title: (27)Al NMR and Raman spectroscopic studies of alkaline aluminate solutions with extremely high caustic content - Does the octahedral species Al(OH)(6)(3-) exist in solution?
abstract: (27)Al NMR and Raman spectra of alkaline aluminate solutions with 0.005M</=[Al(III)](T)</=3M in various M'OH solutions (M'(+)=Na(+), K(+) and Li(+)) were recorded and analysed. Caustic concentrations up to 20M were used to explore whether higher aluminium hydroxo complexes are formed at extremely high concentrations of hydroxide. A single peak was observed on the (27)Al 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)<0.8M. This variation shows a strong dependence on the cation of the solution and practically disappears in systems with [Al(III)](T)>/=0.8M. For Raman spectra of solutions with [Al(III)](T)=0.8M and [NaOH](T)>/=10M, the peak maximum of the symmetric nu(1)-AlO(4) stretching of Al(OH)(4)(-) shifted progressively from approximately 620 to approximately 625cm(-1) and decreased in intensity with increasing [NaOH](T). In parallel, modes centred at approximately 720 and approximately 555cm(-1) (cf. approximately 705 and approximately 535cm(-1) at lower [NaOH](T), ascribed to a dimeric aluminate species appeared, and their intensities increased with increasing [NaOH](T). These variations in the (27)Al 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)(6)(3-)) is not necessary to explain the spectroscopic effects observed.
authors.affiliation: Chemistry-DSE, Murdoch University, Murdoch, WA 6150, Australia; Department of Inorganic and Analytical Chemistry, University of Szeged, Szeged H-6701, PO Box 440, Hungary.
authors.forename: Pál
authors.initials: P
authors.lastname: Sipos
authors.forename: Glenn
authors.initials: G
authors.lastname: Hefter
authors.forename: Peter M
authors.initials: PM
authors.lastname: May