E. HAYON
1502
culate the selective adsorptivity of calcium to sodium ions a t the air-solution interface of Aerosol OT as a function of C1 = 0.0015 mole/l. and Cz. The values are shown by the solid line in Fig. 2 . The agreement between the experimental and calculated values is quite satisfactory, and in both cases the selective adsorptivity tends to decrease with increasing calcium ion concentration. The electrical potential of the ionized monolayer! c$,,= (kT/Xe) In 8, was estimated as about 200-250 mv. for our solutions. We might presume that the monolayer of a nonionic agent at an air-solution interface would not adsorb small ions. However, Judson, et uL,l found that sulfate was adsorbed a t the interface from aqueous solutions of a non-ionic agent, Aerosol NL-10. The amount adsorbed was proportional to the coincentration of sulfate ions in the solution. To obtain further evidence for the adsorption of inorganic ions a t an air-solution interface of a non-ionized layer, we have determined the ratio of calcium ions to adsorbed octyl glucoside. From these results we can conclude that there is some adsorption of calcium ions a t the adsorbed layer of octyl glucoside, and that for a given octyl
Vol. 65
glucoside concentration the ratio of calcium ions to adsorbed octyl glucoside increases roughly with the bulk concentration of calcium ions. The relative adsorptivity of calcium ions is, however, far less a t a non-ionized layer than at an ionized layer. mole/.) In a solution of dodecyl sulfate (1 X containing 1 x 10-6 mole/l. of calcium chloride the ratio of calcium ions to surface active ions in the adsorbed layer was about 0.1, whereas in solution mole/l.) containing of octyl glucoside (2.5 x 1X mole/l. of calcium chloride the ratio of calcium to surface active molecules at the surface was about 0.1. The ability of the adsorbed layer of these two solutions to adsorb calcium differs by a factor of about lo4. The ratio of calcium ions to dodecyl sulfate ions a t the surface was about 100 times greater than the corresponding ratio in the bulk of the solution, whereas the ratio of calcium ions to octyl glucoside at the surface was about half of that in the solution. Acknowledgment.-We wish to thank the Ministry of Education and the Asahi Chemical Industry Foundation for supporting this work. We also wish to thank NIrs. L. H. Weissberger for assistance in preparing the manuscript.
EFFECT OF SOLUTE CONCES'TRATIOn' ON THE RECOMBINATIOS OF H AXD OH I K 7-IRRADIATED AQUEOUS SOLUTIOn'S' BY E. HAY ON^ Chemistry Department, Brookhaven ivataonal Laboratory, Upton, Long Island, Xew York Received January 90,1961
It is shoan that on addition of high concentrations of inorganic solutes reactive to H atoms and OH radicals the recombination of H and OH to form water in y-irradiated aqueous solutions is reduced. The presence of 0.43 M Ce4+, 0.1 M . T1in 0.8 S H,SOa solution increases G - H ~ ofrom 4.50 to 5.24. The yields of molecular hydrogen from air-free solutions of Ce4+-Tl+, T1+, Ce3+, Br- and arsenite have been measured. Gaz 0.4 in 0.8 Ar HzSOl or HC1 solutions as compared t o -0.45 for neutral Br- and arsenite solutes. A mechanism is suggested n-hich could explain the decrease in the molecular yield of hydrogen in acid solutions.
Water is decomposed on irradiation to give H, OH, H2 and H202. The H2 and H202 are thought to result, from combinat'ion of like radicals in regions ('(spurs"), where the concentration of radicals is high, and reaction (3) has been assumed H+H+Hz OH OH -+ H202 H OH +H20
+ +
(1) (2)
(3)
to occur by analogy with (1) and ( 2 ) . The H atom and OH radicals are the reactive intermediates formed in the radiation chemistry of dilute aqueous solutions. Their yields are now fairly well ac~ e p t e d . It ~ was found, however, on irradiation of aqueous systems containing relatively high con(1) Research performed under the auspices of the U. S. Atomic Energy Commission. (2) Department of Physical Chemistry, Cambridge University, Cambridge, England. (3) (a) h'. F. Barr and R. H. Schuler, J . Phys. Chem., 68, 808 (1959); (b) T.J. Sworski, J . Am. Chem. Soc., 76, 4687 (1954). (4) (a) 11. Frioke, E. J. Hart. H. P. Smith, J . Chem. Phys., 6, 229 (1938); C. I t . Maxwell, D . C. Peterson, N. E. Sharpless, Radiation lfes., 1, 530 (1954); ib) G. R. A. Johnson, G. Scholes and J. Weiss, Natuie, 177, 883 (1956); D. M. Donaldson, N. Miller, Radiation Res., 9 , 487 (1958); J. T. 411an, E. Hayon and J. Weiss, J . Chem. Soc.. 3913 (1969).
