 x
18OCa
141
a Value r e f e r s t o a c i d s t a b i l i z e d by hydrogen p e r o x i d e (183). l e a d c h l o r i t e , c h l o r i n e d i o x i d e , and sodium c h l o r a t e . They i n d i c a t e t h a t sodium c h l o r i t e h a s a maximum a t 2600
w i t h a n e x t i n c t i o n c o e f f i c i e n t of 14OM-1 c m - l .
The spectrum o f l e a d c h l o r i t e i s t h e same a s t h a t o f sodium c h l o r i t e .
Only i n t h e f a r - u l t r a v i o l e t
does t h e e f f e c t o f l e a d show up.
region
Chen (31) and
Friedman (61) r e p o r t a maximum a t 261 nm w i t h a molar e x t i n c t i o n c o e f f i c i e n t o f (1.26 ? 0.02) x 1 0 2 K 1 c m - l . Friedman (61) and S c h a e f e r (194) s a y f u r t h e r t h a t a n aqueous s o l u t i o n of c h l o r o u s a c i d h a s a c o n s i d e r a b l e a b s o r p t i o n i n t h e r e g i o n from 3000 t o 2100
[SO
8001, b u t no a b s o r p t i o n maximum i s found i n t h i s
region.
E
206
G. Gordon, R. D.
G. K i e f f e r , and D. H. R o s e n b l a t t
Decomposition of A l k a l i n e NaC102 S o l u t i o n s
Mildly a l k a l i n e s o l u t i o n s of sodium c h l o r i t e a r e s t a b l e f o r p e r i o d s up t o a y e a r , and even w i t h prolonged b o i l i n g no decomposition o c c u r s i f l i g h t is excluded ( 2 4 , 25, 59, 9 7 , 220).
I n hot, strongly
a l k a l i n e s o l u t i o n , c h l o r i n e d i o x i d e i s n o t formed b u t c h l o r a t e i o n i s t h e p r i n c i p a l p r o d u c t of t h e decomposi t i o n ( 2 2 , 193, 226).
+
3NaC102-2NaClO3 E.
(11)
NaCl
Decomposition of NaC102 i n Neutral Solutions
A s t h e pH i s lowered, a c h l o r i t e i o n s o l u t i o n be-
comes l e s s s t a b l e .
N e u t r a l s o l u t i o n s of sodium c h l o r -
i t e a r e reasonably stable i f t h e y a r e k e p t away from l i g h t and h e a t , b u t a s o l u t i o n w i l l decompose slowly i f h e a t e d ( 2 2 , 9 7 , 162, 220).
As i n t h e c a s e of t h e
a l k a l i n e s o l u t i o n , t h e decomposition of a n e u t r a l s o l u t i o n of t h e c h l o r i t e produces no c h l o r i n e d i o x i d e ; only c h l o r a t e i o n and c h l o r i d e i o n a r e formed ( 8 , 2 8 , 220, 2 2 6 ) . C u r t i and Montaldi (36) r e p o r t e d l y have prepared approximately n e u t r a l s o l u t i o n s of c h l o r o u s a c i d by the following reaction: 2C102
+
H202=2HC102
+
02
The e q u i l i b r i u m c o n s t a n t f o r Eq. 1 2 i s 3.2 x
(12)
at
20 7
The Chemistry of C h l o r i n e Dioxide
These " s t a b i l i z e d " s o l u t i o n s of c h l o r o u s a c i d
18OC.
decompose o n l y s l o w l y i n t h e d a r k .
An 0.1M c h l o r o u s
a c i d s o l u t i o n k e p t i n t h e d a r k and a t room t e m p e r a t u r e , decomposes l e s s t h a n 4% i n 24 h r .
A few h o u r s of
e x p o s u r e t o d i r e c t s u n l i g h t , however, w i l l c o m p l e t e l y decompose t h e c h l o r o u s a c i d . The o x i d a t i o n p o t e n t i a l diagram i n d i c a t e s t h a t c h l o r o u s a c i d i s u n s t a b l e w i t h respect t o d i s p r o p o r t i o n a t i o n (127). -1.27
c1-
-1.3595
-C12
-1.63
-HClO
-1.645
-HC102
-1.19
F.
-1.15 -1.21
-1.47
Cl04-
Decomposition of A c i d i c S o l u t i o n s of C h l o r i n e (111)
I n contrast t o alkaline o r neutral solutions, solut i o n s of a c i d i c c h l o r i n e ( I I 1 ) decompose a t measurable
rates and t h e r a t e g e n e r a l l y i n c r e a s e s w i t h d e c r e a s i n g pH (18, 28, 1 2 0 , 1 2 1 , 1 3 0 , 162, 1 6 7 , 217, 220) and i n c r e a s i n g t e m p e r a t u r e ( 2 8 , 99, 1 2 0 , 1 2 1 , 1 6 2 , 2 2 0 ) . The p r o d u c t s o f t h e decomposition r e a c t i o n a r e c h l o r -
a t e i o n , c h l o r i n e d i o x i d e , and c h l o r i d e i o n .
