27Al NMR and Raman spectroscopic studies of alkaline aluminate solutions with extremely high caustic content - Does the octahedral species Al(OH)63- exist in solution?
Sipos, P., Hefter, G. and May, P.M. (2006) 27Al NMR and Raman spectroscopic studies of alkaline aluminate solutions with extremely high caustic content - Does the octahedral species Al(OH)63- exist in solution? Talanta, 70 (4). pp. 761-765.
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27Al NMR and Raman spectra of alkaline aluminate solutions with 0.005 M ≤ [Al(III)]T ≤ 3 M in various M′OH solutions (M′+ = Na+, K+ and Li+) were recorded and analysed. Caustic concentrations up to 20 M were used to explore whether higher aluminium hydroxo complexes are formed at extremely high concentrations of hydroxide. A single peak was observed on the 27Al NMR spectrum of each solution. The chemical shift of this peak shifts significantly upfield with increasing [M′OH]T in solutions with [Al(III)]T < 0.8 M. This variation shows a strong dependence on the cation of the solution and practically disappears in systems with [Al(III)]T ≥ 0.8 M. For Raman spectra of solutions with [Al(III)]T = 0.8 M and [NaOH]T ≥ 10 M, the peak maximum of the symmetric ν1-AlO4 stretching of Al(OH)4- shifted progressively from ∼620 to ∼625 cm-1 and decreased in intensity with increasing [NaOH]T. In parallel, modes centred at ∼720 and ∼555 cm-1 (cf. ∼705 and ∼535 cm-1 at lower [NaOH]T, ascribed to a dimeric aluminate species appeared, and their intensities increased with increasing [NaOH]T. These variations in the 27Al NMR and Raman spectra can be interpreted in terms of contact ion-pairs formed between the cation of the medium and the well-established Al(OH)4- or the dimeric aluminate species. Assumption of higher aluminium hydroxo complex species (e.g., Al(OH)63-) is not necessary to explain the spectroscopic effects observed.
|Publication Type:||Journal Article|
|Murdoch Affiliation:||School of Chemical and Mathematical Science|
|Copyright:||© 2006 Elsevier B.V. All rights reserved.|
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