1429
RADIATION-INDUCED CHAINREACTIOK BETWEEN YITROUS OXIDEA N D HYDROGEN
Table I : The Entropy of the Aqueous Lithium and Fluoride Ions" SOLi+(d
L i & o ~ ( s )= LiOH(s) = LiCl(s) = LiF(s) = NaF(s) a
=
%if Lif Li+ Li+
+ c03-' + OH+ C1-
+ FN a k + F-
* *
4.7 10 2 . 5 rt 0 . 3 2.5 0.3 3 0 f0.4 $OF- ( ~ q )
- 3 . 6 rt 0 . 2
Units: cal./mole "K.
The values for the entropy of the aqueous lithium ion are summasized in Table I. The obvious discrepancy in the value from the data concerning lithium carbonate may be attributed in part to inaccuracies in the solubility products used in the calculations, and in
part to the correction necessary for hydrolysis in the estimation of A H o . The good agreement shown by the other three values substantiates the choice of -2.5 cal./mole OK. for the standard entropy of the hydroxide ion, and therefore, the lower of the two values quoted'for the heat of ionization of water is preferred. Acknowledgment. The authors acknowledge their debt to Professor Lt. G. Hepler, of the Carnegie Institute of Technology, for the use of his laboratory and its facilities, and to Dr. C. Wu for her assistance with the measurements. The partial financial support of the National Science Foundation is gratefully acknowledged. (24) K . Clusius, J. Goldman, and A. Perlick, Z. Naturforsch., 4a, 424 (1949).
The Radiation-Induced Chain Reaction lietween NitrouB. Oxide and Hydrogen in Aqueous Solutions'
by C. H. Cheek and J. W. Swininerton U . S. ,Vaz.al Research Laboratory, Washington 25,D . C. (Receiued December 21, 1963)
The radiation chemistry of solutions containing mixtures of nitrous oxide and hydrogen has been studied. Nitrous oxide oxidizes hydrogen by a radiation-induced chain reaction which proceeds moderately in neutral solution and quite efficiently in stroiigly alkaline solution. Oxygen inhibits the chain reaction in neutral solution, but has no effect in strongly alkaline solution. The lack of a dose-rate dependence is attributed to chainterminating reactions with impurities which predominate over radical-radical combination. Mechanisms are proposed to account for the observations. The results account in part for an alternate interpretation of the sigmoid increase in G(N2) a t high pH in irradiated nitrous oxide solutions. $!
Introduction Recent reports on the yields of the radiolysis products of water in strongly alkaline solution are in disagreement. I n an interpretcationof the radiation chemistry of nitrous oxide solutions, Dainton and Watt2 have
proposed that G,,,- and GOH increase sigmoidally by about one unit in the range above PIX 11, whereas a re(1) Presented in part a t the 144th National Meeting of the American Chemical Society, L~~ ~ ~calif., ~~ ~ ~1963. ~i l , l ~ (2) F. S. Dainton and W. S.Watt, Nature, 195, 1294 ( 1 9 6 ~ ) .
Volume 68, Number 6
J u n e , 1964
~
,
C. H. CHEEKAND J. W. SWINNERTON
1430
cent kinetic analysis of the radiation chemistry of alkaline hypobromite solutions3 purports to show that the yields of the radiolysis products of water are essentially constant in pH range 4 to 14. The radiation chemistry of aqueous nitrous oxide solutions is being studied at this laboratory in an effort to resolve the discrepancy. Exploratory experiments revealed that, in the absence of gas space, G(H2) = 0 in 7-irradiated nitrous oxide solutions a t pH above 3. Dainton and co-workers,2 , 4 however, reported positive values for G(H2) throughout the pH range. Their positive values may be due to escape of radiolytic hydrogen into the gas space above their solutions. The disagreement afforded a clue which led to the work described here. I n order to obtain additional information, we have studied the radiation chemistry of solutions containing mixtures of nitrous oxide and hydrogen.
0.5 -
[NgOlo = 6 m M p H = 13.5
\
0.4
0.3
z
E
..
