ELSEVIER Analytica Chimica Acta 310 (1995) 211-221 ANAIYTICA zyxwvutsr CHIMICA ACM Novel response mechanism and application of sulfite sensitive polymeric membrane electrode based on dithiocarbamate complexes of mercuryt II) Ibrahim H.A. Badr a,‘, Mark E. Meyerhoff a1 * , Saad S.M. Hassan ’ a Department of Chemistry, The University of Michigan, Ann Arbor, MI 48109, USA b Department of Chemistry, Faculty of Science, Ain-Shams University, Cairo, Egypt Received 14 September 1994; revised 12 January 1995; accepted 19 January 1995 Abstract The potentiometric response mechanism of a previously reported sulfite sensitive polymeric membrane electrode employing dithiocarbamate complexes of Hg(I1) 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 potentiomet- ric 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 S&I) 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. Keywords: Ion selective electrodes; Sensors; Sulfite/hydrogen sulfite measurements; Polymer membrane electrodes; Response mechanism 1. Introduction The design of ion-selective polymeric membrane electrodes that exhibit analytically useful response toward specific anions remains a challenging re- * Corresponding author. 1 On leave from Department of Chemistry, Faculty of Science, Ain-Shams University, Cairo, Egypt. search avenue for sensor scientists. In recent years, analytically relevant anion electrodes have been real- ized through the use of various lipophilic metal- ligand complexes (e.g., metalloporphyrins, metallo- phthalocyanines, corrins, etc.) as active membrane ionophores [l-7]. In such systems, anion selectivity is achieved by specific anion axial ligation reactions with the central metal ion of the complex at the organic membrane/aqueous sample interface by ei- 0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0003-2670(95)00154-9