Novel response mechanism and application of sulfite sensitive polymeric membrane electrode based on dithiocarbamate complexes of mercury(II)

The potentiometric response mechanism of a previously reported sulfite sensitive polymeric membrane electrode employing dithiocarbamate complexes of Hg(II) as membrane active components is examined. Unlike conventional polymer membrane ion-selective electrodes (ISEs), the response of the sulfite sensor is shown to deteriorate significantly upon prolonged exposure to solutions containing high levels (10 mM) of sulfite/hydrogen sulfite. The magnitude of potentiometric sulfite response is also shown to be dependent on the amount of dithiocarbamate complex and plasticizer present within the membrane phase. The addition of lipophilic cation or anion sites to the membrane dramatically decreases sulfite response, suggesting that the Hg(II) complex does not act as either a neutral or charged type carrier ionophore. Addition of reducing agents such as Sn(II) to the sample solution yields a potentiometric anion response similar to sulfite. These results are used to postulate a unique potentiometric response mechanism whereby sulfite present in the sample reduces Hg(II) to Hg(I) at the surface of the membrane, concomitantly creating negatively charged anionic sites (of dithiocarbamate ligand) in the organic phase at the membrane/sample interface. Under carefully controlled conditions, such a heterogeneous redox process can yield highly reproducible potentiometric responses to sulfite. Indeed, the practical analytical application of the sulfite sensor is demonstrated by employing it as a detector in a flow injection/gas dialyzer arrangement to accurately quantitate the sulfite (via sulfur dioxide) content of various wine samples.