Potentiometric determination of the first hydrolysis constant of gallium(III) in NaCl solution to 100°C

The hydrolysis equilibrum of gallium (III) solutions in aqueous 1 mol-kg^-1 NaCl over a range of low pH was measured potentiometrically with a hydrogen ion concentration cell at temperatures from 25 to 100°C at 25°C intervals. Potentials at temperatures above 100°C increased gradually because of further hydrolysis of the gallium(III) ion, followed by precipitation. The results were treated with a nonlinear least-squares computer program to determine the equilibrium constants for gallium(III)-hydroxo complexes using the Debye-Hückel equation. The log K (mol-kg^-1) values of the first hydrolysis constant for the reaction, Ga³⁺ + H₂O ⇆ GaOH²⁺ + H⁺ were -2.85 ± 0.03 at 25°C, -2.36 ± 0.03 at 50°C, -1.98 ± 0.01 at 75°C, and - 1.45 ± 0.02 at 100°C. The computed standard enthalpy and entropy changes for the hydrolysis reaction are presented over the range of experimental temperatures.