γ Radiolysis of Liquids at High Pressures. V. Reaction of the Hydrated Electron with Water

Abstract

The γ radiolysis of deaerated aqueous solutions containing 10⁻² or 10⁻³M glucose and 10⁻⁵ to 10⁻⁴M of H₂O₂ or NO₃⁻ has been studied at ∼20° in the applied pressure range 0 to 6.34 kbar. The behavior of H₂O₂ and NO₃⁻ solutions is essentially indistinguishable. In both the H₂O₂ and NO₃⁻ solutions, G(H₂) increases with increase in pressure; at 6.34 kbar, G(H₂) decreases with increase in H₂O₂ or NO₃⁻ concentration. However, the pressure dependence of G(H₂) is not affected by a change in glucose concentration from 10⁻² to 10⁻³M or by a change in atmospheric‐pressure pH from ∼7 to 9. The results are interpreted in terms of an effect of pressure on the competition between Reactions [4] and [5], $$e_{\mathrm{aq}}^{-}+{\mathrm{NO}}_3^{-}(\mathrm{or}\hspace{0.17em}{\mathrm{H}}_2{\mathrm{O}}_2\mathrm{)\to }\mathrm{no}\hspace{0.17em}\mathrm{H}\hspace{0.17em}\mathrm{or}\hspace{0.17em}{\mathrm{H}}_2,$$ $$e_{\mathrm{aq}}^{-}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{H}+{\mathrm{OH}}^{-},$$ with Reaction [5] followed by H+RH→H₂+R. Such an interpretation gives a difference in activation volumes of Reactions [4] and [5] as ΔV₄^‡—ΔV₅^‡ ≈ 15.7 ml mole⁻¹ and a specific rate k₅⁰ ≈ 25M⁻¹·sec⁻¹ at atmospheric pressure. Assumption that Reaction [4] is diffusion controlled permits estimation of ΔV₄^‡ ≈ 1.6 ml mole⁻¹, from the pressure dependence of water viscosity. Thus ΔV₅^‡ ≈ −14.1 ml mole⁻¹. The latter value is used to estimate an absolute value of ${\mathrm{V̄}}_0{\mathrm{(e}}_{\mathrm{aq}}^{-})$ in the range −1.7 to either 2.7 or 9.6 ml mole⁻¹ (dependent on the model assumed for the activated complex); such a range for ${\mathrm{V̄}}_0{\mathrm{(e}}_{\mathrm{aq}}^{-})$ is in satisfactory agreement with a previous estimate.