Aluminium-27 nuclear magnetic resonance studies of the hydrolysis of aluminium(III). Part 3. Stopped-flow kinetic studies

Abstract

The decomposition of two different types of hydrolysed aluminium(III) solutions following the addition of acid has been observed using ²⁷Al n.m.r. and pH measurements. It has been possible to characterise the way in which the ion [AlO₄Al₁₂(OH)₂₄(OH₂)₁₂]⁷⁺ decomposes and to suggest a mechanism to explain the observations. Decomposition is fast and seems to be controlled by the rate of exchange of water ligands on Al³⁺. The major component of a metal-hydrolysed solution behaves similarly if an excess of acid is added, but if the acid addition is limited much more complex behaviour is observed which may indicate the formation and eventual decomposition of a different intermediate species. The high-molecular-weight material present decomposes much more slowly and so is believed to have a different type of structure to that of the above Al₁₃⁷⁺ cation cluster.