Solubility Products of Aluminum Hydroxide in Various Ionic Solutions

Solubility products of Al(OH)3 were determined in various solutions of NH4+, Li+, Na+, K+, and Ca2+ ions. Increasing amounts of these hydroxides were added to the respective chloride solutions containing aluminum chloride. The solutions were aged for 3 months. Activities of the OH− ions were calculated from the pH values of the solutions. The total Al concentrations remaining in solutions were determined by Hsu's modified aluminon method. The Al3+ ion concentrations were computed using the hydrolysis equilibria of Aveston. The ion product, aAl3+(aOH−)3, was found to vary linearly with the [OH]/[Al] ratio. The solubility product of Al(OH)3 was obtained by extrapolating this ratio to 3. The pKsp values thus obtained were 30.55, 33.15, 30.75, 30.36, and 31.00 in NH4+, Li+, Na+, K+, and Ca2+ systems respectively. The anomalously low solubility product of Al(OH)3 in the Li+ system was attributed to the specific "lithium effect".