A systematic investigation of aluminium ion speciation at high temperature. Part. 1. Solution studies

Abstract

Speciation diagrams of aluminium ions in aqueous solution (0.2 M) at high temperature (90 °C) have been obtained from 48 h time-resolved multi-batch titration experiments monitored by ²⁷Al NMR spectroscopy, potentiometry and dynamic light scattering. The quantitative speciation patterns and kinetic data obtained offer a dynamic picture of the distribution of soluble and insoluble Al species as a function of hydrolysis ratio h (h = [OH⁻]/[Al³⁺]) over a very broad range of conditions (−1.0 ≤ h ≤ 4.0). Monomeric, small oligomeric, tridecameric (the ‘Al₁₃-mer’) and the recently characterised 30-meric aluminium species (the ‘Al₃₀-mer’) as well as aluminium hydroxide have been identified and quantified. The Al₁₃-mer species dominates over a relatively broad range of hydrolysis ratios (1.5 ≤ h ≤ 2.7) during the first 6 h of experiment, but are gradually replaced by Al₃₀-mers at longer reaction times. Kinetic profiles indicate that the formation of the Al₃₀-mer is limited by the disappearance of the Al₁₃ species at mildly acidic conditions. The estimated rate constants of both hydrolytic processes show good internal correlation at h ≥ 1.5. The effect of local perturbations leading to the formation of aluminium hydroxide below the electroneutrality point (h = 3.0) has been estimated quantitatively.