APPLICATION OF POTENTIOMETRIC STRIPPING ANALYSIS FOR SPECIATION OF COPPER-COMPLEXES WITH ADSORBABLE LIGANDS ON THE MERCURY-ELECTRODE

Potentiometric stripping analysis (PSA) was used for the determination of equilibrium parameters of complexes of copper(II) with humic acids (HA, adsorbable at the mercury electrode surface), at pH 6.5. The calculations were performed by the DeFord-Hume's method or through the material balance of the metal together with the equation dt/dE = <C(D)over bar>(1/2)[Cu](t) (t is the stripping time, E is the working electrode potential, C is a constant, (D) over bar is the average diffusion coefficient of all forms of metal present in the solution and [Cu](t) is the total copper(II) concentration). For comparison, the study was also carried out by differential pulse anodic stripping voltammetry (DPASV) and by potentiometry with a copper(II) ion-selective electrode (ISE). It was found that for HA concentrations lower than 0.1 mmol/l the adsorption phenomena do not interfere in the determination of the equilibrium parameters (K-def) by PSA, but interfere markedly in DPASV. The values of the function log K-def = f(log theta) obtained by both PSA and ISE techniques fit the same isotherm, the form of which is typical for chemical systems involving heterogeneous ligands. In contrast, the values of log K-def obtained by DPASV fit a non-typical isotherm in total disagreement with that obtained by ISE.