Kinetics of Oxygen Exchange between Arsenic Acid and Solvent Water

The rates (R) of the oxygen exchange of arsenic acid with solvent water were measured over a wide range of hydrogen-ion concentrations. At pH > 4, a 18O tracer method was used ([As(V)] = 0.07 mol dm−3, 0—50 °C). At pH < 5, 17O NMR line broadening technique was used ([As(V)] = 1 mol dm−3, 50—90 °C). Over the whole pH range studied, the variation of R amounts to 1010, and the comprehensive rate law is expressed as: R = k11h[H+][H3AsO4] + k11[H3AsO4] + k12[H2AsO4−] + k13[HAsO42−] + k14[AsO43−] + k21[H3AsO4]2 + k22[H3AsO4][H2AsO4−] + k23[H2AsO4−]2 + k24[H2AsO4−][HAsO42−] + k25[HAsO42−]2. The rate constant for each elementary process in the equation was obtained. The activation parameters of each elementary process were evaluated. The oxygen exchange rate of arsenic acid was compared with that of phosphoric acid. The contrast of an SN2-type exchange of the former acid vs. an SN1-type one of the latter is suggested.