Sodium aluminate solutions have been investigated by infrared absorption and Raman scattering in a broad concentration range in order to detect possible changes in aluminate ion species and distribution. Concentration-dependent changes in spectra include shifts in frequency at maximum band intensity, a loss of proportionality in band intensities, and the appearance of new vibrational bands or the increased asymmetry of existing ones. Changes in infrared spectra correlate well with those observed for Raman spectra and are consistent with the presence of more than one aluminate ion species in equilibrium in concentrated aluminate solutions. Assuming that the observed effects are due to changed solution speciation, it is found that the maximum concentration of Al(OH)4 occurs in solutions of 4-5 M aluminum, and that other species become relatively more abundant as the aluminum concentration is increased. Comparison of solution spectra with spectra of crystalline aluminates indicates that new species are likely to be oligomeric and polymeric anions in which aluminum is mainly 4-coordinate, but the possibility of low concentrations of aluminate species containing 6-coordinate aluminum cannot be excluded.
Helen Watling, "Spectroscopy of Concentrated Sodium Aluminate Solutions," Appl. Spectrosc. 52, 250-258 (1998)