In the
a b s e n c e of added c h l o r i d e i o n , which c a t a l y z e s t h e r e a c t i o n and a l s o a l t e r s t h e s t o i c h i o m e t r y , t h e decomp o s i t i o n can be approximated by ( 6 , 1 0 , 18, 28, 9 9 ,
208
G. Gordon, R. G. K i e f f e r , and D. H. R o s e n b l a t t
120, 1 2 1 , 132, 166, 183, 2 1 1 , 217, 220, 226):
Under most c o n d i t i o n s , more c h l o r a t e i o n i s formed t h a n E q . 1 3 p r e d i c t s (120, 2 1 7 , 226).
Hong (99) ob-
s e r v e d t h a t such s t o i c h i o m e t r i c r e s u l t s are compatible with t h e c o n t r i b u t i o n of an a d d i t i o n a l r e a c t i o n
Considerable v a r i a t i o n i n s t o i c h i o m e t r y i s r e p o r t e d (10, 28, 99, 220, 226) f o r E q s . 1 3 and 14. Chlorine d i o x i d e w a s t h e main p r o d u c t i n t h e decomp o s i t i o n of sodium c h l o r i t e i n a c e t i c acid--sodium
ace-
t a t e b u f f e r (92) o v e r t h e e n t i r e p H r e g i o n s t u d i e d b u t less c h l o r i n e d i o x i d e w a s formed as t h e
(pH 2-71,
p H decreased.
No c h l o r a t e ion was found above pH 5,
b u t below t h i s p H i t s formation became i n c r e a s i n g l y important.
Only about 3% of t h e c h l o r i t e t h a t decom-
posed w a s converted t o oxygen (92, 183) by c102--c1-
+
02
(15)
The a c i d i f i c a t i o n of 1.3M sodium c h l o r i t e (220) with 1 0 %a c e t i c a c i d y i e l d e d almost e n t i r e l y c h l o r i n e d i o x i d e a s t h e major p r o d u c t of t h e d i s p r o p o r t i o n a tion.
A
small amount of oxygen w a s d e t e c t e d b u t no
c h l o r i n e o r p e r c h l o r a t e i o n s were found. Nabar and co-workers
(162) r e p o r t e d t h a t a t pH
3.94 t o 4.38, chlorous a c i d decomposes according t o
The Chemistry of C h l o r i n e Dioxide
Eq. 1 3 a t 75 and 100OC.
2 09
B a r n e t t (10) s t u d i e d t h e de-
composition of chlorous a c i d a t t h e b o i l i n g p o i n t and r e p o r t e d a g r a d u a l t r a n s i t i o n from t h e r e a c t i o n o f Eq. 1 3 t o t h a t of Eq. 16 as t h e c h l o r o u s a c i d concentra-
t i o n decreases.
T h i s r e a c t i o n (Eq. 16) predominates a t low a c i d and high c h l o r i t e concentration.
Some r e p o r t s (28, 162,
226) i n d i c a t e d i f f i c u l t y i n reproducing t h e s e observations. K i e f f e r and Gordon (120) observed t h a t t h e r e l a t i v e amount of c h l o r i n e d i o x i d e formed d e c r e a s e s as t h e hydrogen i o n c o n c e n t r a t i o n i s d e c r e a s e d from 2.0 t o 0.49M.
A s t h e hydrogen i o n c o n c e n t r a t i o n i s f u r t h e r
decreased t o 10'3M,
i s produced.
r e l a t i v e l y more c h l o r i n e d i o x i d e
I n c o n t r a s t t o t h e o b s e r v a t i o n made a t
h i g h a c i d i t i e s (0.7-2.OM),
a t lower hydrogen i o n con-
c e n t r a t i o n s (0.49-1.21 x 10-3M) t h e r e l a t i v e amount of c h l o r i n e d i o x i d e produced d e c r e a s e s with t i m e . K i e f f e r and Gordon (120) used a v a r i e t y o f condit i o n s t o s t u d y t h e s t o i c h i o m e t r y of t h e d i s p r o p o r t i o n a t i o n of chlorous a c i d as a f u n c t i o n of time a t an i o n i c s t r e n g t h of 2.0.
