Thermodynamic Properties of Al(III) Hydroxysalt in Sulfate Aqueous Solution at 333 K

The present investigation has been carried out to examine the thermodynamic behavior of aluminum in the hydrolysis operation of aluminum sulfate aqueous solution with sodium hydroxide solution and to determine the pH-dependence of the solubility of the precipitate produced in the hydrolysis at 333 K. The results obtained are summarized as follows. (1) The hydrolysis proceeds via two steps. In the case where the initial concentration of Al2(SO4)3 was less than 0.05 kmol·m−3, the precipitates produced in the first and second step hydrolyses were identified to be Al7(SO4)2(OH)17·xH2O and Al(OH)3, respectively. These hydrolysis reactions are represented by 7Al3++αSO42−+βHSO4−+17H2O=Al7(SO4)2(OH)17+(17+β)H+, where α+β=2, and Al7(SO4)2(OH)17+4H2O=7Al(OH)3+2H2SO4. (2) For the equilibrium constant, K, of the first step hydrolysis at 333 K, logK was calculated to be −35.6±1.1, from which −8489±7 kJ·mol−1 was obtained as the standard free energy of formation of Al7(SO4)2(OH)17 at the same temperature. (3) The pH-dependence of the solubility of Al7(SO4)2(OH)17 was determined for various choices of a parameter, which can be calculated from the composition of solutions immediately before the beginning of the first step hydrolysis reaction. (4) A method was proposed which enables one to calculate the activity coefficients of H+, HSO4−, and SO42− ions for the ternary system H2SO4–Na2SO4–H2O as a function of ionic strength by using H2SO4-molality as a parameter.