ALUMINUM SPECIATION AND EQUILIBRIA IN AQUEOUS-SOLUTION .2. THE SOLUBILITY OF GIBBSITE IN ACIDIC SODIUM-CHLORIDE SOLUTIONS FROM 30-DEGREES-C TO 70-DEGREES-C

The solubility of gibbsite in aqueous solutions was measured at ten ionic strengths made up of NaCl, HCl, and AlCl3 at 30, 50, and 70-degrees-C with the initial acidity controlled by addition of HCl. The aluminum concentration was determined by ion chromatography, while the final equilibrium pH was measured at temperature. The equilibrium quotients for the reaction Al(OH)3 + 3H+ reversible Al3+ + 3H2O were modeled using both an empirical equation including the Debye-Huckel term and the Pitzer ion interaction treatment which incorporated the relevant single electrolyte and mixing interaction parameters currently available in the literature. In the latter treatment only four independent variables, including theta(AlNa, psi(AlNaCl), and two terms describing the equilibrium constant at infinite dilution, were needed to fit the data well within the projected experimental error. In general, these new equilibrium quotients differ markedly from results of all but the most recently published solubility studies. The thermodynamic parameters at infinite dilution are compared with those calculated from the individual components of the reaction available in the literature. These calculations lead to recommended thermodynamic values for the Gibbs energy of reaction at 25-degrees-C of -44.2 +/- 0.3 kJ mol-1, a DELTA-G(f)0 (Al3+, aq) of -487.7 +/- 1.5 kJ mol-1, and a DELTA-H(f)0 (Al3+, aq) of -540.9 kJ mol-1. No evidence for aluminum chloride complexation was found by comparing solubility experiments in the presence of varying concentrations of sodium trifluoromethanesulfonate and sodium chloride at 50-degrees-C and ca. 5 molal ionic strength.