A hydrogen electrode study of concentrated alkaline aluminate solutions

A detailed study of highly concentrated aqueous alkaline aluminate solutions has been made by hydrogen electrode potentiometry in cells with liquid junction at 25 degrees 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 greater than or equal to [OH-](T)/[AL(III)](T) greater than or equal to 4.3. Nernstian electrode behaviour was observed at [NaOH](T) less than or equal to 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.