The action of X-rays on ferrous and ferric salts in aqueous solutions

Abstract

The action of X-rays on the ferrous-ferric system has been studied under a variety of conditions. The H atoms and OH radicals formed primarily by the action of the radiation on the water react according to Fe³⁺ + H → Fe²⁺ + H⁺ and Fe²⁺ + OH → Fe³⁺ + OH^-. Experiments carried out in the presence of molecular hydrogen, where the latter reaction competes with the reaction H₂ + OH → H₂O + H, permit us to deduce that the specific rate constant of the reaction between OH radicals and ferrous ions is about five times greater than that of the corresponding reaction with hydrogen molecules. The study of the pH dependence of the reaction has led to the assumption that molecular hydrogen ions, H⁺_2(hydr.), intervene in this process undergoing the reaction Fe²⁺ + H⁺_2(hydr.) → Fe³⁺ + H₂, and that these ions exist in the equilibrium: H + H⁺_(hydr.) ⇌ H⁺_2(hydr.). Experimental evidence and some theoretical considerations which have led to the assumption of H⁺₂ in aqueous systems have been discussed in detail. In the presence of molecular oxygen the hydrogen atoms react according to H + O₂ → HO₂, followed by reactions of the latter radical (cf. Haber & Weiss 1934). A comparison of the experimentally determined yields under different conditions with the absolute (chemical) yields as derived from the proposed mechanism has led to the estimation of the energy (WH₂O) required for the production of a radical pair (H + OH) by the action of X-rays on water. This has been found to be WH₂O = 19⋅4 ± 0⋅4 eV.