1866
MALCOLM DAKIELS
Radiation Chemistry of Arsenite Solutions. 111.
Effect of Arsenite
Concentration in Oxygen-Saturated Solution’
by Malcolm Daniels Puerto Rico Nuclear Center, S a n J u a n , Puerto Rico
(Received February 6 , 1064)
Rates of formation of arsenate (at pH 1.3 and 10.2) and hydrogen peroxide (at pH 1.3), following Co60 radiolysis of oxygen-saturated solutions of arsenite, have been determined a t concentrations of arsenite from 1 X lop5to 0.2 M . In acid solution radical scavenging appears to be complete a t 1 mM arsenite and in alkaline solution a t 0.5 mM. In both cases G values show a steady increase a t higher concentrations, the relationships being G(As(V)) = 2.6 26.0(A4s(III))~ at pH 1.3 and G(As(V)) = 7 . 5 51.0(As(III))~”z at pH 10.2. It is suggested that these effects are due to a chain reaction and a unified mechanism is presented fitting both sets of data.
+
Introduction In order that the experimental results of radiation chemistry may be adequately interpreted in terms of “radical yields” g(e-), gH, gOH, and molecular products g(H2),g(Hz02),and appropriate mechanisms, it is necessary to know the conditions which correspond to effective scavenging of the radicals. Previous work2 on the effects of pH and oxygen had been carried out on 1nilw arsenite solutions arid hence, it was necessary to extend this work to a wide range of concentrations a t both acid and alkaline pH. The effect of concentration is also of intrinsic interest as it may lead to further information on the niechanisnis of primary water decomposition and in certain circumstances to elucidation of the chemical consequences of energy deposition in the s01ute.~ Experimental Materials, preparation of solutions, and analysis of products have been described. The highest concentrations attainable at both pH 1.3 and pH 10.2 were in the region of 2 X 10-l M . Arsenite solutioiis were standardized by titration with KBr03.4 Acid pH was maintained satisfactorily with sulfuric acid, but difficulties were experieiieed in alkaline solution at higher concentrations, due to self-buffering of the arsenite. After control experiments had shown the rate of oxidation to be unchanged by a carbonate-bicarbonate buffer, this was subsequently used in most experinients. T h e Journal of Physical Chemistru
+
Radiation Procedure. To eliminate the necessity for energy absorption corrections a t higher concentrations of arsenite, Coco y-rays were used replacing the 200-kv. X-rays of previous work. Dose rate = 5.92 X 1019e.v./l./min., deterniined with the Fricke dosimeter. To decrease the possibility of post-irradiation reaction between the hydrogen peroxide and arsenite a t high arsenite concentrations, the determination of peroxide was carried out immediately after withdrawing the sample from the source. This gave linear yield-dose curves showing no intercept.
Results All yields (As(V) and H20zat pH 1.3, As(V) at pH 10.2) were linear with dose (at the higher concentrations to values corresponding to up to 70% consumption of oxygen). G-Values as a function of concentration are presented in Fig. 1. At pH 1.3, noteworthy features are (i) the constancy of G(As(V)) a t 2.6 over a wide concentration range, M to 10-2 M ; (ii) a t concentrations 134, G(As(V)) shows a continuous increase with concentration which is only limited by the solubility of arsenite. Figure 2 shows that the form (1) Part 11: M . Daniels, J . Phys. Chem., 66, 1475 (1962). (2) IM. Daniels and J. Weiss, J . Chem. Soc., 2467 (1958). (3) D. Smithies and E. J. Hart, J . Am. Chem. Soc., 8 2 , 4775 (1960). (4) R. Belcher and J. Nutten, “Laboratory Manual of Quantitative Inorganic Analysis,” Longmans Green and Co., London, 1955, p. 202.
RADIATION CHEMISTRY
OF
1867
ARSESITESOLUTIONS
,
G(As(V))
=
7.5
+ 51.0(A~(111))~~'~
Discussion The constancy of the yield of peroxide and arsenate over' a wide concentration range in acid solution is clear indication that radical scavenging is complete. Evidence has previously been presented5 showing OH I I
I
-4
-5
-3 -2 Log [As(III)lo M .