centrations of organic4 solutes that the yields of radiation-induced products are dependent upon the concentration of the solute, and cannot be accounted for on the basis of the accepted radical yield^.^ It was suggested4b that at high concentration the solute was reducing the extent of recombination of H and OH in water (reaction 3). Though in certain cases this did seem likely, one could provide an alternative interpretation, e.g., the observed increase in yields with increase in solute concentration could be explained by electronic excitation of the solute molecules by subexcitation electrons5 produced by the radiation, or by direct absorption of energy by the solute resulting in dissociation of the organic molecule. It seemed therefore of interest to investigate the effect of concentration of inorganic solutes since the increase in yields cannot in this case be due to dissociation of the solute, and any contribution due to direct absorption of energy at high concentrations of inorganic solutes could be calculated and corrected for. (5)
J. W e i s s , J . chim.phys.,
t i o n Res., 2, 1 (1955).
5.2, 539 ( 1 9 5 5 ) : R. L. Platzman, Radza-
Sept., 1961
RECOMBISA4TIOS OF
H
AND
OH
I N T-IRRADIATED
Experimental Ceric sulfate, Ce(HS04)4,supplied by G. Frederick Smith Chemical Co., and thallium sulfate, T1804, by A. D. Mackay, Inc., were used without further treatment. A stock solution of ceric sulfate (0.5 rV) in 0.8’ \ I was prepared and allowed to stand in the dark for two weeks previous to irradiation. Water was triply distilled from acid dichromate, alkaline permanganate, and a final distillation.6 A 1100 curie cylindrical Coso y-source was used? with a dose rate of 4.22 X lozo e.v./l.-min. based on the Fricke dosimeter (10-3 ill FeS04, 10-3 M NaC1, 0.8 K HZSO,) taking G(Fes3+) = 15.5. Irradiations of air-saturated solutions were carried out in 1.2 cm. diam., 18 cm. long Pyrex glass tubes, which were preirradiated to a dark brom-n color. Air-free solutions for Hz gas determination were prepared as described by Schware, Losee and Allen.Tb Ceric ion concentrations before and after irradiation were determined by absorption spectrophotometry a t 320 mG using an extinction coefficient of 5580 in 0.8 N HnSO4 solutions .8 Dilution of concentrated ceric solutions wa8 carried out with 0.8 N H2SO4 containing 10 pJ4 Ce4+. Direct interaction of the radiation with Ce4+ and Tlf becomes appreciable a t the relatively high solute concentrations used. The dose absorbed in these solutions was corrected for using the equation
G(Ce3+) correct,ed for direct absorption of the radiation by the thallous and ceric ions present in solution in relatively high concentrations. The error in determining the slopes of the yield-dose plot is a t the highest Ice4+], A37,. Figure 1 shows the yields of G(Ce3f) corrected for absorption of radiation, as a function of Ce(HS04)4concentration. It can be seen that increasing the concentrattion of Ce4* ions at constant [Tl+]results in an increase in G(Ce3+). TABLE I DOSIMETRIC CORRECTION ON IRRADIATION OF Ce4+IN 0.1 Jf TlzS04 IK 0.8 N HzSOc Ce4 + concn.,
iM
Results The ceric-thallous sulfate system developed by Sworski8 was chosen for this study because both constituents have a relatively high solubility in water. On irradiation of air-saturated 10-3 M Ce4+’ M T12S04 in 0.8 N HzS04 solut’ion, G(Ce3+) was found to be 7.80 which is slightly lower than Sworski’s value of 7.92.8 Sworski’s results that addition of up to 10-l M TlzSO4 to 10-e10-3 M Ce(HSO& in 0.8 N H2S04does not affect, G(Ce3+)were also confirmed. It was found, however, on increasing t,he concentration of Ce4+ ions that G(Ce3+) increased over and above the contribution from the decrease in the yield of molecular hydrogen.11f12Table