RADIOLYSIS OF ALKALIHALIDES I N AQUEOUS SOLUTIOXS
2967
On the Radiolysis of Alkali Halides in Aqueous Solutions Saturated with Nitrous Oxide
by M. Anbar, D. Meyerstein, and P. Neta T h e W e i z m a n n Institute of Science and the Soreq Research Establishment, Rehouoth, lerael (Received M a y 7 , 1964)
The radiolytic oxidation of iodide ions in X2O-saturated solutions has been investigated over the range of IOp4 to 1.0 116 KI. G(I2) was found to increase with iodide concentration up to 3.65 at 1 d l KI. This result implies the formation of oxidizing species other than OH radicals in radiolyzed solutions. Fluoride, chloride, and bromide ions were also shown to react with these species.
The radiolytic oxidation of halide ions in aqueous solutions is one of the very simplest oxidation-reduction reactions, as it does not involve any bond cleavage in the oxidized species. I t seems reasonable that a quantitative study of this oxidation process may throw light on the identity and kinetic behavior of the primary oxidizing species formed in water under radiolysis. The reaction of the products of halide oxidation with the reducing species formed in radiolyzed solutions as well as their solvolysis limits, however, the ambiguity of quantitative interpretation. Even in the case of oxidation' of iodide ions in neutral solution, where solvolysis may be neglected, a steady-state concentration of iodine is obtained at very low doses of radiation. l~~ The application of pulsc techniques makes it possible to follow the appearance of oxidized species like Cl2- or 1 2 - before these undergo subsequent rea c t i o n ~ . ~Using conventional radiolytic techniques, conditions had to he found under which no steadystate conditions were attained. I n recent photochemical studies4 of solutions of potassium iodide saturated with nitrous oxidc, it has been shown that under these conditions aquated electrons oxidize iodide instead of reducing iodine. I t has been decided, therefore, to investigate the radiolysis of alkali halides in neutral solutions saturated with nitrous oxide. The results obtained indicate the formation of .primary oxidizing species other than OH radicals.
Experimental Adatem'als. Triple-distilled water was used throughout the experiments. Distilled water was redistilled
over alkaline potassium permanganate and subsequently with dilute phosphoric acid in an all-glass still. All the salts were of Balter Analyzed or Flulta l'uriss grade and were used without further treatinent. The nitrous oxide was obtained from the Mathesoil Co., East Rutherford, IY.J. Pyeparation of the S a m p l e s and Their 7rr.adiation. The p H of the solutions was deterinined using a Metrohm Kolnpensator Type E 148 C, with aii accuracy of *0.05 p H unit. Samples ( 2 nil.) saturated with nitrous oxide in uniforin test tubes were irradiated for 1 and 2 niin. with y-rays from a Coonsource (Gainmacell 200, Atomic Energy of Canada, Ltd.) a t a dose of 7800 r./inin. (dcterinined by the I'riclte dosimeter, 0.1 N H2S0,,takingG(17e+3) = 15.5). A n a l y s i s . The amount of iodine formed was deterniincd by adding 2 nil. of 0.2 N K I and mca,suring the optical density a t 352 nib, using a Hilger Uvispek spectrophotometer. The Iz concentrations were calculated froni the extinction coefficient of 13- ( E 2C,,400).5 The presence of up to 0.25 Ad of other halides in t h e measured solutions was shown not to affect the results (1) E. 11. Johnsoti, J . Chem. P h y s . , 2 1 , 1417 (1953); H. A. Schware, J. 1'. Losee, Jr., and A. 0. Allen. J . Am. Chem. Soc., 76,4693 (1954). (2) A. 0. .411en, "The Radiation Cheti1istr.v of Water and Aqueous Solutions," D. \'an Nostrand Go., Inc., Princeton, N . J . , 1961, ) ) I > ,
108, 109. (3) 11.Anhar and J. K. Thomas, J . P h y s . Chem., in press. (4) J. Jorttier, lf,Ottolenghi, and G. Stein, ihid., 66, 2037 (1962): 68,247 (1904): F. S.Dainton and S.A. Sills, Bull. SOC. chim. Belges, 71, 801 (1962). (5) D. lleyerstein and A. Treiniii, T r a n s . Faraday Soc., 59, 1114 (1963).