They r e p o r t e d t h a t a t t h e
beginning of t h e r e a c t i o n , between 0.7 and 2.OM hydrogen i o n , l e s s c h l o r i n e d i o x i d e t h a n t h e amount p r e d i c t e d by Eq. 1 3 i s formed, and, as t h e r e a c t i o n proc e e d s , t h e r e l a t i v e amount of c h l o r i n e d i o x i d e
210
G. Gordon, R. G. K i e f f e r , and D. H.
Rosenblatt
increases and t h e r e l a t i v e amount of c h l o r a t e ion decreases.
They showed t h a t i f t h e r e a c t i o n proceeds
f o r a s u f f i c i e n t l y long time and t h e a c i d i t y i s r e l a t i v e l y high, t h e r a t i o of C 1 0 2 t o cl03- i s generally g r e a t e r than 2 .
Deviations from E q . 13 are t o be
expected, s i n c e t h e product, c h l o r i d e i o n , not only a c c e l e r a t e s t h e rate b u t a l s o a l t e r s t h e stoichiometry ( 9 9 , 1 2 0 , 1 2 1 , 217) t o approximately Eq. 16.
There-
f o r e , i n any discussion of t h e stoichiometry o r r a t e , the contribution of Eq. 16 must be considered (18, 2 4 , 99, 1 2 0 , 1 2 1 , 1 8 3 ) .
The e f f e c t of added c h l o r i d e ion on t h e stoichiome-
t r y of t h e disproportionation of chlorous a c i d has been extensively s t u d i e d by K i e f f e r and Gordon (120) and Hong ( 9 9 ) .
A t high c h l o r i d e i o n concentrations
(O.lM), t h e stoichiometry i s approximated by Eq. 1 6 . However, a s t h e i n i t i a l chloride ion concentration i s decreased and a s t h e r e a c t i o n proceeds, t h e r e l a t i v e amount of chlorine dioxide produced a l s o decreases. As t h e hydrogen ion concentration decreases from 2.0
t o 0.2M i n t h e presence of 0.04M N a C 1 , t h e r e l a t i v e amount of c h l o r i n e dioxide formed decreases. Bandi ( 9 ) and Beuermann ( 1 4 ) i n v e s t i g a t e d t h e react i o n of hydrochloric a c i d with sodium c h l o r i t e .
They
r e p o r t t h a t , i n t h e range 0.3 t o 1 . 5 M NaC102, t h e r a t e constant of t h e r e a c t i o n depends only on t h e conc e n t r a t i o n of hydrochloric a c i d and, furthermore, t h a t
The Chemistry of C h l o r i n e Dioxide
211 A t pH
1
t h e r e a c t i o n was n o t complete a f t e r 60 min.
Y i e l d s of
100% c h l o r i n e d i o x i d e could be o b t a i n e d by o p t i m i z i n g t h e h y d r o c h l o r i c a c i d and sodium c h l o r i t e concentrat i o n s , t h e r e a c t i o n t i m e , and t h e t e m p e r a t u r e . I t had been r e p o r t e d (220) t h a t sodium c h l o r i t e
r e a c t s with concentrated h y d r o c h l o r i c a c i d t o produce c h l o r i n e and c h l o r i n e d i o x i d e .
Glabisz e t a l .
(75,
76) proposed t h a t t h e decomposition of c h l o r i n e ( I I 1 ) i n c o n c e n t r a t e d h y d r o c h l o r i c a c i d proceeds a c c o r d i n g to:
I t w a s r e p o r t e d t h a t t h e decomposition by 5.5M H C 1 i s
n e a r l y complete and t h a t c h l o r i n e and n o t c h l o r i n e d i o x i d e i s produced.
By c o n t r a s t , Kepinski and
Blaszkiewicz (118) i n d i c a t e d t h a t w i t h 0.1M NaC102 and 5.OM H C 1 t h e decomposition of c h l o r i n e ( I I 1 ) i s complete w i t h i n 1 min, b u t n o c h l o r i n e i s d e t e c t e d . Beuermann ( 1 4 ) found t h a t only small amounts ( c a . 2 % ) of c h l o r i n e were produced when h y d r o c h l o r i c a c i d rea c t e d with sodium c h l o r i t e .
Although c h l o r i n e h a s
been r e p o r t e d among t h e r e a c t i o n p r o d u c t s ( 1 0 , 1 8 , 75, 76, 1181, t h e bulk of evidence a p p e a r s t o b e a g a i n s t t h e formation of any c h l o r i n e (91, 99, 120, 162, 2 1 7 ,
212
G . Gordon, R.
