Resumen

MERCE, Ana Lucia Ramalho et al. Model compounds of humic acid and oxovanadium cations. Potentiometric titration and EPR spectroscopy studies. J. Braz. Chem. Soc. [online]. 1999, vol.10, n.6, pp. 463-468. ISSN 0103-5053.  http://dx.doi.org/10.1590/S0103-50531999000600008.

The stability constants and the isotropic EPR parameters A_o (hyperfine splitting constant) and g_o (g value) were obtained by potentiometric titrations and EPR spectroscopy, respectively, of 85%v/v aqueous solutions of model compounds of humic acids - salicylic acid (SALA) - and both nitrohumic acids, a laboratory artifact - nitrosalicylic acids, 3-nitrosalicylic acid (3-NSA), 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 VO³⁺ (V(V)), the values for the logarithms of the stability constants ranging 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 the acidity of the carboxylic and the hydroxyl groups in the aromatic ring of the model compounds studied as the -NO₂ substituents were added. Species distribution diagrams were also obtained for the equilibria studied. The EPR parameters showed that as the logarithm of the overall stability constants increase, g_o values also increase, while A_o values show a tendency to decrease.

Palabras llave : humic acid; model compounds; equilibrium constants; oxovanadium cations.