Time-Dependent Solution Speciation of the AlIII−Citrate System: Potentiometric and NMR Studies

Abstract

Time-dependent potentiometric and NMR spectroscopic measurements were carried out in the AlIII−citric acid system in an equimolar solution and at an excess of ligand in order to monitor the changes in speciation as a function of time. In fresh solutions, the mononuclear 1:1 species [AlLH]+, [AlL], [AlLH−1]− and [AlLH−2]2− and the 1:2 complexes [AlL2]3−, [AlL2H−1]4− and [AlL2H−2]5− are formed. In agreement with earlier findings (Inorg. Chem. 1988, 27, 2565), these complexes are converted to a large extent into a thermodynamically more stable trinuclear species as the solutions age. Depending on the composition of the initially formed mononuclear species, the formation of the trinuclear species is accompanied either by the consumption or by the liberation of hydroxide ions. NMR spectroscopy was used to confirm the considerable time dependence of the species distribution. The 1H, 27Al and 13C NMR spectra indicated that at equilibrium the trinuclear species [Al3(LH−1)3(OH)]4− predominates in the pH range 4−7. Oligomerization of the mononuclear species takes place through at least one intermediate complex. At equilibrium, the ligand-exchange reactions between the trinuclear species, the mononuclear species and free citrate are slow on the NMR time scale.