-
n
r"
0.2
Experimental Reagent grade chemicals were used without further purification. Solutions of the nongaseous constituents in triply distilled water were saturated with the desired gaseous mixture and transferred to the irradiation cells in a manner such that no gas space remained above the solutions. The apparatus and procedure for sample preparation have been described.j The relative amounts of gases in the mixture were controlled by the use of flow meters, and mixing occurred at the entrance to the purging vessel. The Co60y-ray source was calibrated by ferrous sulfate dosimetry. Dissolved gases were determined by a gas chroniatographic method which has been describeda6
Results and Discussion Figure 1 shows the chain disappearance of hydrogen with dose in a typical experiment a t pH 13.5. Figure 2 presents the variation of G(-H,) and G(N2) with pH for solutions initially containing -0.5 niM H2 and -6 mM XZO. The results show that no chain reaction occurs in strongly acid solution, a moderate chain reaction occurs in neutral solution, and a more efficient, strongly pH-dependent chain reaction occurs in the range above pH 12. G(NJ parallels G(-H2) in the chain reactions, and no oxygen is formed. Hydrogen peroxide quickly reaches a steady-state concentration < 2 MLM in neutral solution and a t p H > 12, but rises to higher steady-state concentrations a t 10 < pH < 12, with a maximum value of 40 MLM a t pH 10.5. Variation of the dose rate does not affect the yields In neutral or alkaline solutions. A trace of added oxygen causes a brief induction period before the chain reaction begins in neutral soluThe Journal of Physical Chemistry
0.I
b 5
I
IO
15
MINUTES Figure 1. Chain disappearance of hydrogen in strongly alkaline solution.
tion, but appears to have no effect on the chain reaction in strongly alkaline solution. The following mechanism is proposed to account for the results in neutral solution.
HzO -+ eaq-, H+, OH, H,, HzOz N20
+ eaq- -+Nz+ OH + OHH, + OH + H + HzO N2O + H + N2 + OH HzOz + H HzO + OH H + il/I -+-term
(0) (1)
(2) (3)
(4) (51
Essentially all the eaq- and OH are assumed to disappear by reactions 1 and 2 under the experimental condi(3) C. H. Cheek and V. J. Linnenbom, J . Phys. Chem., 67, 1866 (1963). (4) F. S. Dainton and D . B. Peterson, Proe. R o y . Soe. (London), 267, 443 (1962). (5) C. H. Cheek, V. J. Linnenbom, and J. W. Swinnerton, Radiation Res., 19, 636 (1963). (6) J. W. Swinnerton, V. J. Linnenbom. and C. H. Cheek, A n a l . Chem., 34, 483 (1962); ibid., 34, 1509 (1982).
RIIDIATIOX-IXDUCED C H A I X
REACTION BETWEEK NITEOUSO X I D E AND HYDROGEN
eaq-
6ot
40
*
O
O
I
47
2 L d PH
Figure 2 . Effects of pH, dose rate, and carbonate on G(-Hz) and G(X2). Initially -0.5 mM H2 and -6 mM XnO. Dose rate, 3.11 x 1020 e.v./l. min.: X, G(N2); 0, lC(-Hz); . , G(-H2) with 0.01 M carbonate present. Dose rate, 2.75 X IO19 e.v./l. min.: A, G(-H,). Dose rate, 1.75 X loz1 e.v./l. min.: e,G(N2): 0,G(-Hz).
tions. Reactions 2 and 3 comprise the chain oxidation of hydrogen by nitrous oxide. In a similar manner, hydrogen peroxide is reduced to water by reactions 2 and 4. The absence of a dose-rate effect shows that chain termination occurs principally by processes other than radical-radical combination. The reaction HzOz
+ OH
+ HOz
+ HzO
(6)
leads to chain termination, but it occurs much too infrequently to account for the chain length of -4, since IC2 = IC2 and (HZ) >> ( H 2 0 2 ) . No additional deconiposition products have been found, and it is assumed that chain termination is effected by impurities. The principal chain-termination process is represented schematically by eq. 5. The results in strong1,y alkaline solution are accountable by the follom~ingseyuence. OH -
HzO --+
eaq-, OH, HP,€IO2-
+ eaq- --+S,+ OH + OHHz + O H + H + HZO H + OH- --+ eaqeaq- + HOz--+2OH- + OH
NzO
(0')
1431
+ RI +term
(9)
Reactions 1, 2, and 7 constitute a chain sequence in which the oxidation of hydrogen by nitrous oxide is strongly augmented as the pH becomes high enough for reaction 7 to compete effectively for H atoms. Chain termination is considered to occur principally by the action of impurities, as discussed below. The observed decrease in reaction yield in the pH range 8 to 12 is not expected on the basis of the mechanisms proposed above, and the effect is attributed to reaction of reducing radicals with bicarbonate or carbonate introduced as an impurity in the sodium hydroxide. The lower curve in Fig. 2 shows that 0.01 M carbonate completely suppresses the chain reaction up to pH 12. The pH dependence of the carbonate effect suggests that the impurity competes effectively with reaction 3 for hydrogen atoms, but not with reaction 7 above pH 12. It appears that bicarbonate (pK = 10.2) is much more effective than carbonate. This is in agreement with the report* that the absorption spectrum of the hydrated electron has been