March5, 1958
EXCHANGE OF HYPOCHLORITE AND OF HYPOBROMITE IONSWITH WATER
1073
neous idea of the amount of decomposition, it does analogous to those advanced for methyl n-butyl not seem likely that pressure measurements will k e t ~ n e . ~ It is likely that the carbon dioxide reported by have a simple relationship to the amount of methyl propyl ketone decomposed in any part of the reac- Waring and Garik4ais produced by the reactions of ketene since carbon dioxide is a major product of the tion. Ethylene and propylene also react and decrease ketene decomposition a t these temperatures.I7 greatly in amount during the d e c o m p ~ s i t i o n . ~ ~ Combining the analyses here with those of WarSince propylene inhibits the d e c o m p ~ s i t i o n that , ~ ~ ing and Garik,4a.7one can see that, if the above steps effect will operate to increase the complexity of the 1-4 represent the decomposition, steps 2 and 3 are most important. ~ process. As with methyl n-butyl k e t ~ n e ,one must conclude that a complete kinetic analysis of Acknowledgment.-The author wishes to acthe free radical chain mechanism is not possible knowledge the many helpful comments of Dr. IN. D. with the data available. However, the great simi- Walters who suggested this problem. The gaselarity of the behaviors of these two ketones is ap- ous infrared analyses were performed by Mr. C. A. parent. I t is seen that the chain steps above are Whiteman.
[COSTRIBUTION FROX
THE
GEORGEHERBERTJONES LABORATORY, UNIVERSITY O F CtIICAGO]
The Exchange of Hypochlorite and of Hypobromite Ions with Water BY M. ANBARAND H. TAUBE RECEIVED FEBRUARY 23, 1956 The reaction of C10- and BrO- with SO2- proceeds with almost complete transfer of hypohalite oxygen to iiitritc. This reaction has been applied in the isotopic analysis of C10- and BrO- undergoing exchange with water. The exchange Paths of C10- and water in alkaline solution proceeds by the paths kA[(ClO-)/(OH-)] and kB[[(ClO-)(Cl-)]/(OH-)]. of similar kinetic form are indicated for the exchange of BrO- and water, and C1- as well a s Br- can catalyze the exchange, The reactions reverse to the k B paths are the attack of Clz, BrE or BrCl by OH-, and the specific rates for these reactions have been calculated. The mechanisms of the reactions and the mode of action of H + in reactions of oxy-ions are discussed.
In the course of tracer work on the reactions of oxy-anions with nitrite ion in water, it was observed that when C10- or BrO- are the oxidizing agents, almost complete transfer of oxygen to the reducing agent takes place. The reactions with nitrite provide a means of isotopic analysis of the hypohalites and therefore of following the rate a t which oxygen exchanges between them and water. While the method thus made available is far from ideal, the exchange reactions in question are of such interest in themselves, as well as for comparison with other substitution or oxidation-reduction reactions of the hypohalites, that it was considered worthwhile to obtain even data of the quality of those reported in this paper. This decision was not reached without making an extensive survey' of other reactions which might be exploited for oxygen isotope sampling, but none was discovered superior to that adopted here. Some of the disadvantages of the present sampling method could be avoided by sampling the solvent by distillation. This method, however, is not accurate for dilute solutions of the solute. Experimental Reagents.-The hypochlorite solution was prepared by passing Cl? through ice-cooled 5 M NaOH, then gradually adding freshly prepared AgzCO3 until AgzO, recognizable by its color, began to form. The solution at pH > 13 was stored in a refrigerator. T h e hypobromite was prepared in a similar manner, but because of the greater rate of decomposition, was used within 24 hr. Other chemicals were of A . R . quality, used without further purification. Procedure.-Solutions in 0I8-enriched water, each of total volume ca. 20 ml. were prepared, containing the hypohalite as well as appropriate amounts of HzSO, and NaH2P04and (1) The results of these tracer experiments will be described in a separate article.
