Model compounds of humic acid and oxovanadium cations. Potentiometric titration and EPR spectroscopy studies

The stability constants and the isotropic EPR parameters Ao (hyperfine spli tting constant) and go (g value) were obtained by potentiometric titrations and EPR spectroscopy, respectively, of 85%v/v aqueous solutions of model c ompounds of humic acids - salicylic acid (SALA) - and both nitrohumic acids , a laboratory artifact - nitrosalicylic acids, 3-nitrosalicylic acid (3-NS A), 5-nitrosalicylic acid (5-NSA) and 3,5-dinitrosalicylic acid (3,5-DNSA) and oxovanadium cations. It was possible to record EPR spectra of those model compounds and the ion VO2+(V(IV)), and the stability constants were obtained from a solution of V O3+ (V(V)), the values for the logarithms of the stability constants rangin g from 12.77 +/- 0.04 to 7.06 +/- 0.05 for the species ML, and from 9.90 +/ - 0.04 to 4.06 +/- 0.05 for the species ML2 according to the decrease in th e acidity of the carboxylic and the hydroxyl groups in the aromatic ring of the model compounds studied as the -NO2 substituents were added. Species d istribution diagrams were also obtained fur the equilibria studied. The EPR parameters showed that as the logarithm of the overall stability co nstants increase, g(o) values also increase, while A(o) values show a tende ncy to decrease.