Aluminum speciation and equilibria in aqueous solution: III. Potentiometric determination of the first hydrolysis constant of aluminum(III) in sodium chloride solutions to 125°C

Abstract

The first molal hydrolysis quotient of aluminum(III) was measured potentiometrically from 25 to 125°C at 25° intervals at ionic strengths of 0.1, 0.3, 1.0 and 5 mol · kg −1 with sodium chloride as the supporting electrolyte. The experimental method involved using a hydrogen-electrode concentration cell modified to compensate for any intrinsic potential offset between the two electrodes. The initial concentration of Al 3+ was varied to test for the presence of multinuclear aluminum species while being kept to a maximum of 10 −3 mol · kg −1 to minimize their occurrence. Similarly, the maximum degree of hydrolysis of Al 3+ reached in each titration was ca. 30%, after which polymerization and/or precipitation became apparent. The equilibrium quotients obtained in this study and selected values from the literature were fitted by an empirical equation incorporating a linear dependence of log K 1,1 on the reciprocal temperature (Kelvins) over the range 10–200°C and three ionic-strength-dependent parameters. Comparisons are made between the results of this study and the literature values.