Quantitative determination of an aluminate dimer in concentrated alkaline aluminate solutions by Raman spectroscopy
Abstract
Raman spectra of concentrated alkaline aluminate solutions in various M′OH media (M′+ = Na+, K+, Cs+ and (CH3)4N+) have been investigated systematically as a function of concentration and water activity, aw. All spectra at [Al(III)]T ≤ 1 M and at 0.1 ≤ [M′OH]T/M ≤ 5 (where the square brackets denote concentrations and the subscript T totals) exhibit one significant Raman mode in the low frequency region, at ca. 620 cm−1, due to the symmetric Al(OH)4− stretch. At higher [Al(III)]T and [M′OH]T new modes appear at 530–550 and 700–720 cm−1. The intensities of these new bands depend on [Al(III)]T and aw but are independent of [OH−]T and are only slightly cation-dependent. All three bands shift towards higher wavenumbers at [M′OH]T > 10 M, probably due to ion-pairing. Spectra at [M′OH]T < 10 M have been interpreted quantitatively by assuming that the integrated peak area of the 620 cm−1 mode is linearly proportional to [Al(OH)4−] at constant aw and that the only significant equilibrium in these systems is the formation of a dimer that can be represented as (Al(OH)4)22−(aq), although it may exist in an oxo-bridged form such as [(HO)3Al–O–Al(OH)3]2. The (aquated) species Na+, OH−, Al(OH)4−, the dimer, and their ion-pairs, were sufficient to interpret all the Raman observations. No evidence was found for various other species that have been claimed to exist in concentrated alkaline aluminate solutions.