kept in a thermostat. The adjustment of PH was made, using a Beckman PH meter, with a special electrode for alkaline solution. Measurements were reproducible t o ca. 0.03 p H unit. Aliquots were withdrawn at intervals for isotopic assay of the hypohalite. Each sample immeliiately after being withdrawn was mixed with enough sodium nitrite solution to leave the oxidizing agent in slight excess ar,d then was added with stirring to 100 ml. of water buffered t o a pH of 1. After reaction, which was found to be complete within 10 sec., HIOz was added t o remove the excess hypohalite. Solid Ag?S04and Ba(OH)* were now added, and the solution was adjusted t o PH 9 using NaOH. After filtration, which removes sulfate, phosphate, halide and silver from the system, the pH of the solution was adjusted t o betweer 5 and 6 with acetic acid, and nitron (diphenylenedianelohqdrotriazole) reagent was added. T h e solution was left for about 12 hr. for crystallization of nitron nitrate. This was filtered off, digested with ammonium hydroxide and was reprecipitated. After digesting the precipitate again with ammonium hydroxide, the remaining nitron base was extracted with ethyl acetate. The solution was then evaporated t o dryness, the final drying being done under vacuum. T h e NzO for isotopic analysis was obtained by decomposition of the NHdN03 comprising the residue, the gas firs: being purified by storage over solid XaOH. I n spite of pains t o improve the method of isotopic analysis, specific rates calculated from results for successive aliquots of a given reaction mixture are found to diffe- by as much as a factor of 2 . These fluctuations do not correspond t o real changes in rate, since they follow n o trend with time, and furthermore the data xithin a precision of a factor of 2 or so conform to a simple and reasonable rate law, over a very wide range in rate. I t has been shown by direct experiments t h a t the handling of the nitrate is not responsible for the fluctuations. The principal error seems to be incurred in the reaction of nitrite and hypohalite, for the conditions necessary to make this reaction rapid also increase the rate of exchange between hypohalite and water. Partial exchang- intervenes from time to time. This causes an apparent reduction in the rate of exchange, because the environment during the quenching reaction has nearly the isotopic composition of the initial hypohalite. T o conform t o this analysis, the theoretical line in Fig. 1 has been drawn through a high density of points corresponding t o the higher
M. ANBARAND H. TAUBE
1074
rates of exchange. Positive deviations are stillleft, attributable to random errors in isotopic analysis, the measurement of pH and possibly to stray catalytic effects. I n spite of the irreproducibility of the data, important conclusions can still be drawn from them, consisting as they do of a large number of determinations over a wide range of variation in some of the parameters to which the rate is sensitive. Definitions and Conditions.-Specific rates were calculated from the results for individual aliquots. Since one of the exchanging forms, HsO, is so much more abundant than the other, the specific rates calculated from the change in isotopic composition of the hypohalite are equivalent to the specific rates, first order in hypohalite, for total rate of the exchange reaction. Lower case k is used for specific rate, modified by subscript if reserved to represent a particular reaction; upper case K is used for equilibrium quotients. Total hypohalite, whether X or HXO, is represented by the symbol X(1). E represents the O'*-enrichment ratio of the NTOin question, ; . e . , the ratio Nz018/N2016compared to that obtained for the reaction of isotopically normal XO- corresponding to zero exchange, with the nitrate used for analysis. Unless otherwise indicated, the units used are minutes f u r time and moles per liter for concentration. All experiments were done a t 2 5 " .
-
Vol. 80
10.0
\
I
6 ,--. w
I
C
a
'. w
+
&
1.0
\
Results Table I contains a summary of the experiments on the transfer of oxygen from C10- to NOp-. Exchange of NOz- or C10- with the solvent before reaction wiII decrease the apparent transfer. Thus the values in the last column of Table I are minimum values. The actual transfer may well be close t o loo%, as indicated by experiment 1.04, the defect being caused by the exchange with water during the analysis. The bulk of the results obtained on the exchange of C10- and water are not reported in detail b u t are entered in the graph of Fig. 1. Some of these experiments are summarized in Table I1 to illustrate the dependence of specific rate on the time of reaction and the concentration of C10-.
