UV-visible and synchronous fluorescence spectroscopic investigations of the complexation of Al(III) with caffeic acid, in aqueous low acidic medium

Aluminium-caffeic acid complex equilibrium in aqueous solution was studied by both UV-Vis and synchronous fluorescence spectroscopies. The stoichiometry of the different complexes has been established using the molar ratios method at pH 5 and 6.5. The pure electronic spectrum of the main species present in the solution has been estimated by chemometric methods and stability constants of these complexes have been calculated. The results show that the chelation power of the catechol site increases with pH. At pH 5, the formation of a 1: 1 complex is largely predominant for low amount of metal, in this complex Al(III) coordinates to the catecholate group, but rather rapidly, the carboxylic function is involved in the chelation mechanism to give a 2:1 complex. At pH 6.5, the simultaneous formation of complexes 1:2 and 1:3 occurs and only the catechol site is involved in the process at this pH. (C) 2006 Elsevier Ltd. All rights reserved.