I
-I
30
Figure 1. Variation of G(As(V)) and G(H202) with [As(III)lo: 9,As(V) a t pH 1.3; 0, As(V) a t pH 10.2; 0,H20za t pH 1.3.
h
g"z o
t .
rn
4
G 8.0
6.0 0 0
E;
I
I
10
20
[As(III)],I/*
4.0
I
I
30 X 102,M"1.
40
50
Figure 3. G(As(V)) as linear function of [ A S ( I I I ) ] ~a /t ~pH 10.2. 2.0
0.0
0
--
100 [As(III)]omiM.
200
Figure 2. G(As(V)) as linear function of [As(III)]oat pH 1.3: 0, As(V); 0,HzOz.
radicals to react with arsenite to form an intermediate oxidation state As(1V) ; under the conditions prevailing here this would react alniost 100% with oxygen to foriii the peroxy radical As(IV).02.
OH 4- As(1II) As(1V)
of this dependence is linear, and the results can be represented by the equation
G(As(V))
=
2.6
+ 26.O(As(III))
(iii) the rate of peroxide formation ib largely independent of concentration from M As(II1) (3.80) to lo-' 114 (3.50) after which it exhibits a precipitous drop to -0.5 at 2 X lo-' M . At pH 10.2 the results are qualitatively similar but the region in which pronounced concentration dependence is found extends to such a low concentration that there is effectively no independent zone. In agreement with previous observations, no hydrogen peroxide is found, due no doubt to its rapid reaction with As(II1) a t this pH. Again a t higher concentrations, G(As(V)) shows a rapid increase up to the solubility limit, but in this ca:3e the functional dependence is on the square root of concentration (Fig. 3) from M to 2 X IO-' M.
+
+ HzO
0 2
+ As(IV)
+As(IV).Oz
(1)
(2)
Subsequent reactions of As(IV).02 are a t present ambiguous, but the following reaction will (a) quantitatively account for the results a t lower arsenite concentrations when scavenging is completed (-1 niM) and (b) fit in to the scheme accounting for the results at higher concentrations.
+ HzOz+ O2 (3a)
2As(IV).O2 + 2As(V)
Under present conditions, H atoms will form HOz
H
+ Oz
--+
HOz
(4)
Contrary to previous suppositions, there seems to be no experimental evidence requiring a reaction between the hydroperoxy radical and arsenite, and it appears that these radicals may simply form peroxide 2H02 + HzOz ( 5 ) M. Daniels,
+
0 2
(5)
J. Phgs. Chem., 66, 1473 (1962).
Volume 68, Number 7 July, l.%4
MALCOLM DANIELS
1868
Reactions 1-5 indicate that a t complete radical scavenging G(As(V))
=
tration is increased, reaction 6, an oxygen atom transfer, becomes important As(IV).02
g(OW
and
+ As(II1) +As(1V)'O + As(V)
OH G(H202)
=
0.5(gH
+ gOH) + g(H2Oz)
(where small g's symbolize primary radical and molecular yields). The present data are adequately accounted for by the following values: gH = 3.30, gOH = 2.6, gH2 = 0.45, and g(H202)= 0.8. In the same concentration region, vix., 0.5-1.0 mM, it seems that the G(As(Y)) in alkaline solution can be approximated by the sum of arsenate formed by radical reactions and that formed by thermal reaction with hydrogen peroxide (this reaction is rapid, stoichiometric and nonchaiii a t alkaline pH, although slow in acid)6 As(II1)
HO
+ RH --+ RO + ROH R O + RH ROH + R
+
0 2
--+
RO2
in the course of Fvhich no hydrogen peroxide is formed. Accordingly it is suggested that as the arsenite concenThe Journal of Physical Chemistry
OH\
HO
+
As
--f
I
HO HO
OH
\ / As
/ \
0
HO
+
OH
(7)
I
0
Termination of this chain sequence is of course by reaction 3, or by reaction 8.
