Effect of Anions on the Hydrolysis of Aluminum Ion

Abstract

The effect of anions on the hydrolysis of aluminum ions has been studied to understand the roles of anions in the coagulation treatment of water with aluminum salts. A modified 8- quinolinolate extraction procedure reported earlier was used for the determination of mononuclear aluminum species in the presence of the polynuclear species. Anions can be classified according to their function into three groups: (1) those which have no effect on the hydrolysis of aluminum ions, (2) those which shift the optimum pH range for coagulation to the acid side, (3) those which shift it to the alkaline side. It is shown that anions of group 2 stabilize polynuclear aluminum species while anions of group 3 stabilize mononuclear aluminum species. Anions of group 2 exert a greater influence on the coagulation reaction. At high pH region (>7) anions have little effect on the hydrolysis equilibrium. The following solubility products were obtained:. precipitate Polynuclear hydroxo-aluminum sulfate Aluminum hydroxide solubility product [Al³⁺][OH^-]^2.68±0.04[S04^2-]^{0, 22±0.02}=10^-29.6±0.4(25°C, A=0) [Al³⁺][OH^-]³=10^-32.94±0.06(23±2°C, u=0.1) [Al(0H)4^-][H⁺]=10^-12.28±0.07(12 min.20°C. u=0.1)