SPECTROSCOPIC STUDIES OF THE CHEMICAL SPECIATION IN CONCENTRATED ALKALINE ALUMINATE SOLUTIONS

The Tl-205 NMR and UV/VIS spectral changes associated with the hydroly sis of thallium(I) have been employed to investigate hydroxide-sensiti ve 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 med ium of 8 M Na(ClO4) at 25 degrees C. Spectroscopic titrations were per formed at [OH-](T)/[Al(III)](T) ratios of 6.08, 5.06, 4.59 and 4.33 an d 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 previo us 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 fo r the formation of at least one Tl(I)-aluminate solution species. Howe ver, unequivocal interpretation of the data is difficult because of po ssible ion pairing, solvation and activity coefficient effects occurri ng in these extremely concentrated electrolyte solutions.