PHOTOCHEMICALLY-INDUCED OXIDATIONOF ARSENITE
PHOTOCHEMICALLY-INDUCED OXIDATION OF ARSENITE : EVIDENCE FOR THE EXISTENCE OF ARSENIC(1V) BYMALCOLM DANIELS Radioisotope Applications Division, Puerto Rico Nuclear Center, Rfo Piedras, P. R., and Chemistry Department, University of Puerto Rico, Rio Piedras, P. €2. Received March 7, lg68
Hydrogen peroxide has been photolyzed by 2537 A. radiation in the presence of arsenite ion under conditions of complete radical scavenging. I n the absence of oxygen, hydrogen peroxide is consumed and arsenate formed in equivalent amounts which show an exponential dependence on the duration of photolysis, +i(-HzOz) = 0.55, &i(As(V))= 0.47. When oxygen is added to the system, consumption of hydrogen peroxide ceases, formation of arsenate becomes linear with time and the quantum yield effectively doubles, +i(As(V))02/+(+.(V)02-free = 1.9. It is shown that these results are evidence for the existence of an intermediate oxldation state of arsenic, As(IV), which is capable of reacting with oxygen to form a peroxy radical As( IV).02. Implications concerning the photolysis of HzOz are discussed.
Previous investigation of the radiation chemistry of arsenite solution1 gave results which were interpreted in terms of a transitory intermediate oxidation state, As(IV), capable of reaction with oxygen. It obviously was desirable to get further evidence from an independent method to support this postulate. Accordingly oxidation of arsenite has been initiated by O E radicals generated by the photolysis of hydrogen peroxide solutions under suitably chosen conditions, and the effect of oxygen on the course of the reaction has been investigated.
Conditions in which unambiguous results could be obtained were limited by properties of the system. Thus, to eliminate chain decomposition of hydrogen peroxide and to facilitate efficient scavenging of OH radicals by arsenite, the hydrogen peroxide concentration was kept low. The arsenite concentration, however, was limited by thermal reaction with hydrogen peroxide. Preliminary experiments on arsenite-peroxide mixtures showed the following concentrations to be stable for several hours at room temperature, and they were used in all subsequent experiments.
Experimental (i) Apparatus.-Photolyses were carried out in a fused quartz vessel of the type previously described,Z such that solutions could be readily degassed without contamination. A 550-w. Manovia, low pressure mercury arc supplied from a Ferranti voltage stabilizer was used in most of the work. The radiation, chiefly 1849 and 2537 A., was condensed and filtered b a cobalt-nickel filter solution*in st spherical quartz flask a n l t h e photolysis vessel was cooled by 2 centrifuged blower. Under these conditions, no 1849 A. radiation reached the photolysis vessel. Chemical actinometers were compared using a Mineralight lamp. (ii) Actinometry.-Both the monochloroacetic acid4 and ferrioxalate6 systems were used as chemical actinometers. Intensity of incident light was determined under conditions of complete abborption, giving the value 0.96 X einstein/l./min. From this the fraction of light absorbed by the peroxide-arsenite mixture was found to be 5.3%. Optical density of the mixture was calculated from literature values for ~ ( 1 3 ~of019.6. ~)~ In view of uncertainties in the quantum yield of the monochloroacetic acid a ~ t i n o m e t e r , ~the , ~ incident light was redeti:rmined by the ferrioxalate method. Using ‘$2537 = 1.20 for the latter16agreement was obtained between the two methods if +GI- = 0.25, this being a short extrapola; tion of the results of Smith, Leighton, and Leighton to 19 (the temperature of the photolyses). (iii).-Reagents and preparation of solutions were as previously described.’ 90% unstabilized H~02(Laporte) was suitably diluted with triple-distilled water. The procedure for degassing before photolysis, and the collection of gases afterwards, has been described,2 as have the analytical methods.$ Gas analysis was carried out on a Metropolitan Vickers M.S.2 mass spectrometer.
(H202)o = 200 p M , (AS(III))~= 2 mM, HzS04 = 0.1 N It was essential to be certain that chemical reactions should be ascribable only to light absorbed by the hydrogen peroxide. Consequently the absorption spectrum of arsenite in acid solution was measured, aGd showed no appreciable absorption above 2200 A., in agreement with Goldfinger and von Schweinitz.8 As a further check, acid arsenite solutions alone were irradiated in the presence and absence of oxygen. I n no case was any chemical reaction observed. Accordingly it was demonstrated that the presence of hydrogen peroxide was necessary for photolysis to occur. (i) Photolysis in the Absence of Oxygen.Results for oxygen-free solutions are shown in Fig. 1. Hydrogen peroxide was consumed and arsenate formed in equivalent amounts. At any one time, the sum of arsenate formed and peroxide consumed is constant, indicating that no other product is formed. This was checked by carrying out gas analyses and a typical gas analysis after 60 min. was
coz H 2
(1) M. Daniels and J. Weiss, J . Chem. SGC.,2467 (1958). (2) T.Rigg and J. Weise, J . Chem. Phys., 20, 1144 (1952). (3) M.Kasha, J. Opt. Soc. Am., 38,929 (1048). (4) R. N. Smith, P. A. Leighton, and W. (2. Leighton, J . Am. Chem. Soc., 61,2299 (1939). ( 5 ) C. A. Parker, Proc. Zoy. Soc. (London), 8236, 518 (1956). (6) W. C. h h u m b , C. N. Satterfield, and R. L. Wentworth, “Hydrogen Peroxide,” A. C. S. Monograph, 1956. (7) L. Kuohler and E. Pick, Z . phusik. Chem., B46,116 (1940).
