Molecular fluorescence analysis of the effect of the pH on the complexation of Cu(II), Ni(II) and Fe(III) ions by the stronger binding sites of a soil fulvic acid

The effect of the pH (in the range between 3 to 6) on the stability of the complexes formed between the stronger fluorescent binding sites of a soil fulvic acid (FA) and Cu(II), Ni(II) and Fe(III) ions, at a concentration of 25 mg L-1 in KNO3 0.1 M, was studied The analytical methodology consisted on monitoring the synchronous fluorescence spectra of FA as function of the metal ion concentration. Spectral data were preprocessed by SIMPLISMA, a self-modeling mixture analysis method, to obtain improved quenching profiles to be used in the estimation of equilibrium parameters. Two methods were used in the interpretation of the quenching profiles: (i) graphical procedures based in Stern-Volmer analysis; and (ii) a non-linear least-squares procedure. Both two methods allowed estimation of the conditional stability constants (Kc). However, the Stern-Volmer analysis originated useful results only when the logarithm of Kc is relatively small (Log Kc < 5). Besides the calculation of Kc, the two methods allowed the estimation of the percentage of fluorescent structures not accessible for the complexation of the three metal ions. The Log Kc were: Cu(II) ion, 3.42 +/- 0.04, 4.12 +/- 0.07, 4.63 +/- 0.07 and 5.1 +/- 0.3 respectively for pH = 3, 4, 5 and 6; Ni(II) ion, 3.1 +/- 0.1, 3.48 +/- 0.06, and 4.03 +/- 0.07 respectively for pH = 4, 5 and 6; Fe(III) ion, 5.7 +/- 0.2 and 5.86 +/- 0.09 respectively for pH = 3 and 4.