A voltammetric method for determining free metal activities in aqueous solutions. Part 2: anodic stripping voltammetry of Cd, Cu, Pb and Zn in synthetic and soil solutions
Theory and methodology for determining the free metal activity, (M2+), of Cd, Cu, Pb and Zn by anodic stripping voltammetry is presented. Metal-buffer solutions were prepared to control (M2+), which was varied by using different ligands and by changing pH. Total metal aqueous concentration of 10-6–10-7M and metal-ligand ratios of 1:20M, 0.010M acetate medium were used. Equations for reversible and irreversible systems were interpreted to explain the results. For Cd and Pb (pPb < 10), agreement was found among theory, experimental ASV and (M2+), calculated by using a speciation chemical equilibria programme and stability constants reported in the literature. Kinetic effects were observed for values of pPb > 10. For Cu and Zn, good agreement was found between the theory and calculated (M2+), and between experimental ASV results and calculated (M2+). Experimental ASV results showed Nernstian parallel shifts with regard to theory. Free metal activities in the order of pCd = 12, pCu = 18, pPb = 10 and pZn = 9 were measurable under the experimental conditions tested. Results too showed good agreement between Cu-ISE and Cu-ASV when measuring the free Cu2+ activity in aqueous extracts of four soils. Values of soil-(Cu2+) measured were in the order of 10-5–10-9.
Keywords: chemical speciation, chemical activity, metal buffer solutions, heavy metals, trace metals, electrochemistry, soil pollution, chemical speciation, free metal activities, aqueous solutions, anodic stripping voltammetry, DCV, cadmium, lead, copper, zinc