On the mechanism for phosphate-mediated increase of Cl2− yield in irradiated, neutral chloride solution

The effect of phosphate buffer components on the radiation induced oxidation of chloride to Cl₂⁻ is studied by pulse radiolysis. The yield of Cl₂⁻ in neutral phoshate-free 0.13 mol dm⁻³ NaCl solution is much lower than in the corresponding acidic solution (pH < 3), but increases rapidly with increasing chloride concentration. Addition of 0.2 mol dm⁻³ phosphate buffer (pH 6.8) to 0.13 mol dm⁻³ NaCl results in about a five-fold increased yield of Cl₂⁻ in N₂-saturated solution, and about a 10-fold increase in N₂O. At higher chloride concentrations, the total yield both in N₂ and N₂O is near that at pH 2 when G(Cl₂⁻) is suggested to be equal to G(OH). The phosphate mediated enhancement of G(Cl₂⁻) in neutral solution is suggested to be caused by a proton transfer from H₂PO₄⁻ to the intermediate chloride oxidation product ClOH⁻. From the effect of H⁺ and phosphate on G(Cl₂⁻ we conclude that the rate of the proton donation is about 100 times slower than that for the direct reaction between protons and ClOH⁻.

The results are discussed in the light of previously suggested mechanisms for radiation-induced oxidation of chloride.