Thermodynamic Model of Inorganic Arsenic Species in Aqueous Solutions. Potentiometric Study of the Hydrolytic Equilibrium of Arsenious Acid

A potentiometric study of the hydrolysis of arsenious acid was carried out to define the thermodynamic model of the inorganic arsenic species in aqueous solutions. The protonation equilibrium of arsenious acid was determined at 25^°C. The variation of the stoichiometric formation constant with the ionic strength was also studied up to ionic strength 3.0 mol-dm^–3 in aqueous NaClO₄, NaCl, and KCl. The thermodynamic formation constant of arsenious acid (log K ^o = 9.22 ± 0.01) and the various interaction parameters were computed using the Modified Bromley Methodology (MBM), for both the molar and molal concentration scales at constant temperature (25^°C). The results showed the importance, not only of ionic strength, but also of the composition of the ionic medium on the distribution of the acid-base As(III) species as a function of pH in natural waters.