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

摘要

Raman spectra of concentrated alkaline aluminate solutions in various M'OH media (M~('+) = Na~+,K~+,Cs~+ and (CH_3)_4N~+) have been investigated systematically as a function of concentration and water activity,a_w.All spectra at [A1(III)]_T <= 1 M and at 0.1 <= [M'0H]_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 A1(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 alpha_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 [A1(OH)_4~-] at constant alpha_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)_3A1-O-A1(OH)_3]~2.The (aquated) species Na~+,OH~-,A1(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.