Spectroscopic studies of the chemical speciation in concentrated alkaline aluminate solutions

Abstract

The ²⁰⁵Tl 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 °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^–]. 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.