1
0
\ 0
0.1
10
12
11
pH. Fig. 2.---The specific rate of escliaiigc o f C10 aiid 1I,O catalyzed by C1-, as function of pI-1.
ordinates were obtained by subtracting from those directly calculated the specific rate for the noncatalyzed path, as taken for the appropriate pH from the line in Fig. 1. In all but one experiment, the correction was less than 25y0 of the total. In TABLE I TRACER EXPERIMESTS ON THE REACTION OF NO2- ASD C10(In all experiments, C10- and NOz- mixed a t pH indicated added to acid buffer. In 1.03 to 1.06, the enriched oxygen was contained in the water of acid buffer, i n 1.07 and 1.08, in the C10-, in 1.01 and 1.02 in the water of the original solution.) 7
pII
YO*-
11 1.01 0 . o 5 0 . 2 1.02 .05 .02 1 1 103 ." .1 12 12 1.04 .0L5 1 1.05 ,05 1 12 s 1.06 ,I 2 1.07 , I .-I 13 1:3 1.08 1 .2
1.0
1.11
1.0 1.0
l.(!
:3.1 5.5
1 0
3.5
1.0 1.0 1 0
1 0 1 0 l.(t
5.5 5.0 ii.2
1.0 G O 1 . 0 711
I.],,
SCI.( S O * - , ( C l O - ,
0
8
9
10
11
1"
PH . Fig. 1.-The
specific rate of the exchange of C10 \\'iter, as function of PH.
-
md
'The exchange of C10- and water is strongly catalyzed by C1-. In Fig. 2 are shown the results of experiments devoted t o studying this effect. The specific rates (divided by (Cl-)j plotted as
II20 fafter mixinaj
CIO
L(J3
S(13-
Transfer from C1C)
1.43 1.61 1.20
;is
1.11 1.95 111 2.21 2.5:)
!).'3
11 7: 2:) 77 7.'i i!)
the majority of experiments, ((2) was 1 .OO X or 2.00 X IO-* M , but in the experiment reprcsented by the filled-in symbol, it was 0.100 111, and in the experiment a t highest pH i t was 2.00 X M . Only a limited variation in (Cl(1)) was undertaken, from 0.20 to O.G8 21,and the specific rates are not affected by this change. \?'ithin the precision of the data, there is no \-ariation ( i f specific rate with the extent of exchange. 'The test of this factor was of the same quality for the presciit scrics of experiments as for those reported in Tablc 11. The results on the exchange of BrO - and water are less coinpletc tilati for C l W . 111Fig. 3 , the re-
March 5, 1958
EXCHANGE O F
HYPOCHLORITE AND
TABLE II THERATEO F EXCHANGE O F c10- AND (Phosphate buffer at 0.1 M) E - Em NO.
#H
(cy))
t
2.01 2.02 2.03 2.04 2.03 2.06 2.07 2.08 2.09 2.10
9.55 9.75 9.75 9.75 9.85 9.85 9.85 9.90 9.90 9.90
0.20 1.02 1.02 1.02 0.68 .68 .25 .34 .34 .34
15 10 20 30 10 20 10 15 30 45
m7.6 3.5 4.6 8.5 2.1 3.4 2.3 2.8 4.2 15.7
HYPOBROMITE I O N S WITH
OF
WATER
0
WATER
k -
k
0.13 .12 .077 .071 ,076 ,061 .OS2 .067 ,047 ,061
( H +)
x
10-8
4.6
6.8 4.3 4.0 5.4 4.4 5.9 5.4 3.7 4.8
sults obtained for the direct exchange of BrO- and water are shown, as well as those for the reaction catalyzed by C1- and by Br-. To some extent, the data for the direct reaction and t h a t catalyzed by C1- constitute a test of the order with respect to (Br(1)). The variation of (X-), 1 X to 2 X 10-2 was too slight, and the data were too imprecise for the results to be taken as demonstrating the order of the reaction with respect to (X-) .