Accordingly the stoichiometry is simply H202
+ As(II1) --+HzO + ASW)
As would be expected for such a small fractional light absorption, change of concentrations is an (8) P. Goldfinger and G. von Sohweinitz, ibid., B19 219 (3932).
arsenate concentration is 530 piW-approximately double the initial oxygen concentration of airequilibrium solution. Accordingly, the over-all process and stoichiometry now follom the equation 02
+ 2As(III) --+- 2As(V)
with the quantum yield +(As(V))o, = 0.9. Thc results are shown in Fig. 2.
20 40 60 80 100 120 Duration of photolysis (min.). Fig. 1.-Photolysis of oxygen-free arsenite-peroxide mixtures: 0 , H202; 0, As(V); A = 2Hd.32 As(V). 0
Discussion A. Photolysis of Oxygen-Free Solutions.-It is now satisfactorily establishedg-ll that under suitable conditions of high light intensity and low concentration, hydrogen peroxide can be photolyzed in a non-chain manner, at a quantum yield +(-HzOz) of 1.0 at room temperature. The commonly accepted mechanism accounting for this is the reaction sequence (1)-(3) (consideration of an alternative
HzOz * 20H HzOz +HzO
+ HOz 2H02 +HzOz + 02;
(1 1 (2)
(3) formulation is given below), which infers that the primary quantum yield dp(-HzOz) is 0.5. This value is the same as that reported here for photolyses in oxygen-free solution. I n view of the evidence from radiation chemistry1 that arsenite reacts readily with OH radicals, together with the evidence presented here for complete scavenging of OH radicals (ie., absence of O2 formation), it is suggested that the behavior in oxygen-free solution can be satisfactorily represented by reaction 1 fol-
2 o o p,I y i
60 120 180 240 Duration of photolysis (min.). Fig. 2.---Photolysis of arsenite-peroxide mixtures in presence of ox gen: 0,As(V) (air-saturated); 8 , As(V) (oxygensainratedy; 0 , H202
+ As(II1) +HYO + As(1V) 2As(IV) -+ As(lI1) + As(V)
(5) exponential function of the time of photolysis, -d(H20z)/dt = kAbs. and in very dilute solution lon-ed by (4) and ( 5 ) , noting that existence of the inIAbs. = IO^, so that -d ln(Hz02)/dt = kIoel. termediate oxidation stateAs(1V) again is presumed. From the initial slopes, the following quantum This will lead to +;(-HzOz) = di(As(V)) = 0.5 = +,(-HzOz), in reasonable agreement with the yields were obtained values reported here. It also should be noted that +i(-H202) = 0.55 +i(As(V)) = 0.47 reactions 4 and 5 are utilized in accounting for the radiation chemistry of oxygen-free sdution. (ii) Photolysis in the Presence of 0xygen.However, in itself this no proof of the existence of When photolysis of the mixture is carried out using air-equilibrated solutions ( [ 0 2 ] 0 255 p M ) the As(1V); this arises from consideration of the effect behavior changes completely. No hydrogen per- of oxygen. B. Photolysis of Oxygenated Solutions.-It oxide is coiisuxned at all, although its presence has has been shown (see Results) that peroxide is been shown to be iiecessary for oxidation to occur. A h e n a t eformation is now a linear function of time necessary for oxidation of arsenite to occur when and occurs ai an increased rate compared to the oxygen is present. Clearly then, as there is no net 02-free case until available oxygen has been con- consumption of peroxide, it must be regenerated in the course of the secondary reactions. Furthersumed. Results for As(V) and H20zin air-equilibrated more, as oxygen has no effect on the photolysis of solutions obtainrd after approximately 120 min. peroxide alone, such secondary reactions must inu-(wrather uratic and it is suggested that this is volve the arsenic species, and as neither arsenitc due to coiitinued absorption of O2 from the gas nor arsenate react with oxygen at this pH, it mubt phase above the photolyzed solution. These re- be concluded that the only other possible species, sults therefore are not considered in the discussion. As(IV), is the reactive entity. In the presence of That the change in slope is indeed due to oxygen oxygen, then, it is suggested that reaction 4 is sur(9) J. P. Hunt and H. Taube, J . Am. Chem. Soc., 74, 5999 (1952) consumption was shot$-n by photolyzing oxygen (10) J. L. Weeks and AI. S.hIatheson, i b z d , 78, 1273 (lcJ5b) saturated solutions ([O2l0 1250 p L M ) when the ( I 1) ,I. €1. Baxendale and J. A. Wilson, Trans F m n d a g Soc , 63,344 initial rate was maintained well past the previous (1957) point of inflection. Furthermore, at this point the (12) M. Daniels, J. Phys. Chem.. 6 6 , 1475 (1902).