Volume 88, Number IO
Octoher. 1904
by more than l%.5 The G values obtained were calculated froin the mean of a t least four experiiiierits and had a standard deviation < +0.10. In order’to calculate the dose absorbed in the more concentrated solutions (>O. I 1%’) , the formula
D, =
L)F
x
6.3
-CF
was used, where D, is the corrected dose, D F the dose as measured by the Fricke dosiineter, and es and e~ the electron densities of the irradiated solutions and of the dosinwter, respectively, as calculated from the densities of the solutions.6 No correction was made for the photoelectric effect7 nor for the absorption of low energy scattercd radiation inside the CosOsource by materials of high atomic n u ~ i i b e r . ~The ~ ~ G(Iz) obtained in a 0.5 M CsCl solution as compared with a 0.5 ’14 S a C l solution (Table 11) verified the adequacy of the applied approximation in the range of concentrations under study.
Results and Discussion The rcsults for pure potassiuiii iodidc solutions are given in Tablc I. It can be seen that G(Iz) changes from 1.17 at M I- up to 3.65 a t 1 M I-. The mechanism suggested for the forination of iodine under the experimental conditions is OH+I-+OH-+I
I
(1)
+ I - -+ Iz-; Iz- + Iz---+I,- + I SzO+ eaq- + H + -+KZOH XZOH + I- + I T 2 + OH- + I
(2)
(3)
(4)
alternatively
+ OH + HOz + H + + 2H+ +
XZOH + S z
Iz-
+ HzOz
13-
+ HzOz + 31
Is 12-
+ HOz
--+
21-
0 2
+ H+ + + HOz +21 + H + + 02 H + H + + 12--+ 1 2 -
0 2
(5)
(6)
Table I : The Formation of Iodine in Polassium Iodide Solutions Saturated with ru’itrous ( ~ x i d e “ WII, M .
I x 10-4 2 x 10-4 5 x 10-4 1 x 10-3 2 x 10-3 5 x 10-3 1 x 10-2 2 x 10-2 5 x 10-2 1 x 10-1 2 x 10-1 5 x 10-1 1.0
G(Iz)~
G(Idc0:
1.17 1 25
1 1 1 1 2 2 2 2 3 3 3
41 60 78 83 16 20 30 86 28 66 94
2 3 3 3
83 21 52 65
a All solutions a t pH 6. The observed value for (?(I2). Value obtained after correcting for mass effect.
GII,O,taking G, = 2.8, GOH = 2.8,lo-l3GH20, = 0.8, and GH = 0.6. G(Iz) = 1.70 is expected, which is the value obtained for the 1 X lo-, M solutions. Increasing the concentration of the iodide above 1 X lop3M , the yield of iodine is still growing. This effect is not a mass absorption effect, above the correction included, as is evident by coinparing the results in the presence of cesium chloride with those of sodium chloride (Table 11). Under similar irradiation conditions it has been shown by Anderson14 that the photoelectric effect does not have an appreciable specific contribution up to 4 M iodide, which is four tiines our maximum Concentration. Further, it can be seen from Tablc I that the per cent increase in G(Iz) when ricreasing iodidc concentration is 37, 3;i, 35, and 28%, respectively, when (I-) is increased from to 10-3, lop1, and 1.0 M , respectively. If some kind of “direct” action would be involved, the increase in G (Iz) going from 0.1 to 1.0 M I - would be niuch larger than that when changing iodine concentration froin
(7) (6) “Handbook of Chemistry and Physics,” 44th E d . , Chernical Iluhber I’uhlishing Co., Cleveland, Ohio, 1963, pp, 2056-2062.