Rosenblatt
G. K i e f f e r , and D. H.
I f any c h l o r i n e w e r e formed it would r e a c t
220, 226).
immediately w i t h c h l o r i n e ( I I 1 ) , s i n c e t h e c h l o r i n e ( I I 1 ) c h l o r i n e r e a c t i o n i s known t o b e r a p i d i n an a c i d i c solution (48). Nakamori e t a l . (166) r e p o r t e d t h a t t h e s t o i c h i o m e t r y
i s independent of t h e a c i d used t o a c i d i f y t h e c h l o r i t e solution.
N e i t h e r t h e a d d i t i o n of c h l o r a t e i o n (120,
166) nor c h l o r i n e d i o x i d e (120) a f f e c t s t h e stoichiometry. Bohmlander (18) p o i n t e d o u t t h a t t h e d i s s o c i a t i o n r e a c t i o n must b e a, combination of E q s . 1 3 and 16.
He
a l s o r e p o r t e d t h a t i n i t i a l l y t h e r e a c t i o n i s second order with r e s p e c t t o chlorous a c i d , with a s l o w t r a n s i t i o n occurring t o a f i r s t - o r d e r reaction.
The
i n i t i a l r e a c t i o n a t pH 0.5 t o 2.0 i s proposed t o be 2HC102-Cl03'
+
H+
+
(19)
HOCl
and 2HC102
+
C1'-CC102
+
HOCl
+
OH-
+
C1.
(20)
followed by 2HC.102
+
Cl*-.LC102
+
2HOC1
(21)
A f t e r 10 min, Bohmlander s u g g e s t s , t h e r e a c t i o n s of E q s . 19 and 20 are no l o n g e r rate determining b u t t h e f o l l o w i n g r e a c t i o n becomes important:
I t i s improbable, however, t h a t a measurable amount o f
The Chemistry of C h l o r i n e Dioxide
213
c h l o r i n e could b u i l d up, s i n c e t h e r e a c t i o n of c h l o r i n e with c h l o r o u s a c i d i s so r a p i d ( 4 8 ) . The r a t e o f d i s p r o p o r t i o n a t i o n of a c i d i f i e d c h l o r i n e ( I I 1 ) s o l u t i o n s v a r i e s with t h e p H .
The r e a c t i o n
i s v e r y slow a t a pH g r e a t e r t h a n 4 ; l e s s t h a n 10’5M
c h l o r i n e d i o x i d e i s formed i n 2 h r i f t h e i n i t i a l sod-
i u m c h l o r i t e c o n c e n t r a t i o n (28) is 3 x 10-3M.
Only i f
t h e pH i s less than 3 w i l l more t h a n 1% o f t h e sodium c h l o r i t e decompose w i t h i n 1 0 min.
K i e f f e r and Gordon
(120) found t h a t t h e r a t e of decomposition o f c h l o r o u s a c i d d i d n o t vary a p p r e c i a b l y w i t h hydrogen i o n conc e n t r a t i o n i n t h e 2.0 t o 0.2M r a n g e i n t h e absence of i n i t i a l chloride ion.
The h a l f - l i f e f o r t h e decompos-
i t i o n was approximately 3 h r f o r 1 . 2 M hydrogen i o n , 7.2 x 10-3M sodium c h l o r i t e , 2 . O M i o n i c s t r e n g t h , a t 25OC.
The r a t e was about t h r e e t i m e s f a s t e r a t 3.33 x
10-3M hydrogen i o n c o n c e n t r a t i o n .
H e f t i (92) s t u d i e d
t h e decomposition of a sodium c h l o r i t e s o l u t i o n i n an a c e t i c a c i d sodium a c e t a t e b u f f e r .
A t 95OC w i t h 0.2M
b u f f e r , t h e decomposition r e a c t i o n was slow a t pH 5 t o 7 , b u t below p H 5 t h e r e a c t i o n w a s f a s t . The decomposition r e a c t i o n of c h l o r o u s a c i d i s approximately second o r d e r w i t h r e s p e c t t o c h l o r o u s a c i d ( 1 0 , 1 8 , 9 9 , 120, 1 2 1 , 130, 132, 162, 167, 183, 2 1 1 , 217, 2 2 6 ) .
A t low a c i d i t i e s , where t h e c h l o r i t e
ion concentration is g r e a t e r than t h e chlorous a c i d c o n c e n t r a t i o n , t h e r a t e l a w (10, 120, 1 2 1 , 162, 217)
214 G. Gordon, R. G. Kieffer, and D. H. Rosenblatt is
where k = 0.023K’ sec” at 25OC, 0.033K’ sec-’ at
3OoC, 0.043M-1 sec” at 35OC and AH$ = 11 kcal mole”
[Activation enthalpies and entropies are desig-
(10).
nated by a double dagger (f).] Launer and Tomimatsu
(130) studied the rate of decomposition of chlorine(II1)
at pH 2.4 to 3.4, maintained with an orthophosphoric acid-dihydrogen phosphate buffer, at 5OoC, with 8 x sodium chlorite present initially.
lO-%f
posed the following rate law (10):
They pro-
where h = [H+l/( [H+l + KHclo2) KHclo2 = dissociation
.
