The Dissociation Constant of Antimonic Acid at 10–40 °C

Abstract

The hydrolytic behavior of antimonic acid, Sb(OH) 5 o , was experimentally investigated, at fixed temperatures within the range 10–40 °C, by both titration of dilute Na-antimonate solutions with HClO4 and single-point pH measurements of diluted Sb(OH) 5 o solutions. The thermodynamic constants, K a, for the reaction: $$\mathrm{Sb(OH)}_{5}^{\mathrm{o}}+\mathrm{H}_{2}\mathrm{O}\,\rightleftharpoons\,\mathrm{Sb(OH)}_{6}^{-}+\mathrm{H}^{+}$$ were derived at different controlled temperatures, based on pH measurements, applying suitable mass and electrical balances and correcting the concentrations of ionic species for medium effects. From the resulting log 10 K a values, those of the corresponding isocoulombic equilibrium reaction: $$\mathrm{Sb(OH)}_{5}^{\mathrm{o}}+\mathrm{OH}^{-}\,\rightleftharpoons\,\mathrm{Sb(OH)}_{6}^{-}$$ were computed and used to derive its thermodynamic properties. These were finally combined with the corresponding thermodynamic properties of the water association reaction, to obtain robust estimations of ΔG r o , ΔS r o and ΔH r o for the ionogenic reaction. These are the first thermodynamic data at temperatures different from 25 °C for the ionization reaction of Sb(OH) 5 o . The results of the present work confirm that Sb(OH) 5 o is a moderately weak and monoprotic acid with a pK a of 2.848 at 25 °C.