Ionisation constants of ˙OH and HO ˙2 in aqueous solution up to 200 °C. A pulse radiolysis study

Abstract

The ionisation constant of ^˙OH has been determined up to 200 °C from measurements of its rate of reaction with CO^2–₃ in alkaline solution, where O^˙– is unreactive. The Van't Hoff plot shows the same curvature as that for K_w, so that ^˙OH and H₂O have the same heats of ionisation. ΔH for the equilibrium ^˙OH + OH^–⇌ O^˙–+ H₂O is independent of temperature and equal to –15.4 ± 0.5 kJ mol^–1. ΔH for the ionisation of HO^˙₂ has been measured as zero within experimental error up to 175 °C by determining the concentrations of HO^˙₂ and O^˙–₂ at equilibrium from optical absorbance measurements. The activation energies for the reaction of ^˙OH with CO^2–₃ and HCO^–₃ are independent of temperature and equal to 23.6 ± 0.4 and 21.2 ± 0.2 kJ mol^–1, respectively, although k(^˙OH + CO^2–₃)≈ 50 k(^˙OH + HCO^–₃). The G values for e^–_aq and ^˙OH increase by 27% between 20 and 200 °C in the pH range 4.4–9.2, but appear to be independent of temperature at pH 2.