EFFECT OF THE CONCENTRATION ON THE COMPLEXATION AT COPPER[II]-ION BY A SOIL FULVIC-ACID

The nature of the binding of Cu(II) ion by a sample of a soil fulvic a cid was studied by potentiometric titration of the ion at variable and constant pH and 0,1 M ionic strength. Free Cu(II) ion and H+ concentr ations were read after each metal ion addition. The influence of the f ulvic acid concentration on the mean equivalent weight of the ligand ( g.mol(-1)) and on the (macroscopic) stability constants (calculated ac cording to the literature and by a proposed new method) of its Cu(II) complexes was investigated. It was found that the equivalent weight in creases (from 1.10(3) to 4.10(3) g.mol(-1)) and stability constants de crease (for 1:1 complexes from ca 10(2) to 10(-7)) when the fulvic aci d concentration increases ca 30 to 600 mg.L(-1). The present results c an be explained by conformational changes on the fulvic acid when the concentration increases and the ionic strength is high: the stronger b inding sites are involved in changes of the fulvic acid molecules, and the Cu(II) ion have to bind to weaker coordination positions.