A rapid non-equilibrium critical precipitation assay to assess aluminium-ligand interactions

In many natural situations (e.g. environmental and biological) aqueous metal-ligand interactions occur in complex, dynamic solutions and do not adhere to true equilibrium. Nonetheless, equilibrium-based assays in simple solutions are generally used to model metal-ligand interactions of natural systems. Moreover, these are time consuming and not easily applied or understood by many applied scientists. Here, a 'critical precipitation assay' was used to investigate the interaction of common ligands with aluminium at pH 7.0, under non-equilibrium conditions. Results obtained were correlated with literature-derived stability constants for the aluminium-ligand interactions, while high-resolution 1H nuclear magnetic resonance spectroscopy (1H NMR) was used to confirm the nature of observed interactions. Weak interaction with aluminium was confirmed for traditional weak ligands (e.g. bicarbonate) as these were unable to compete with the hydroxide ion for aluminium at pH 7.0. Two types of interaction were seen for the 'stronger' ligands that could compete with hydroxy-polymerisation. Firstly, distinct aluminium:ligand stoichiometric ratios were observed for ligands such as ethylenediaminetetra-acetic acid (1:1) or 1,3,5-trideoxy-1,3,5- tris(dimethylamino)-cis-inositol (1:2). Secondly, most ligands, including citrate and maltol, did not prevent hydroxy-polymerisation but did maintain more aluminium 'in solution' (approximately 2.5:1 aluminium:ligand) than permitted by acceptable aluminium:ligand stoichiometric ratios, suggesting the formation of dynamic metastable hydroxy-bridged aluminium-ligand complexes. 1H NMR with aluminium and maltol or citrate, supported this idea as complex spectral patterns were observed prior to precipitation. Aluminium maintained in solution at pH 7.0 correlated, with literature-derived stability constants suggesting that non-equilibrium aluminium-ligand interactions approximate to equilibrium and that this assay could be used as a quick screening method for investigation of aluminium-ligand interactions.