= 0.11 f 0.007M-’ sec’’ D Experimentally, the decomposition was found to be pro-
constant of HC102 and k
portional to h1.5.
stitt and co-workers (211) ob-
tained the same results with an acetate or a citrate
buffer.
They reported an activation energy of 20.0
kcal mole-’.
In an acetate buffer, pH 3.4 to 4.0, at
4OoC, Launer, Wilson, and Flynn (132) report a value
of k
D
(Eq. 24) of 0.67
k 0.05M-’
sec”.
Nakamori et
al. (167) found a second-order rate constant of 5 x 10-3K1 sec” at 25OC.
Kieffer and Gordon (120) ob-
tained the following second-order rate constant for
The Chemistry o f C h l o r i n e Dioxide t h e disappearance of H C 1 0 2 :
215
(1.17 f 0.06) x 10-2M-1
sec-I a t 1 . 2 t o 2.OM hydrogen i o n , 2.0M i o n i c s t r e n g t h , and 25OC.
The second-order f i t s became p o o r e r as t h e
r e a c t i o n proceeded, i n t h a t c h l o r i d e i o n w a s produced during t h e reaction. Hong
( 9 9 ) determined t h a t , a t p H 0 t o 3 and i n t h e
absence of c h l o r i d e i o n , t h e i n i t i a l r a t e l a w i s
where k = 5.5 x 10-3FT1 sec-l.
s e c - l , k' = 2.3 x 10-2W1
When t h e s e v a l u e s f o r t h e appearance of C 1 0 2
a r e m u l t i p l i e d by a f a c t o r of 2--since
-d(HC102)/4 =
d(C102)/2--and compared t o t h o s e of K i e f f e r and Gordon (120) f o r H C 1 0 2 d i s a p p e a r a n c e , we s e e t h a t t h e agreement i s q u i t e good. A s mentioned e a r l i e r , c h l o r i d e i o n a c c e l e r a t e s t h e
decomposition of chlorous a c i d and a l s o a l t e r s t h e s t o i c h i o m e t r y (10, 25, 9 9 , 120, 1 2 1 , 2 1 7 ) .
Most o f
t h e evidence i n d i c a t e s t h a t , i n t h e p r e s e n c e of apprec i a b l e amounts of c h l o r i d e i o n , o n l y v e r y small amounts of c h l o r a t e i o n a r e formed and t h e s t o i c h i o m e t r y approximates E q . 16.
The d a t a i n Table I1 i n d i c a t e t h e
marked e f f e c t t h a t added c h l o r i d e i o n h a s on t h e r a t e (120).
The r a t e law f o r low pH
f i t s t h e s e k i n e t i c d a t a b o t h i n t h e absence and i n t h e
216
G . Gordon, R.
G. K i e f f e r , and D. H.
Rosenblatt
presence of i n i t i a l c h l o r i d e i o n under t h e f o l l o w i n g conditions:
1 . 2 t o 2.OM hydrogen i o n c o n c e n t r a t i o n s ,
2.OM i o n i c s t r e n g t h , (1.0-7.2)
x 1 0 - 3 M sodium c h l o r i t e ,
0.0 t o 0.1M sodium c h l o r i d e , and 25OC.
For 1 . 2 M hydro-
gen i o n c o n c e n t r a t i o n , t h e b e s t v a l u e s f o r kl, k2, and K were 1 . 1 7 x 10'2M-1 s e c - l ,
1.57 x 10'2M-1
and 1.20 x 10-3M, r e s p e c t i v e l y .
For 2.OM hydrogen i o n
sec-l,
c o n c e n t r a t i o n , t h e v a l u e s were 1.17 x 1 0 - 2 ~ 1 sec-l, 3.00 x 10-2M-1sec-1,
and 1.20 x 10-3M, r e s p e c t i v e l y .
Apparently, kl and K are independent of t h e hydrogen i o n c o n c e n t r a t i o n i n t h e 1 . 2 t o 2.OM range and k 2 i s d i r e c t l y p r o p o r t i o n a l t o t h e hydrogen i o n concentration. Hong (99) found t h a t c h l o r i d e i o n could have both an i n h i b i t i n g e f f e c t and an a c c e l e r a t i n g e f f e c t on t h e formation of c h l o r i n e d i o x i d e from c h l o r o u s a c i d .
