Determination of lead complexation in lake water by cathodic stripping voltammetry and ligand competition

A procedure is presented to determine lead complexing ligands in lake water by cathodic stripping voltammetry (CSV) with ligand competition using Calcein-blue (CB) (8-[N,N-bis (carboxyl-methyl)aminomethyl]-4-methylumbelliferone). The optimised conditions to determine dissolved lead in lake water by CSV using CB entails a CB concentration of 500nM, a solution pH of 7.0 adjusted with TES buffer, an adsorption potential of -0.15V, and an adsorption time of 90s. Using these optimised conditions the 3sigma limit of detection was 100pM Pb. The complex stability of lead complexation by CB was calibrated by ligand competition against EDTA giving a value of 104.8 for alphaPbCB2 (=[PbCB2]/[Pb2+]), and of 1016.5 for the conditional stability constant (log betaPbCB2´) in lake water. The reactive lead concentration in lake waters was found to be lowered by complexation with unknown ligands. The concentration of these ligands in natural waters was determined by titrations with lead whilst monitoring the reactive lead concentration. The method was used to determine the concentration of lead complexing ligands in samples from a mountain lake (the Gossenkoellensee, Austria) revealing the presence of an excess of complexing ligands. The ligand concentration in the water column of this lake was found to vary between 1.6 and 2.7nM, compared to lead concentrations between 0.6 and 1.1nM, and with a value of 13.8± 0.3 for log KPbL´. The ionic lead concentration in equilibrium with the ligands was 14-33fM. It is likely that this very strong complexation controls the geochemistry of lead in lakes.