observed in carbonate solution, but is suppressed completely in bicarbonate solution. There can be little doubt that impurities played a major role in chain termination in neutral a n d alkaline solutions, Experiments with carbonate-free solutions are clearly desirable and are being pursued. A value of G(Xz) = 10 was obtained a t pH 11.2 in a very recent experiment in which the carbonate concentration was -5 p M . This suggests that the depression in reaction yields a t pH 8 to 12 may be eliminated by the use of carbonate-free solutions and provides further credence to the assignment of chain termination to bicarbonate. It is noteworthy that the maximum steady-state concentration of peroxide is obtained in the pH range of maximum suppression of the chain reaction, as is to be expectcd if the chain-terminating reaction also suppresses the removal of peroxide by reaction 8. The increased efficiency of the chain reaction a t high pH is attributable to the large difference between the rate constants of reactions 1 and 3. Dainton and 10s-lOgM - I see.-', and CzapPeterson4 estimated IC1 ski and Jortnerg reported k 3 103-104M-' see.-'. The occurrence of reaction 7 has been demonstrated unequivocally. Io Inhibition of the chain reaction by oxygen in neutral
--
(1) (2)
(7) (8)
(7) A. 0. Allen, "The Radiation Chemistry of Water and Aqueous Solutions," D. Van Nostrand Co., New York, K. Y., 1961, p. 86. (8) E. J. H a r t and J. W. Boag, J . Am. Chem. Soe., 84, 4090 (1962). (9) G. Czapski and J. Jortner, S u t u r e , 188, 50 (1960). (10) 3. Jortner and J. Rabani, J . Am. Chem. SOC.,83,4868 (1961).
Volume 68, Number 6
J u n e , 1964
ROBERT L.CLELAND
1432
solution is attributed to competition of reaction 10 with reaction 3.
H
+
0 2
+ HOz
(10)
Takingll kin = 1.2 x 1O1o M-l set.-', one obtains kIo/k3 > l o 6 , which indicates that as little as 10-6 M oxygen will suppress reaction 3 completely. The chain reaction commences as the oxygen 'is depleted. Small amounts of oxygen, however, do not conipete effectively with the very efficient chain sequence in strongly alkaline solution. These observations suggest
that oxygen produced in the radiolysis of K 2 0 solutions. prevents the net reaction between X20and H2in neutral solution, but permits G H , to contribute to G(NJ in strongly alkaline solution, thereby accounting in part for the observed increase in G(N2)a t high pH. This point and additional details will be considered in a publication on the radiation chemistry of n T Z 0 solutions. Acknowledgrnent. The authors are grateful to E . 0. Davis and G. R. Gauvin for technical assistance. (11) 9.R. Anderson and E. J. H a r t , J . P h y s . Chem., 6 6 , 70 (1982).
Adsorption from Nonelectrolyte Solutions on Porous 96% Silica (Vycor) Glass
by Robert L. Cleland Department of Chemistry, Dartmouth College, Hanover, N e w Hampshire
(Receieed December 21, 1965)
Adsorption studies on porous (Vycor) glass for the binary liquid systems: (I) C6H6(l)CC14, (11) C6He(l)-C6H12, and (111) CCl4(1)-CsHl2showed that component 1 was preferentially adsorbed a t all compositions. The difference, Aw,between the potential energies of adsorption of a given component pair estimated from a linear plot of a Langmuir isotherm is too large when the surface shows normal heterogeneity of site-adsorbate interaction energy. Estimated surface compositions indicate that Aw is smaller on a porous glass surface than on silica gels of larger specific surface area. The difference is ascribable to larger adsorption potentials in the smaller pores of silica gels and to partial thermal dehydration of the porous glass surface.
Introduction The supposition that transport properties of binary liquids in porous 96% silica (Vycor) glass1 (hereafter referred to as porous glass) were affected by selective adsorption led to an experimental study of adsorption froin binary solutions of interest. The data are compared with similar results for adsorption on silica gels having smaller pores. Reduction in pore size in silica gels has been shown2 to cause an increase of total adsorption of benzene and n-hexane vapors a t low surface coverage. Similar effects occurred when the gel surface was dehydrated by heating.2 The effect of reduction in pore size was atThe Journal of Physical Chemistry
tributed to an increased adsorption potential in smaller pores. The dehydration effect was explained by Kiselev as due to decreased interaction of the a-electron syst,eni of benzene with surface hydroxyl groups as the latter were removed. An increase in the adsorption of benzene per unit surface from benzene-n-heptane solutions also occurred with decreasing pore size in silica g e k 3 This effect was ascribed to a greater increase
(1) R. L. Cleland, "Binary Liquid Flow in Porous 98% Silica (Vycor) Glass," to be published. (2) 4.V. Kiselev, Proc. Intern. Congr. Surface Actinity, O n d , London, 2 , 179 (1957).
