Spectroscopic studies of the chemical speciation in concentrated alkaline aluminate solutions

The Tl-205 NMR and UV/VIS spectral changes associated with the hydrolysis of thallium(I) have been employed to investigate hydroxide-sensitive equilibria in highly concentrated alkaline aluminate solutions {0.1 M < [Al(III)](T) < 2.6 M; 0.15 M < [NaOH](T) < 4.0 M} in an ionic medium of 8 M Na(ClO4) at 25 degrees C. Spectroscopic titrations were performed at [OH-](T)/[Al(III)](T) ratios of 6.08, 5.06, 4.59 and 4.33 and also at notionally constant [OH-](T). The data obtained demonstrate conclusively that the hydroxide concentration at high [OH-](T) and [Al(III)(T)/[OH-](T) ratios is significantly higher than if only the well established Al(OH)(4)(-)(aq) ion is present. In agreement with previous potentiometric results, the data are consistent with the formation of one or more oligomeric species of general formula Al-q(OH)(3q-r)(r+)(aq) (with q = 4-7 and q + r = 1 or 2). Evidence is also presented for the formation of at least one Tl(I)-aluminate solution species. However, unequivocal interpretation of the data is difficult because of possible ion pairing, solvation and activity coefficient effects occurring in these extremely concentrated electrolyte solutions.