Mercury film electrodes coated with negatively charged polymer films in speciation studies of trace amounts of lead

Glassy carbon electrodes modified with Nation or poly(3-thiopheneacetate) and subsequently plated with mercury where used for lead speciation studies in buffered (0.1 M Tris, pH 8) sodium chloride (0.6 M) solutions. The complexing agents studied were nitrilotriacetic acid, ethylenediamine, oxalic acid and the well known electrode 'foulant' humic acid. Most measurements were performed with square wave anodic stripping voltammetry. Films of Nafion were spin-coated and heat-cured, and poly(3-thiopheneacetate) films were produced by base hydrolysis of electropolymerized poly(ethyl 3-thiopheneacetate). The results are, to a certain extent, supporting the permselective model which has been suggested for Nafion modified electrodes. However, they do not allow the conclusions that negative species in general are not sensed by these ionomer coated electrodes, because the negatively charged groups of the ionomers may compete successfully for metal cations with chelating agents forming labile species. The nonlabile, negatively charged lead-NTA, where hardly measurable at the polymer-coated electrodes, and the lead-oxalate complexes were also to a certain extent excluded by the ionomers. The response of the Nafion-coated electrode to the positively charged lead-ethylenediamine complexes was in accordance with the earlier reported permselective properties of the polymer when the literature values of the stability constant were used to calculate the lead species distribution. At the poly(3-thiopheneacetate) film, the response to lead-ethylenediamine complexes were higher than to the lead-oxalate complexes, but not as high as one could have expected if charge dependent permselectivity prevailed. As a means to understand the partition of ligands between bulk solution and polymer film, stability constants for the lead complexes of these were determined at the polymer-coated electrodes as well as at the bare mercury film electrode.