Reaction 6 is an equilibrium reaction, but in the case of neutral solutions it is shifted totally to the right. (It has been shown that changing the p H from 5.5 to 9.0 has no effect on the forination of iodin(. in pure iodidc solutions.) The interaction of aquatcd electrons
(7) E. Hayon, J . Phys. Chem., 6 5 , 1502 (1961). ( 8 ) W. Rernstein and 11. H. Schuler, Nucleonics, 13, N o . 11, 110 (1955). (9) 11. Anhar, R. A. Murioz, and 1’. Iiona, J . P h y s . Chcm., 67, 2708 (1963). (10) I n the cases where GOH = 2.2 has been reported,ll-12 GH was n o t taken into consideration and it is obvious t h a t the H atoms formed cancel an equivalent yield of OH radicals.
with nitrous oxide was shown to yield S r O H (reaction an interiiiediatc which has a lifetiinc long enough to perinit chemical reaction. The expected G(IJ according to this schcnic is G(12) = I ’2(Gr GoH - GH) -
(11) See ref. 2, 11. 47. (12) M .S.Matheson, Ann. Re?. P h y s . Chem., 13, 77 (1962). (13) 1;. S. n a i n t o n and 1%‘. S.W a t t , Satitre, 195, 1294 (1962). (14) 11. Anderson and B. Knight, Ahstracts of the 2nd Internat,ional Congress of Radiation Research, 1962. p . 71.
12 +
+
The Joicrnnl of Physical Chemistry
(8)
I ~ A U I O L YOFS ~ALKALI S HALIDES I N AQGEOUS SoLvrioNs
0.01 to 0.1. I t may be concluded that the increase in G(I2) with iodide concentration has to be explained in tcrriis of "indirect action." Table 11: The Effect of Added Halide Ions on G(I$' Additive
0
None KF
1.60
~___
_ _ Concn., 'V
,-__ 10-2
3 X 10-2
10-1
NaCl
1.75
1.82
1.68" 1.87
CSCl NaBr
2.06
2.29
2.55
5 X 10-1 5 X 1 O - x b
1.76d 2.08 2.15 2.89
A' potassium iodide solutions a t pH 6. ' In for mass effect. pH 6.8. pH 7.5.
1.74 2.04 2.06 2.84
* Corrected
The increase in G(Iz) at high iodide concentration may be interpreted, a t first sight, by scavenging of the precursor of the molecular hydrogen peroxide, possibly OH radicalJ15by the iodide, leading to the forination of iodine instead of its reduction. Assuming that a11 priniary OH radicals are scavenged by iodide ions, this effect tnay lead to an upper liinit of G(1,) = l/g(G, GOH - GH) G H ~ = o ~3.30. G(Iz) = 3.65 obtained for the 1.0 M potassiiiin iodide solution cannot be explained by this mechanism alone since it significantly exceeds G(I2) calculated for this mechanism. Iodide was shown to scavenge the formation of molecular hydrogen peroxide to the smile extent as broinide.l6* Taking the value of GH202 = 0.4 obtained in the presence of 1 M bromide,lfih G(1,) is not expected to exceed 2.50 up to 1 M iodide. It must be concluded, therefore, that a substantial fraction of iodine is formed by a mechanisin which does not involve OH radicals. I n order to get a better understanding of the observed increase in G(I2), the effect of added fluoride, chloride, and bromide on the forination of iodine has bcen examined. All the X atoms or Xz- radicals formed3 are expected to oxidize I- to Iz. The results are suinniarized in Table 11. The results with added sodiutn bromide show a siniilar behavior to those of concentrated iodide solutions, though the yields of Iz are lower, probably due to the lower reactivity of broniide in the oxidation processes. The increase in G(Iz) in the case of added sodiutn chloride is niuch smaller, but this effect is of special interest owing to the fact that chloride ions are not oxidized by OH radicals in neutral solution^.^^^^^^^ 1;luoride ions have also shown a small, but significant, effect on G(12). A mechanism which might account for the increase in G(Iz) at high iodide concentration was suggested
+
+
2969
by Anderson, who attributed the increase in G(H2) in iodide solutions to the interaction of I - with subexcitation electrons. 1 4 s 1 * This inechariisn~is however inconsistent with the results of Kuppermann on the possible excitation energies of these electrons.lg Another process which may contribute to the increased yield of iodine is the IHzO+ + I HzO reaction which will partially inhibit the eaq- HzO++ HZO combination process, resulting in a n over-all increase in G - H ~ o . It should be noted that inhibition of the recombination of H and OH radicals does not affect G(1,). The interaction of halides a t high concentrations with H 2 0 + has bcen suggested as a mechanism for the formation of CIZ- in neutral ~ o l u t i o n . ~The effect of chloride ions on G(In) presented in Table I1 may be due to this reaction. HzO+ has also been postulated in other radiolytic proccsses.2n An alternative mechanism by which G(Iz) may be increased a t high iodide concentrations is the oxidation of iodide by excited water molecules.
