MULTIWAVELENGTH ANALYSIS OF SYNCHRONOUS FLUORESCENCE-SPECTRA OF THE COMPLEXES BETWEEN A SOIL FULVIC-ACID AND CU(II)

The variations observed in the synchronous fluorescence spectra of a s oil fulvic acid, collected in the north of Portugal, with pH (between 2 and 7) and with Cu(II) concentration (at pH 3, 4, 5 and 6 and total copper concentrations between 0.005 and 4 mM depending on the pH) were studied by SIMPLISMA, a self-modelling mixture analysis technique. Th ree components were deeected when varying the pH, corresponding to thr ee acid-base equilibria with pK(a) values around 3.0, 5.0 and 6.0, and their individual spectra were obtained. Two components were detected when varying the Cu(II) concentration, corresponding to two different ligand sites, and their spectra and concentration profiles (based on t he two detected pure variables) were obtained. The spectrum of one bin ding site is similar to the component associated with the acid-base eq uilibrium with pK(a) = 3.0 and that of the other binding site is simil ar to the sum of the spectra of the components associated with the oth er two acid-base equilibria. Using a 1:1 stoichiometry for complex for mation, complexation capacities (C-L) and stability constants (K) were calculated for the two binding sites at the four pH values. The calcu lated stability constants were log K-1 = 3.42(4), log K-2 = 3.05(5) at pH 3, log K-1 = 4.17(3), log K-2 = 3.71(1) at pH 4, log K-1 = 4.51(3) , log K-2 = 3.95(2) at pH 5 and log K-1 = 4.57(3), log K-2 = 4.03(1) a t pH 6.