ALUMINUM SPECIATION AND EQUILIBRIA IN AQUEOUS-SOLUTION .1. THE SOLUBILITY OF GIBBSITE IN THE SYSTEM NA-K-CL-OH-AL(OH)4 FROM 0-DEGREES TO 100-DEGREES-C

The solubility of gibbsite, Al(OH)3, has been measured in a large number of solutions in the system Na-K-Cl-OH-Al(OH)4 from 6.4 to 80-degrees-C and 0.01 to 5.0 molal ionic strength. These results have been coupled with selected literature data and modeled using the Pitzer ion interaction treatment. An eight-parameter expression is derived for the molal concentration quotient of the reaction Al(OH)4- reversible Al(OH)3 + OH- in this system from which the equilibrium constant at infinite dilution, log K4, and the stoichiometric molal activity coefficient ratio, log (gamma-OH-/gamma-Al(OH)4-), can be evaluated over the range 0-100-degrees-C and 0-12 molal ionic strength at 1 bar with a precision of approximately +/- 0.02 log units. Standard thermodynamic properties of the reaction at 25-degrees-C and 1 bar are log K4 = 1.143 +/- 0.006, DELTA-H-degrees = -22.5 +/- 0.3 kJ/mol, DELTA-S-degrees = -54 +/- 1 J/K/mol, and DELTA-C(p)0 = -148 +/- 6 J/K/mol. From these quantities and the standard thermodynamic properties of gibbsite, the values of DELTA-G(f)0, DELTA-H(f)0, and S0 of [Al(OH)4-](aq) at 25-degrees-C and 1 bar are computed to be -1305.6 +/- 1.2 kJ/mol, -1500.6 +/- 1.5 kJ/mol, and 111.4 J/K/mol, respectively. Approximate values of the pure electrolyte parameters beta(0), beta(1), and C(phi) for NaAl(OH)4 and KAl(OH)4 as well as the mixing parameters for aluminate ion with OH-, OH- + Na+, and OH- + K+ are also obtained.