γ Radiolysis of Liquids at High Pressures. X. The Reaction H+OH− and the Partial Molal Volume of the Hydrated Electron

$\mathrm{G(}{\mathrm{H}}_2)$ is independent of pressure over the range 0–6.2 kbar in $\gamma$ radiolysis of the following deaerated aqueous solutions at 23°C: (a) solutions with $\text{0.001 M}\mathrm{or}\text{0.1 M}$ hydroxide, isopropanol at concentrations in the range ${\text{9\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−4}}M\mathrm{to}\text{0.1 M}$ , and nitrate at one‐fiftieth of the isopropanol concentration; (b) solutions with $\text{0.01 M}$ formate, $\text{0.001 M}$ nitrate, and $\text{0.01 M}\mathrm{or}\text{0.1 M}$ hydroxide; (c) near‐neutral solutions with $\text{0.01 M}$ formate and $\text{0.01 M}\mathrm{or}\text{0.002 M}$ benzyl alcohol. From pressure independence of $\mathrm{G(}{\mathrm{H}}_2)$ for such solutions and the established pressure independence of the primary yields of water radiolysis, it follows that activation volumes for the reactions of H with hydroxide, isopropanol, formate, and benzyl alcohol are equal. Consequently, ${\mathrm{\Delta V}}_1^{‡}\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}5.9}{\mathrm{ml\; mole}}^{\text{−1}}$ is obtained for Reaction [1], $$\mathrm{H}+{\mathrm{OH}}^{-}\to {\mathrm{H}}_2\mathrm{O}{\mathrm{+e}}_{\mathrm{aq}}^{-}.$$ Combination of ${\mathrm{\Delta V}}_1^{‡}$ with the previously determined ${\mathrm{\Delta V}}^{‡}\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}14.2}{\mathrm{ml\; mole}}^{\text{−1}}$ for the back reaction gives $\mathrm{\Delta V}$ for Reaction [1]. From $\mathrm{\Delta V}$ , with a previous estimate of $\mathrm{V̄(}\mathrm{H})$ in the range 5.9–10.3 ml mole⁻¹, the absolute partial molal volume of $e_{\mathrm{aq}}^{-}$ (based on a value of − 5.4 ml mole⁻¹ for H⁺) is estimated to be in the range − 1.7–2.7 ml mole⁻¹. With an estimated electrostriction contribution of − 3 ml mole⁻¹, the electron cavity volume is in the range 1–6 ml mole⁻¹. In effect, such estimates of the electron cavity volume and ${\mathrm{V̄(e}}_{\mathrm{aq}}^{-})$ are obtained from the pressure dependence of an equilibrium constant.