Comparison of the hydrolyses of gallium(III) and aluminium(III) solutions by nuclear magnetic resonance spectroscopy

Abstract

A quantitative ⁷¹Ga n.m.r. study of the base hydrolysis of gallium(III) salt solutions has been undertaken. The resulting spectra were very similar to the ²⁷Al n.m.r. spectra obtained for aluminium solutions hydrolysed under analogous conditions. No evidence was observed of any species different from those seen for aluminium, despite the fact that the eventual outcomes of the hydrolyses are different, i.e. precipitation of an oxyhydroxide of diaspore structure in the case of gallium, and the eventual formation of a gibbsite hydroxide phase in the case of aluminium. For aluminium, the formation of the [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ tridecamer is well known, and the formation of an analogous species for gallium has also been proposed. Although the ⁷¹Ga n.m.r. studies undertaken herein appear to show that much less of the gallium polyoxycation is formed than is the case for aluminium, clay mineral pillaring studies have suggested that this apparent difference is not real. Through consideration of both the extremely broad nature of the tetrahedral gallium peak and the high quadrupole moment of the ⁷¹Ga nucleus, it is proposed that the gallium tridecamer is much more distorted than the aluminium species, and that the integrated area of the n.m.r. peak assigned to this species does not accurately represent the amount actually present in solution.