Raman and Infrared Spectra and Structures of the Aluminate Ions

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Abstract
The Raman and infrared spectra of the aluminate ions in alkaline solutions have been measured. Two ionic species, one predominant from pH 8 to pH 12 and the other predominant above pH 13, may be identified from the spectra. At intermediate pH's both forms are present. The selection rules and the polarization of the lines of the low-pH species are consistent with either a square-planar Al(OH)4− structure or a polymeric structure in which each metal ion is surrounded by six OH− ions and the perturbations on the Oh symmetry are not great enough to be resolved. The ion does not appear to be tetrahedral Al(OH)4−, as has been previously thought. Pronounced aging effects observed with the low-pH anion support a polymeric structure rather than the square-planar Al(OH)4−. The spectra show that the high-pH species is AlO2− with D∞h symmetry. The observation of two distinct species and measurements of the pH ranges over which they are stable, confirm the results of earlier measurements by some of these authors using Leden's techniques of studying complex-ion formation. The Raman spectra of deuterated aluminate solutions were measured. The low-pH species shows the expected frequency shifts and the frequency of vibration of the high-pH form is not affected by deuteration, in agreement with the supposition that the aluminate ion contains hydrogen at low pH but does not contain hydrogen at high pH.