Determination of stability constants of labile heterogeneous complexes using differential pulse polarography and anodic stripping voltammetry

Abstract

Labile complexes of cadmium with Chlorella marina cells, alive or killed with formaldehyde, and of lead with dissolved organic matter (DOM) from an estuarine sample (both ligands examples of heterogeneous macromolecules) have been studied at constant pH using differential pulse polarography (DDP) and differential pulse anodic stripping voltammetry (DPASV). In this context, stability constants have been determined either from peak currents or peak potentials, using both techniques. It has been noticed that K ̄ i p and K ̄ E p values obtained from DPP are related with bulk concentrations, whereas the results obtained by DPASV are related to surface concentration and cannot be used for the determination of metal species in the bulk. In a medium NaNO 3 0.01 M and at pH=7.0, C. marina cells form complexes with Cd(II) for surface coverages ( θ) between 10 −5 and 3×10 −5 with log K ̄ values between 3 and 4, probably due to aminoacids, proteins and polycarboxylic groups. Log K ̄ values of the order 2.5 with θ between 3×10 −5 and 3×10 −4 corresponding to more homogeneous groups have also been determined and are probably due to carboxylic sites. Major groups in a concentration 3×10 −4 M and with log K ̄ <2.0 probably due to polysaccharides are also present, but cannot be detected by voltammetric methods. DOM present in estuarine sample, buffered at pH=8.0 has log K ̄ values between 3.5 and 5.0 with an average value of log K ̄ =4.2 , being a much less heterogeneous system.