As(EV)*O
+ As(IV),02 ---+2As(V) +
0 2
(8)
At the higher concentrations involved hsre it may be anticipated that all As(IV).O reacts with As(IlI), and hence termination steps 3a may be preferred. Application of the steady-state approximation to the sequence I, 2, 6,7, 3a, leads to the expression
which has the form required by Fig. 2 . However, it will be seen below that this scheme will be slightly modified in order to accommodate the effect of pH. The same basic mechanism can be applied to the results obtained at pH 10.2, the only change required being that the termination reaction should formally include the arsenite species, and be written as
-----f
R
I
OH OH HO
+ As(1II) +As(V) + As(IV)
As(IV).O
--+
R02
----f
and is followed by reaction 7 (hydrogen abstraction)
ROz
has previously2 been suggested to account for high G values in oxygenated arsenite. There was a t the time no further evidence either for or against this suggestion. However the results reported here definitely do not support this, for it implies that peroxide and arsenate should be formed in equal amounts, whereas it is clear (Fig. 1) that the increase in arsenate is not accompanied by any change in peroxide. (When the peroxide yield does change abruptly between 1 X 10-' N and 2 X 10-1 '$1 As(III), it causes no change in the arsenate slope (Fig. 2 ) , and it is probably due to an unrelated mechanism, vix., effective competition of arsenite for H atoms a t high ratios of (As(I11))/(02). The present results do, in fact, seem to be differentiating evidence in favor of the alternative process of chain oxidation
/OH As
+
The G values found a t lower concentrations cannot be profitably discussed in the absence of further information on g(H202)and g(H2). However, plausible suggestions can be made concerning the linear increase in G(As(V)) a t arsenite concentrations M. A chain reaction of the type
0 2
\
(CO2
+ H2Oz --+ As(V) + H2O
+ RH --+ RO2H + R R+ ROz R02K "eo_ ROH + Hz02
HO
2As(IV).02
(6)
+ As(II1) 2As(V) + H2Oz + As(II1) + Oz
M. Daniels, unpublished observations.
(3b)
1869
RADIATION CHEMISTRY OF ARSEKITE SOLUTIONS
This leads to the expression
The difficulties involved in postulating a terinolecular reaction such as 3b have been encountered previously in studies on the radiolysis of hydrogen peroxide solutions’ and in the radiation chemistry of permanganate18 in both of which half-power dependence on concentratiln was demonstrated. The same type of solution seeins applicable here. It simply involves postulating the propagation readion 6 to proceed, not directly, but via a conzplex of lifetime comparable to that of radicals. Thus, reaction 6 is written as As(IV).02
+ As(II1)
9
A 4 ~ ( I V ) . 0 y A ~ ( I I(9) I) -9
This complex can then either break up to propagate the chain As(IV).O~.AS(III)--+ As(IV).O
+ As(Y)
(10)
or react with another As(1V). 02 to terminate it.
+
mechanisms (see also ref. 5 ) which would lead to a first-power-half-power transition. The most reasonable of these in chemical terms, involving increasing dimerization of arsenite in alkaline solution, is not support,ed by the availablc Thus, on the basis of the mechanism proposed here the only significant difference between the reactions in acid and alkaline is that the rate constants governing the reactions of the complex vary with pH in such a way as to make reaction 3a terniination in acid solution, whereas in alkaline solution reaction 10 becomes predominant. It is hoped to investigate further these reactions by determining the functional dependence on radiation intensit,y when suitable facilities are ready. Aclinowledgrnents. This work was carried out at King’s College, Sewcastle-on-Tyne, and Argonne Xational Laboratory, Argonne, Illinois. Thanks are due to Professor J. Weiss and Dr. E. J. Hart for encouragement and interest, and to the Graphic Arts Department, Argonne National Laboratory, for assistance in preparation of the manuscript.
As(1V) .O~.AS(III) As(IV)’On + 2,4s(V)
+
H202
+
0 2
4-As(II1) (11)
All reactions are now bimolecular. Consideration has, of course, been given to alternative
(7) E. J. Hart and M. Matheson, Discussions Faraday Soc., 12, 169 (1952). (8) M . Daniels, J. Phys. Chem., 64, 1839 (1960). (9) A. B. Garret, 0.Holmes, and A. Laube, J. Am. Chem. Soc., 62,
2024 (1940).
Volume 68, Number 7
J u l y , 1964
