Quantitative determination of an aluminate dimer in concentrated alkaline aluminate solutions by Raman spectroscopy

Raman spectra of concentrated alkaline aluminate solutions in various M'OH media (M'(+) = Na(+), K(+), Cs(+) and (CH(3))(4)N(+)) have been investigated systematically as a function of concentration and water activity, a(w). All spectra at [Al(III)](T) < or = 1 M and at 0.1 < or = [M'OH](T)/M < or = 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))(2)(2-)(aq), although it may exist in an oxo-bridged form such as [(HO)(3)Al-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.