+
X-
+ HzO*
--j
X
+
+
+ eaq-
Excited water molecules were suggested to be formed in radiolyzed s o l ~ t i o n s . ~ ~It- ~was ~ suggested that these species are the precursors of the so-called residual h y d r ~ g e r i . ~ It ~ - ~has ~ been. suggested that excited water ~noleculesoxidize halide ion^**^^* and thus G- B1O may be increased again. Fluoride ions, which obviously do not react with OH radicals, were shown not to compete for H 2 0 + with chloride ions.3 On the other hand, these ions have been shown to be oxidized in radiolyzed solutionsz4 and this process was attributed to their reaction (15) M . Burton and K. C. Kurien, J . Phys. Chem., 63, 899 (1959) (16) (a) See ref. 2. p. 64; (b) M.Anbar, J . Chem. P h y s . , 34, 703 (1961).
S.Guttniann, and G. Stein,
(17) (a) See ref. 2, p. 63; (b) A. 0. Allen, C. J. Ilorhanadel, J . A. Ghorrnley, and T. W. Davis, J . Phys. Chem., 56, 575 (1952). (18) H. C. Sutton in "ltadiation IXects in Physics, Chemistry and Biology," M. Ebert and A. Howard, Ed., North Holland I'ublishing Co., Amsterdam, 1963, p . 56.
(19) See ref. 18, p. 67. (20) J. *J. Weiss, Radiation Rea. S u p p l . . 4 , 141 (1964). (21) F. 6. Dainton and D. B. Peterson, Proc. R o y . Soc. (London), A265, 443 (1962). (22) 1,'. S. Dainton and W. S. Watt, ibid., A275, 447 (1963). (23) D . N. Sitharamaro and J. I;. Duncan. *J, /-'h,ys. Chem., 67, 2126 (1963). (24) XI. Anbar and D. Meyerstein. Israel AEC Reports, IA-851 (1963). (25) J. T. Allan and G . Soholes, .Vaticre, 187, 218 (1960). (26)
S.Nehari and J. Itabani, .I. P h y s . Chem., 67,
1609 (1063).
(27) 13. Hayon, ,Vatwe, 196, 533 (1962). (28)
&I. Anbar and D. Xfeyerstein, J . f'hys. Chem.,
68, 1713 (1964).
2970
F. A. M, DE HAAN
with H,O*. The oxidation of Tlf to T1+Zby H,O* may account for the increased apparent yield of OH observed by Hayon.’ The formation of eaq- by the reaction of iodide with H20* may also explain the increased yield of “niolecular hydrogen” in concentrated iodide solutions.14r18 The existence of oxidizing species other than OH radicals in neutral irradiated solution has been reported recently by different investigators29s30and their identity has not been finally established. These species manifest themselves only in rather concentrated solutions, in analogy to the experinients described in the present study. It is suggested here that either HzO+ or H20* or most probably both are liable for these findings. Both species are undoubtedly primary products of the act of radiation on water.31 The only question is whether these will
not dissociate or undergo deactivation before having a, chance to interact with solutes. Our postulation is that mater molecules which are both in thc inner and outer hydration shells of a given ion and which undergo a radiolytic ionization or excitation are liable to interact with their central ion rather than dissociate to Hf OH or H OH, respectively, or alternatively undergo neutralization or de-excitation. The rate of interaction of H 2 0 +or H20* formed in the hydration shell of a given solute with thcir central atom or molecule niay be of the order of electronic transition within a molecule, ie.,
Introduction The predoniinantly negative charge on l,,ost soil colloids leads to a decrease of the aniorl concentration in the close vicinity of the colloidal particles as compared with the concentration in the equilibrium solution. Thc .Joirrnal
OJ
P h p i c a l Chcmistrlj
This deficit of anions has been termed negative adsorption or anion exclusion. An expression for the negative adsorption, based on theoretical model calculations, was derived by Schofieldl and I
