A Hydrogen Electrode Study of Concentrated Alkaline Aluminate Solutions

Abstract

A detailed study of highly concentrated aqueous alkaline aluminate solutions has been made by hydrogen electrode potentiometry in cells with liquid junction at 25°C in 8 M Na(ClO₄) media. Measurements were performed over the total concentration ranges 0·1 M < [NaOH]_T < 4·0 M and 0·02 M < [Al^III]_T < 3·2 M, and at total concentration ratios of 6·92 ≥ [OH^-]_T/[Al^III]_T ≥ 4·3. Nernstian electrode behaviour was observed at [NaOH]_T ≤ c. 2 M in pure NaOH/NaClO₄ mixtures. At low [Al^III]_T the data were consistent with the formation of only the well known Al(OH)₄^-(aq) species but there is clear evidence of a systematic release of hydroxide as [Al^III]_T and [NaOH]_T increase. The data are consistent with the formation of polymeric aluminate complexes having a stoichiometry of Al_q(OH)_{3q - r}^r+ (q = 4–7, q + r = 1 or 2). However, unequivocal identification of these species is difficult on account of strong correlations in the data and the possible effects of ion-pairing.