Kinetics of pyroarsenate hydrolysis in aqueous solution

Abstract

The rate of hydrolysis of pyroarsenate in 0.1 mol dm^-3 NaClO₄ solutions, pH 6-9, was measured by a pH-stat technique at temperatures ranging from 278 to 298 K. The pK_a of HAs₂O₇^3-, found to be the predominant reactant under these conditions, was 7.3 and 7.6 at 283 and 298 K, respectively. The hydrolysis rate constant for HAs₂O₇^3- was 0.05 s^-1 at 298 K with activation parameters ΔH^? = 49 ± 9 kJ mol^-1 and ΔS^? = -107 ± 30 J mol^-1 K^-1. An associative mechanism is indicated.