BINDING-CAPACITY OF CASEIN TO LEAD AND VOLTAMMETRIC SPECIATION OF LEAD IN MILK WITH A NAFION COATED ELECTRODE

The complexation of lead with casein (CS) was studied by several voltammetric methods, as was the selective determination and speciation of lead in milk. First, cyclic voltammetry (CV) was used to examine the influence of adsorbed casein and complexation on the electrochemical response of lead at neutral and alkaline pHs. Second, by titrating solutions of CS with lead, two mathematical methods based on differential-pulse anodic stripping voltammetry (DPASV) for evaluating the binding properties of CS to lead were compared with a hanging mercury drop electrode (HMDE) and a Nafion-covered mercury film electrode (Nf-MFE). In all cases, the conditional stability constants for the Pb-CS complex agreed well and were close to log beta' = 5.5 at pH 7.0. Finally, lead was directly determined with the chemically modified electrode in acidified samples (almost-equal-to pH 0.3) of whole and skim milk at the ng/ml level, with no need for preliminary acid digestion.