Forms of H and OH produced in the radiolysis of aqueous system

Abstract

As the pH of an aqueous solution of nitrous oxide is increased from 0.1 to 4, G(N$_2$) for the $\gamma$-irradiated system increases sigmoidally from 0.75 to 3.1 and G(O$_2$) from 0.1 $\pm$ 0.1 to 1.23 while G(H$_2$) decreases from 0.45 to 0.2 and G(H$_2$O$_2$) decreases slightly. At all higher pH values G(H$_2$) is constant, but G(N$_2$) and G(O$_2$) are constant only up to pH 11.2 when G(N$_2$) increases suddenly by about one unit and G(O$_2$) possibly undergoes a small increase. These data and the effect of pH on G(Fe$^{3+}$), G(N$_2$) and G(H$_2$) for N$_2$O-containing, de-aerated solutions of ferrous sulphate are explicable in terms of a primary act for $\gamma$-irradiated aqueous systems represented by the equation $4.5 H_2O\rightsquigarrow^\gamma_{100 eV}0.45 H_2+(0.8-0.015 pH)H_2O_2+(2.05+0.03 pH) OH+2.75 H_a+0.75 H_2O*,$ where H$_a$ is an entity, stoichiometrically equivalent to a hydrogen atom, which can react with N$_2$O ultimately forming N$_2$ and an entity stoichiometrically equivalent to a hydroxyl radical. H$_a$ reacts with H$^+$ to form H$_b$ which reacts with N$_2$O much more slowly than H$_a$. H$_2$O* is an excited water molecule which can revert to the ground state (lifetime > 10$^{-9}$s) or react with sulphuric acid to form OH or its equivalent and H$_c$, equivalent to a hydrogen atom and which can react with N$_2$O in a manner similar to H$_a$. All three species, H$_a$, H$_b$ and H$_c$ can oxidize ferrous ions and reduce ceric ions in a stoichiometrically equivalent manner. Their possible identities are discussed and it is concluded that probably H$_a$ = e$^-_{aq.}$, H$_b$ = H and H$_c$ = H$^+_2$. The postulated existence of H$_2$O* gives a rational description of the hitherto unexplained increases in G$_{OH}$ and `conventional' G$_H$ which occurs at pH < 3.5. The `step' in the curve of G(N$_2$) against pH at 11.2 is attributed to ionic dissociation of the hydroxyl radical ($\rightarrow$ H$^+$ + O$^-$) and the reaction O$^-$ + N$_2O \rightarrow$ (N$_2$O$^-_2) \rightarrow \frac{1}{2}N_2$ + NO$^-_2$.