At
l o w a c i d i t i e s t h e i n i t i a l rate ro passes through a minimum as a f u n c t i o n of c h l o r i d e i o n c o n c e n t r a t i o n . The i n h i b i t i n g e f f e c t o n l y predominates a t low a c i d -
i t i e s and l o w c h l o r i d e i o n c o n c e n t r a t i o n s .
A t 0.97M
n i t r i c acid, the r a t e increases with increasing chloride ion concentrations.
The i n h i b i t i n g e f f e c t of
c h l o r i d e i o n i s observed with 0.05M s u l f u r i c a c i d and t h e minimum o c c u r s a t (1-2) x 10-2M sodium c h l o r i d e concentrations. The r a t e of formation of c h l o r i n e d i o x i d e goes through a maximum around pH 2 ( 2 8 , 99, 120, 1 2 1 1 ,
The Chemistry of C h l o r i n e Dioxide
217
TABLE I1 E f f e c t of C h l o r i d e I o n on t h e Rate o f t h e Reactiona i n 1.2M H C l O 4
tl/2,
min
6.77
10-5
397
6.77
10-4
296
6.77
10-3
0.01
0.01 0.04 b 0.1
t3/4
, min
d
389
0
0.04
C
b b
a
90.5 f 1 . 5
197 i- 9
64.1 f 1 . 6
137.4 f 7.7
1 7 . 2 f 0.4
36.9 f 0 . 8
68.5 5 0 . 7 18.2 ? 1 . 9 6.85 k 0.05
38.4 ? 0.9 14.35 f 0.05
Conditions: (3.60 f 0.02) x 1 0 - 3 M N a C l O 2 a n d 2.OM k o n i c s t r e n g t h w i t h N a C 1 0 4 a t 25 f 0.5OC. Conditions: (1.99 f 0.01) x 10-3M N a C l O 2 a n d 2.OM i o n i c s t r e n g t h w i t h N a C 1 0 4 a t 25 2 0.5OC. C t 1 / 2 i n d i c a t e s t h e t i m e it t a k e s f o r o n e - h a l f of t h e g r i g i n a l c h l o r i n e ( I I 1 ) t o decompose. t 3 / 4 i n d i c a t e s t h e t i m e it t a k e s f o r t h r e e - f o u r t h s of t h e o r i g i n a l c h l o r i n e ( I I 1 ) t o decompose. p r o b a b l y b e c a u s e t h e r e l a t i v e amounts o f c h l o r o u s a c i d and c h l o r i t e i o n change markedly i n t h i s r e g i o n a n d b e c a u s e t h e r a t e s of r e a c t i o n f o r c h l o r o u s a c i d w i t h c h l o r o u s a c i d a n d c h l o r o u s a c i d w i t h c h l o r i t e i o n are different.
The o b s e r v a t i o n o f s u c h a maximum i s con-
s i s t e n t w i t h E q . 25, w i t h t h e maximum o c c u r r i n g a t a p H g i v e n by
218 G. Gordon, R. G. Kieffer, and D. H. Rosenblatt
where b = [C102'l ] ' H [
1.
[HC1O2I = 3.18 x lO'3EI,
Kc =
[C102-l/[HC1021 , and k and k' are from Eq. 25.
This corresponds to a maximum at pH 1.68, which is in
good agreement with the experimental value of 1.70 obtained by Buser and Hanisch (28).
Consistent with the rate law and the mechanism to
be given, when sodium chlorite is in great excess, the reaction between chlorous acid and chlorite ion will
predominate and the order with respect to hydrogen ion
will be approximately 1. On the other hand, if the
acid used for acidification of the sodium chlorite is in excess, the bimolecular reaction of chlorous acid
will predominate and the reaction order with respect to total sodium chlorite will be approximately 2. low acidity the reaction will be first order in
chlorous acid, chlorite ion, and hydrogen ion.
At
Since
Barnett (10) determined the rate law at low acidities
where [C102'1
k[HC102] [C102'1,
P
[HC1021, it should be rewritten r = rather than r = k[HC10212.
rate constant of 0.023M'1
Barnett's
sec-l appears to agree well
with Hong ' s value f o r k' of 0 . 0 2 2 $ P 1
sec-l.
Hong reports the followlng empirical rate expres-
sion for the decomposition of chlorous acid, with the
effect of chloride taken into account:
The C h e m i s t r y o f C h l o r i n e D i o x i d e
where m = 2 I k l IHC102 l 2
+
2 19
k 2 I C 1 0 2 - I [ H C 1 0 2 11
n = 4 k 5 [H+l [ H C 1 0 2 1 k3 ( k 7
P =
+
k& I H C 1 0 2 ] kgk7 [ H F
x = [Cl’l This rate l a w i s consistent w i t h the following
mechanism:
HOCl
+
k3 H C 1 0 2 V H 2 0
H+
+
C1’
+
k5 HC102-2HOC1
H+
+
C1-
+
k6 HOCl-Cl2
+
(21202
(33)
+
H20
(341
220 G. Gordon, R. G. Kieffer, and D. H. Rosenblatt The first four reactions suffice for the reaction
in the absence of chloride: Eqs. 29 and 30 are the
rate-determining reactions, and Eqs. 33 to 36 are re-
quired to explain chloride effects.
