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, a_w. 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 a_w 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 a_w 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.