The photochemical evolution of hydrogen from aqueous solutions of ferrous ions. Part II. Effect of changing pH

The photochemistry of the ferrous ion in the presence and in the absence of oxygen was investigated in the pH region 0.35-3.0. The pH dependence of the initial quantum yield up to pH 2.5 in the presence and absence of oxygen is interpreted as arising from the dependence on H⁺ ion concentration of the rate of introduction of H atoms into the bulk. This results in the dependence of the quantum yield on √[H⁺]. A mechanism to account for this is proposed. Another pathway leading to the pH independent formation of H atom also is postulated. The oxidation and reduction processes of the H atoms thus formed are investigated and it is shown that the oxidation of Fe²⁺ by H atoms may proceed by way of a hydride intermediate. Specific velocity constants are derived. The results of the photochemical experiments are correlated with those obtained in radiation chemistry.