RADIATIW CHEMISTRY OF ARSENITE
ceeded by format,ion of a peroxy radical according to
+As(1V) - 0 2
the relevant structures being represented as OH
At the beginning of this Discussion an alternative mechanism for the photolysis of hydrogen peroxide was mentioned and it is appropriate to consider the implications of this now. On the basis of oxygen fractionation experiments it was pointed out by Hunt and Taubeg thatj the net primary step of photolysis may be
As the quantum yield is irot changed by increasing the oxygen concentration fivefold, it seems that reaction 6 is lOOyo effective in the present conditions. Subsequent reactions of As(1V) are not so clear. On the basis of the present work we cannot distinguish between the following two modes of reaction. ils(1V) .O, +As(V)
+ OH) +HzO + 0
followed under non-chain conditions by
In this case, then, the primary quantum yield is identical with the observed quantum yield of 1.0. Arsenite would be expected to be a ready scavenger for oxygen atoms, according to As(II1)
+ 0 -+As(V)
so that if photolysis followed this pattern, then according to the evidence and arguments presented above, arsenate should be formed a t a ZAs(1V) - 0 2 --+ZAs(V) HZ02 0 2 (7b) uantum yield of 1.0 in oxygen-free solution. Furand indeed distinction is irrelevant for the present t er, as arsenite should (according to (10)) proceed purpose. Both of these reactions lead to the re- directly in a one-step reaction to arsenate, oxygen generation of Hz02together with a doubling of the should have no effect on the quantum yield. quantum yield of arsenate compared to the 0 2 - Neither of these expectations is fulfilled, and it is free solution (experimentally qj;(As (V))o,/qji- concluded that the results reported here are evi(As(V))02-free= 1.9) and it is concluded then that dence against the photolysis of hydrogen peroxide reactions 1, 4, 6, and 7 give an adequate account of producing kinetically significant quantities of freely diffusing oxygen atoms. the behavior of oxygenated solution. It) is hoped to obtain further evidence concernAcknowledgments.-This work was carried out ing reaction 7 from two sources: (a) rapid reaction a t King’s College, Newcastle-on-Tyne, Argonne whether the peroxy radical disappears by either National Laboratory, Argonne, Illinois, and Unifirst-order or second-order kinetics, and (b) the versity College of the West Indies, Kingston, kinetics of the radiation-induced chain reaction1J3 Jamaica. Thanks are due to Professor J. Weiss, having reaction 7 as termination steps should show for encouragement and interest while the major either first power or half power intensity depend- part of the experimental work was carried out and ence, respectively. to A.E.R.E. (Harwell) for support and permission to publish. (13) M. Danicls, communication in preparation.
folloivrcl b,y reaction 3, or
CHEMISTRY OF ARSENITE. FREE SOLUTION BY
Radioisotope Applications Division, Puerto Rico Nuclear Center, BOPiedras, P. R., and Chemistry Department, University of Puerto Rico, Rio Piedras, P. R. Received March 7,1968
Irradiation of oxygen-free solutions of arsenite a t pH 2.7 produced hydrogen and arsenate in small, equivalent amounts G(H2) = 0.46, G(As(V)) = 0.48. Hydrogen peroxide was not formed. Irradiation of oxygen-free mixtures of arsenite and hydrogen peroxide showed increased formation of arsenate and consumption of hydrogen peroxide. G-values increased with the ratio (H20z)o/(As(III))ato the limiting values of G(As(V)) = 3.3 and G(-HzOz) = 2.8. The hydrogen yield is unaffected
and no oxygen is formed. Results are interpreted in terms of an intermediate oxidation state As(IV), and competition between arsenite and hydrogen peroxide for H atoms.
Introduction Previous work1 on the radiation chemistry of the arsenite system concerned the effect of pH in oxygenated solutions, and showed, not surprisingly, that oxidation w a s the major process. The present (1) M. Daniels and J. Weiss, J . Chern. Soc., 2467 (1958).
work, which was a natural continuation, was aimed a t determining (1) whether net oxidation or reduction of arsenite would take place in the absence of oxygen, (2) the yields of the “mo1ecu1ar products” Hz and HzOz, (3) the mode of reaction of the Primary radicals H and 013 and the secondary radical As(1V).