Potentiometric determination of the first hydrolysis constant of gallium(III) in NaCl solution to 100 degrees 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 hydrog en ion concentration cell at temperatures from 25 to 100 degrees C at 25 de grees C intervals. Potentials at temperatures above 100 degrees C increased gradually because of further hydrolysis of the gallium(III) ion, followed by precipitation. The results were treated with a nonlinear least-squares c omputer program to determine the equilibrium constants for gallium(III)-hyd roxo complexes using the Debye-Huckel equation. The log K (mol-kg(-1)) valu es of the first hydrolysis constant for the reaction, Ga3+ + H2O reversible arrow GaOH2+ + H+ were -2.85 +/- 0.03 at 25 degrees C, -2.36 +/- 0.03 at 5 0 degrees C, -1.98 +/- 0.01 at 75 degrees C, and - 1.45 +/- 0.02 at 100 deg rees C. The computed standard enthalpy and entropy changes for the hydrolys is reaction are presented over the range of experimental temperatures.