As constantes de estabilidade e parâmetros isotrópicos de EPR (Ao e go) foram obtidos por titulações potenciométricas e espectroscopia de EPR, empregando-se soluções aquosas, 15% v/v etanol, de compostos modelo de ácido húmico -ácido salicílico (SALA) -e do artefato de laboratório, ácido nitrohúmico -ácidos 3-nitrossalicílico (3-NSA), 5-nitrossalicílico (5-NSA) e 3,5-dinitrossalicílico -e cátions oxovanádio.Espectros The stability constants and the isotropic EPR parameters Ao (hyperfine splitting constant) and go (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 VO 2+ (V(IV)), and the stability constants were obtained from a solution of VO 3+ (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, go values also increase, while Ao values show a tendency to decrease.
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