10-1
I
i
Y
i
2 10 -2
Discussion The data on the exchange of hypochlorite and water establish the kinetic law for the principal path under the conditions of our experiments to have the form k[(ClO-)/(OH-)] and for the C1catalyzed path to have the form k [(ClO-) (Cl-)/ (OH-)]. The rate laws for the exchange of hypobromite with water have not been tested in all their features as thoroughly as those for hypochlorite, and in adopting the forms shown in Table 111,to a large extent analogy with the hypochlorite system has been relied on. The highest rates have been chosen to fix the values of the coefficients. The quantitative conclusions from the work are summarized in Table 111. Assuming the analysis of errors to be correct, the coefficients in the empirical rate laws A, B, C and D are believed to be accurate within 30y0 and t h a t for rate law E, to within a factor of 2. The coefficients in the derived rate laws depend also on the equilibrium quotients for the hypohalite hydrolysis reactions. The values of these quotients as calculated from the dissociation constants of the acids and of water a t zero ionic strength have been used. For the dissociation constant of HOC1,2 3.4 X lo-* has been selected and for that of HOBrIS 2.4 X The assumption made in calculating the concentrations of OH- and the concentration hydrolysis quotient may have introduced additional error, perhaps by as much as a factor of 1.5, into the coefficient of the derived forms of the rate laws but leaving their ratios almost unaltered. Although the rate law terms of Table I11 represent the principal exchange paths for the ranges of concentration covered in our work, other paths must be considered and the data have been used to calculate upper limits for the specific rates by rival (2) As calculated by R . E. Connick, THISJOC'RNAL, 6 9 , 1509 (1947), from d a t a a t lower temperatures. (3) Measured as 2.0 X 10-8 a t 20' and low ionic strength (L. Farkas and hl.Lewin, ibid.,72, 5766 (1950)) and a s 2.1 X 10-S a t 2 2 O (E. A . Shilov, Zhur. Fin. Khim., 24, 702 (1950); C . A . , 4 4 , 10445 (1950)).
12
13
14
PH . Fig. 3.-Specific rates for the exchange of BrO- and H20 as a function of pH: 0, uncatalyzed, ordinate scale on left; 8 , catalyzed by C1-, ordinate scale on right; 0 , catalyzed by Br-, ordinate scale on right.
paths. The conclusions concerning some other important and simple paths are summarized in Table IV. The upper limit for the coefficient of rate law li was calculated assuming t h a t a t PH 8.7, a t which acidity the rate of exchange is fairly well documented, exchange by path F does not exceed t h a t given by the theoretical line in Fig. 1; for rate law G, by making a similar assumption a t the highest flH of measurement, 12.4; for H and K assuming that the rates by these paths in the experiments a t (ClI) = 1 M and + H 9.75 do not exceed the rates actually measured. The only reasonable mechanism for excha:nge by paths B, D and E is attack by X- on the halogen part of HOY. Substitution by X- on oxygen does not lead to the exchange measured (although it does lead to halide exchange) and exchange by an electron transfer process in which HO-, X and U are formed is not energetically possible. A straightforward interpretation of the present data would X - = HO- Y X . This appear to be: HOY interpretation4 is, however, ruled out by the observation that when X = Y = C1, exchange of chlorine between C1- and C10- can actually be slower than that observed for HzO-C1O - as catalyzed by Cl-.j Thus, whatever the nature of the
+
+
(4) This obvious interpretation was given t o t h e results by H. Taube, Rec. C h o n . Propress, 17, 25 (1956), b u t in t h e light (of t h e rein exchange, must be rejected. sults of t h e experiments on CI'CIO(5) This observation was first made b y R. A. Silverman in these laboratories. Complete results on t h e CI--CIO- exchange have been obtained by (Sa) hZ. Anbar, S. Gutmann and R . Rein and will soon be published.
1070 TABLE
111
SPECIPIc R A T E S FOR 'THE P R I N C I P A L E X C I i A N C l s I'AT€I
Rate coefficients k
Euipiriral Ian.
(C1O --I k.k---(OW)
A
k e -(ClO-)(CI--) . (OH -) k,:.(BrO-) ( O H --) Cl-.) k u (BrO-)( --~EIY - ~
B C
D
'L
ktr
;;.o x
kc'( IIOBr)
10-3
Term
F
k V ( H +)(HOCl) kc( O H -1 (€IOCl) k H (C10 -) (I-I 0 C 1) k j (HOC1) (fioc1)
c H
J
'( €IOc1) ( c1- )
x
10'
1.3 x
l(l3
1.0
kl)'(IIOI3r)(Cl- )
5 x' 10'
2
kt.;'(HOBr)(Er- )
2
1 )
x
lip
I I J.
TABLE IV SPECIFIC RATELiar1 rs Path
11'