These reactions
are essentially those proposed by Kieffer and Gordon (1201, except that for Eq. 32 these investigators
would substitute
2c1202-c12
+
(37)
2Cl02
Kieffer and Gordon (120) also used the following
reactions to explain the chloride ion effect on the
rate law of Eq. 26:
HC1202-
+
k2
Cl’-products
(rate determining) (39)
The value of Kc1 corresponds to 1/K or 833M-1 at 25OC. Hong (99) proposed that the presence of chloride
ion initiates two reactions which occur in parallel
with the reaction o f Eq. 13. Eq. 16 and
They are represented by
Equation 40 occurs to only a minor extent.
The reac-
tions of Eqs. 13 and 16 both appear to be independent
of the primary kinetic salt effect, but the contribu-
tion of Eq. 16 increases with an increase in the chloride ion concentration.
The Chemistry of Chlorine Dioxide
221
Both Kieffer and Gordon (120) and Hong (99) ob-
served that the reaction order with respect to chloride ion at high chloride ion concentration is one.
The order is also one with respect to chlorous acid
and one with respect to hydrogen ion.
Equation 34 is
necessary in order to explain the inhibiting effect of chloride ion.
Summations of (Eq. 31 x 2 + Eq. 32 x 2
Eq. 33) and (Eq. 29
+
+
Eq. 34 + Eq. 35) give, respec-
tively, Eqs. 16 and 40.
In light of the foregoing findings, the mechanism
for chlorous acid decomposition proposed by Robson (1831
+
c1203-C10
H2C102 2c10-02
+
Clo2
ClO-HC103
+
c12
should probably be abandoned.
(421
+
HC1
(441 (45)
He explained the stabil-
izing effect of hydrogen peroxide on an acidic chlor-
ate solution (152) by postulating the formation of a
complex between hydrogen peroxide and the intermediate
Cl2O3.
He suggested that the rate at which the com-
plex releases chlorine dioxide is slower than the release rate characteristic of the intermediate C1203.
The effect of hydrogen peroxide, however, can easily
222 G. Gordon, R. G. Kieffer, and D. H. Rosenblatt be explained by Eq. 12, even though the mechanism is not readily apparent.
In agreement with the observations of Taube and
Dodgen (217) and White, Taylor, and Vincent (226),
Kieffer and Gordon (120) showed that low chlorate ion
concentrations had only a very small effect on the rate
of disproportionation of chlorous acid, and that such
concentrations caused an increase, rather than inhibition, in the rate, Hong found that 0.324M sodium
chlorate only increased the rate of initial decomposi-
tion by 6.7% with 0.0207M sodium chlorite and 0.0375M sulfuric acid.
He found the same to be true if
0.00402M chloride ion were present.
acidities and high IC103'1~1C102'1
Only at high
ratios would the
rate be appreciable as compared with the disproportionation of chlorous acid.
Robson (1831 reported that the following compounds
are known to repress the formation of chlorine dioxide from chlorous acidi
pyrophosphates (4); amines,
especially ethylenediamine (1841, and hydrogen peroxide, The repressors probably complex, or perhaps
they react with one of the intermediates formed during the decomposition.
The following compounds, on the
other hand, are known to promote the production of chlorine dioxide from chlorous acid:
aldehydes,
especially formaldehyde, and organic anhydrides such
as acetic anhydride ( 3 , 154). According to Masschelein
The Chemistry of Chlorine Dioxide
223
( 1 5 4 ) , the principal reaction that occurs with the
acetic anhydride is
2NaC102 + (CH3C0)20
+
+
+ H20-Cl02
CH3COOH + H+ + CH3COONa
+
NaCl (461
O2
This is an unbalanced equation; only the yields of
chloride and chlorine dioxide were determined, and not
all reaction products were definitely identified.
Subsequent examination (40) of this reaction has indi-
cated that the proper stoichiometric reaction is most likely
4NaC102
f
(CH3COI20-2ClO2
+
+
NaClO3
+
NaCl
2CH3C02Na
(471
In an orthophosphoric acid-dihydrogen phosphate
buffer (1301, the rate of the acidic decomposition of chlorous acid appears to be independent of the
concentration of added ferric ion, whereas in an ace-
tate buffer (1671, ferric ion was found to catalyze the decomposition of chlorous acid.