I1 gives values of (6) E. R. Johnson and A. 0. Allen, J . Am. Chem. Soc., 74, 4147 (1952). (7) (a) H. ‘4. Schwarz and A. 0. Allen, Nucleonics, 12, #2, 58 (1954); (b) H. A. Schn-ara, J. P. Losee and A. 0. Allen, J . Am. Chem. Soc.. 76, 4693 (1954). ( 8 ) T. J. Sworski, Radiation Res., 4 , 483 (1956). (9) “Handbook of Chemi8t.ry and Physics,” 39th ed., p. 2469. (10) W. Bernstein and R. H. Schuler, Nucleonics, 13, # l l , 110 (1955). (11) J. A. Ghormley and C. J. Hochanadel, Radiation Res., 3, 227 (1955). (12) J. T. Harlan and E. J. Hart, Sucleonics, 17, #8, 102 (1959).
Density, g./ml.
ES/ED
T
X
correction, lo3
0.06
a
where D, is the corrected dose, D F ~ zthe + dose as measured by the Friclre dosimeter, eg and ED the electron density of the irradiated solutions and dosimeter, respectively, T the correct,ion for the photoelectric effect,Qa the correction for the absorption of low-energy scattered radiation inside t,he Co60 source by materials of high atomic number.l0 r was calculated using the $ass absorption coefficients for -/-rays a t wave length 0.01 A . for Pb and Sn, these having about the same atomic numbers as T1 and Ce, respectively. I n order to calculat,e the electron density the specific gravities of the solutions were determined, and the results and the calculated corrections are shown in Table I. The corrected doses are good to +a%. The implied assumption made here is that the “extra” energy absorbed by the Ce4+and T l + is transferred to the solvent (water), but that no energy is transferred from the water to the solutes. Arsenite solutions were prepared by dissolving 5 X M arsenious oxide in 0.1 NaOH and rapidly neutralizing the solution with sulfuric acid. Hydrogen %-as measured h y combustion with oxygen on a platinum filament.
1503
AQCEOUS SOLUTIONS
1.086 1.041 7.6 .10 1.104 7.9 1.048 .16 8.4 1.122 1.056 .25 1.145 1.065 8.8 .43 1.208 1.090 10 .15a 1.081 1.04 4 Containing 2 X 10-2 M T12S04.
a
X 103
20 21 22 24 28 6.7
70
6.8 7.8 8.8 10.0 13.0 5
Table I1 gives the corrected yields of molecular hydrogen as mea’sured in air-free 0.8 N HzS04 solutions of ceric sulfate in lo-’ M TlzS04. The presence of thallous ions does not affect G(H2), as seen from the irradiation of a 0.11 M Ce4f solution in the absence of TI+. The value of G(HJ ext’rapolated to infinite dilution from the plot of G(HJ us. is 0.395
*m
TABLE I1 G(Ce3+) AND G(H2) YIELDS FROM ?-IRRADIATED 0.8 N H2S04 CERICSOLUTIONS IN 10-1 M TlzSOI [Ce“], .+I
G(Cea+)cor.a
[Ce’+], M
C(Hdcor.4
8.15 2 x 10-4 0.364 8.56 5.26 X .355 9.19 7.77 x 10-3 ,313 9.40 8 . 2 X lod2 ,236 9.70 3.74 x 10-1 ,187 ,1506 8.98 1.1 x .23 a Yields corrected for direct interaction of the radiation with the solutions (see Experimental section). Containing M Tl~S04. S o thallous ion added. 2X 0.062 ,095 .165 .257 ,431
The value of 0.395 for the molecular yield of
Hzis lower than that recently obtained by Mahlman and Boyle13from 0.8 N HzSO4 solutions of bromide and by Schmrz and Hritz14 from ferrous chloride solutions in 0.4 :1f HCl, GH* = 0.45. The yield in neutral solution, G H ~= 0.45, seems t,o be generally agreed on. It seemed worthwhile t,o check this discrepancy. The yield of H2 from air-free HzS04 solutions of KBr, T1+, Ce3+ and amenitre, and of KBr and arsenite in neutral solutions are shown in Table 111. The values of G(H2) measured in every run are included. As can be seen, in all cases the yields of Hz are slightly lower in acid than in neutral solut’ion. Figure 2 gives a plot of G(H2) from air-free bromide solutions us. pH. The yield of H2 in 0.8 N HC1 in the presence or absence of M KBr also gives a value near 0.4. 113) H. A . Malilinan and J. W. Boyle, J . A n . Chem. Soc., 80, 773 (1958). Schwarz and J. hI. Hritz, ibid., 80, 5636 (1958).
