HYDROLYSIS OF THE ALUMINUM(III) ION IN SODIUM-CHLORIDE MEDIUM

The hydrolysis of the aluminium(III) ion in 3.0 M sodium chloride medium has been studied by the EMF method at 25-degrees-C. The data collected show that 2.5-500 mM Al(III) hydrolyzes in the pH range 2.5-4.5. At a definite pH, the extent of hydrolysis increases with increasing aluminium concentration, thereby indicating the presence of polynuclear complexes. The least-squares treatment of the data indicates the formation of complexes [Al2(OH)2]4+, [Al2(OH)4]2+ and [Al3(OH)4]5+ with the following stability constants: log beta-2,2 = -7.53 +/- 0.15, log beta-2,4 = -16.50 +/- 0.25 and log beta-2,4 = -13.44 +/- 0.05. The complex [Al2(OH)2]4+ was observed only at lower values of the hydroxide number. Distribution curves indicate that the complex [Al3(OH)4]5+ is predominant.