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 soil 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. Three components were deeected when varying the pH, corresponding to three 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 the two detected pure variables) were obtained. The spectrum of one binding site is similar to the component associated with the acid-base equilibrium with pK(a) = 3.0 and that of the other binding site is similar to the sum of the spectra of the components associated with the other two acid-base equilibria. Using a 1:1 stoichiometry for complex formation, complexation capacities (C-L) and stability constants (K) were calculated for the two binding sites at the four pH values. The calculated 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) at pH 6.