E. HAYON
1504
Vol. 65
This mechanism holds up to nearly complete consumption of the ceric ions present in solution as long as the concentration of thallous ions is greater than that of cerous ions formed, since h/ke = 38.8 G(Cea+) is therefore equal to ~ G H * o * GH 4OH
L
7
-
10-4
-
io-*
_
_
I
i
-
1
1
+
(9)
GOH. Increasing the thallous concentration decreases GH,O*by reaction of TI+ with OH radicals in the spurs before they recombine to form HzOz. But the sequence of reactions 6 and 7 is equivalent to (8) so that G(CeS+) remains unchanged.* Increasing the ceric ion concentration in the absence of thallous ions has been shomn11v12 to reduce the yields of molecular hydrogen appreciably, as a result of reaction 5' occurring in the spurs, leading to an increase in G(Ce3+). From the mechanism proposed by Sworski
10-2 10 -1 1.0 Ce4+,M . Fig. l.-G(Cea+) plotted a8 a function of [Ce4+] obtained from irradiation of air-saturated 0.1 M TlzSO4, 0.8 N HaSO4, except for one run ( 0 ) in 0.02 M TlzSO,.
0.45
+ Ce*+ +Ce4+ + OH-
+ +
G(Ce3+)= 2&,02 GH Goa: = 2G-~,o - 2 G ~ 1
since from the stoichiometric balance G-H,o ~ G H , o4 , - GOH= ~ G HI, GH,Or A(~_H,o = I/2AG(Cea+)
+ AGE,
=
(10)
si
where AGH, is negative since the yield of Hz has decreased. On a statistical basis one would expect that the increase in G-H*o (reaction 3) with increase in concentration of the inorganic solutes 6 0.4 t- %----E t5 would reach an optimum value equal to ~ G H or , I 8 ~GH,o*.But since G H ~ and GH,O~are not equal,a the maximum increase in G-H*o would to a first 0.35 _____ - ~ - approximation be equal to G H ~ GH~O,,or 1.20 in 0.8 N H2S04. Ignoring the difference in the 1 2 3 4 5 6 molecular yield of hydrogen in air-saturated soluPH. Fig. 2.-Yields of G(H2) us. pH obtained from Cofio tion compared to air-free solution, due to scaveng7-irradiaicion of air-free KBr solutions. All runs are ing of H atoms by 0 2 , one can to a first approxiincluded on the graph. mation calculate AG-H~O. G(H2) = 0.18 for airTABLE I11 free and G(Ce3+) = 9.70 for air-saturated 0.43 HYDROGEN YIELDSFROM 7-IRRADIATED AIR-FREE AQUEOUS M Ce4+in 10-1 M TlzS04, 0.8 N H2S04 solutions, giving a value for AG-K~o = 0.735 according to SOLUTIONS equation 10. Solute PH G(Hd G(Ce3+) seems to be independent of [T1+] (Fig. lo-* M KBr 5 . 5 0.438; 0.445; 0.442 1) in concentrated ceric ion solutions. One must, 10-8 M ElBr 3.0 ,432; .437; ,438 therefore, assume that the reaction of H atoms with M KBr 2.4 .422; .425 TI2+must be as fast as the reaction of H atoms with 10-8 M KBr 1.2 .405; .401; .404 M I
