Photochemistry of Ferrous Ions in Aqueous Solution

The photochemistry of ferrous ions in aqueous solutions has been investigated, and it has been confirmed that irradiation in the quartz ultraviolet, in the absence of oxygen, leads to the formation of ferric salt and molecular hydrogen. It has been concluded further that the primary absorption of a light quantum results in the formation of excited ions, (Fe²⁺·H₂O)^*, a considerable proportion of which undergo processes of deactivation, while the remaining (about one‐tenth) dissociate according to $$({\mathrm{Fe}}^{\text{2+}}·{\mathrm{H}}_2\mathrm{O}{)}^{*}\to {\mathrm{Fe}}^{\text{3+}}+{\mathrm{OH}}^{-}+\mathrm{H},$$ leading to the formation of hydrogen atoms. The latter can undergo back reactions of the type $${\mathrm{Fe}}^{\text{3+}}+\mathrm{H}\to {\mathrm{Fe}}^{\text{2+}}+{\mathrm{H}}^{+}.$$ Investigation of the pH dependence of the quantum yield has led to the assumption that hydrogen atoms can also react according to $$\mathrm{H}+{\mathrm{H}}_{(\mathrm{hydr})}^{+}\rightleftharpoons {{\mathrm{H}}_2}_{(\mathrm{hydr})}^{+},$$ leading to the formation of hydrated hydrogen molecular ions which intervene in this process according to the reaction $${\mathrm{Fe}}^{\text{2+}}+{\mathrm{H}}_2^{+}\to {\mathrm{Fe}}^{\text{3+}}+{\mathrm{H}}_2.$$ Other experimental evidence and some theoretical considerations which have led to the assumption of ${\mathrm{H}}_2^{+}{\hspace{0.17em}}_{\mathrm{(hydr)}}$ in aqueous solution are referred to.