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

Labile complexes of cadmium with Chlorella marina cells, alive or killed wi th formaldehyde, and of lead with dissolved organic matter (DOM) from an es tuarine sample (both ligands examples of heterogeneous macromolecules) have been studied at constant pH using differential pulse polarography (DDP) an d 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 tha t (K) over bar(ip) and (K) over bar(Ep) Values obtained from DPP an related with bulk concentrations, whereas the results obtained by DPASV are relate d to surface concentration and cannot be used for the determination of meta l species in the bulk. In a medium NaNO3 0.01 M and at pH=7.0, C. marina cells form complexes with Cd(II) for surface coverages (theta) between 10(-5) and 3 x 10(-5) with lo g a values between 3 and 4, probably due to aminoacids, proteins and polyca rboxylic groups. Log (K) over bar values of the order 2.5 with theta betwee n 3 x 10(-5) and 3 x 10(-4) corresponding to more homogeneous groups have a lso been determined and are probably due to carboxylic sites. Major groups in a concentration 3 x 10(-4) M and with log (K) over bar < 2.0 probably du e to polysaccharides are also present, but cannot be detected by voltammetr ic methods. DOM present in estuarine sample, buffered at pH=8.0 has log (K) over bar va lues between 3.5 and 5.0 with an average value of log (K) over bar = 4.2, b eing a much less heterogeneous system. (C) 1999 Elsevier Science B.V. All r ights reserved.