Nakamori et al.
(167) reported that ferric ion, cupric ion, cobalt(II1
ion, and nickel(I1) ion change the stoichiometry and
catalyze the rate of decomposition of chlorous acid at pH 3.5 in an acetate buffer.
Their conclusions,
however, are subject to question, since they used the chloride salts of the metals as catalysts.
In view of
the known effects of chloride ion on the stoichiometry
224 G. Gordon, R. G. Kieffer, and D. H. Rosenblatt and rate of decomposition of chlorous acid, it is
difficult to ascribe the observed changes to the metal ion alone.
G.
Photochemical Decomposition of Chlorine (111) Solutions
Nabar and co-workers (161) studied the photodecom-
position of sodium chlorite solutions. They found
that the extent of photodecomposition decreased with
an increase in pH and an increase in the initial sodium chlorite concentration.
Chloride ion had no
effect on the photodecomposition reaction.
The photo-
decomposition reaction was found to be much more rapid than the decomposition of acidified sodium chlorite
solutions. For example, a chlorous acid solution that
decomposed 2.5% in the dark in 3 hr decomposed 100% in 1 hr in the presence of a carbon arc.
Equations
48 and 49 qualitatively represent the stoichiometry found at pH 4.0 and 8.43, respectively 10NaClO2 = 2NaC103
f
6NaC1
6NaC102 = 2IJaClO3
+
f
2WaClO4
4NaCl
f 302
f 302
(48) (49)
Some evidence for the formation of chlorine dioxide
was also observed.
It should be noted that the sodium
chlorite used was quite impure and contained 78% sod-
ium chlorite, 15% sodium chloride, 1.3% sodium chlor-
ate, and 5.7% water.
Launer and Tomimatsu (129) report that at pH 2.4,
The Chemistry of Chlorine Dioxide
225
maintained with an orthophosphoric acid-dihydrogen
phosphate buffer, at 5OoC and 8 x 1 0 - 4 ~sodium chlorite, even low levels of light illumination increase
the rate of decomposition of chlorous acid.
The rate
of acidified chlorite decomposition was increased
four times by decreasing the buffer concentration from 0.5 to 0.1M (129).
Thermal Decomposition of Sodium Chlorite
H.
According to Taylor and co-workers (2201, thermal
decomposition of solid sodium chlorite begins at 175OC
according to the following reactions:
+ NaCl
3NaC102 -2NaClO3
(principal
reaction) NaC102-cNaCl
+
O2
(50)
(contributes less
than 5%)
(511
Solymosi et al. (2071 report that in general, at
lower temperatures, solid chlorites disproportionate into chlorate and chloride ions. At higher tempera-
tures, up to 4OO0C, sodium chlorite still undergoes
disproportionation, whereas lead and barium chlorites
explode violently to give mainly chloride ion and
oxygen.
226 G. Gordon, R. G. Kieffer, and D. H. Rosenblatt IS:. REACTIONS OF CHLORINE ( 0 ) AND CHLORINE (I) WITH CHLORINE (111) Stoichiometry
A.
Chlorine reacts rapidly with acidified or neutral
solutions of chlorite ion (24, 25, 48, 165, 217).
In
alkaline solution, however, the reaction is very slow,
on the order of hours (2171.
The stoichiometry of the
reaction varies with pH and the initial concentrations
of the reactants, but the only products of the reac-
tion are chloride ion, chlorine dioxide, and chlorate ion (40, 48, 59, 82, 99, 217, 220, 226). As we can see from Eqs. 52 to 55,
C12
+
2HC102-2C102 A F O
C12
+
HC102
+
+
=
=
HOCl t Clop’
+
+
(52)
+
2C1’
i-
3H+ (531
C1-
f
OH’
= -27,000
OH’--ClO3’
A F O
2H’
-7050
2C102--C2C102 A F O
+
-6490
H2O-ClO3A F O
HOCl
C1’
$.
+
(541 c1-
+
H20
= -31,870
(55)
the equilibria €or all the possible reactions lie far to the right (217).
Equations 52 and 53 are for acidic
solutions; Eqs. 54 and 55 are for neutral solutions.
Taube and Dodgen (217) studied this reaction with
tagged chlorine species and noted that the chlorine
The Chemistry of Chlorine Dioxide
227
atoms from chlorine or hypochlorous acid at all stages in the reaction remained distinct from the chlorine
atoms in the chlorite ion.
Thus they proposed that
the intermediate must be unsymmetrical, that is, or
\0
C1-0-C1-0
rather than symmetrical 0
-
c1- c1- 0
Equations 56 through 59 c12*
-I-
HOC1*
f
c102--c[cl*-c~
