CONSTRUCTION AND RESPONSE CHARACTERISTICS OF A SULFITE HYDROGENSULFITE-SELECTIVE ALL-SOLID-STATE CONTACT ELECTRODE BASED ON THE 4-METHYLPIPERIDINEDITHIOCARBAMATE COMPLEX OF MERCURY(II)/

An all-solid-state contact electrode selective for sulfite/hydrogensul fite was constructed using a wire coated with a mixture of graphite, e poxy and hardener and covered by 4-methylpiperidinedithiocarbamate-mer cury(II) complex. Its preparation and the response characteristics wer e examined. With an optimum thickness of the complex at the surface of the all-solid-state contact (which appears to affect the response tim e, slope and lifetime), the electrode exhibited a short response time and high selectivity for reducing species, and also was found to be re producible for 3 weeks when kept in distilled water or dry, For electr odes exposed to solutions containing high levels (0.1 mol dm(-3)) of s ulfite/hydrogensulfite or reducing species, e.g., Sn-II or Cu-I, the r esponse was significantly lower, The observed response of the electrod e only to reducing species can be attributed to a heterogeneous redox reaction (sulfite in the sample reduces Hg-II to Hg-I at the electrode -sample interface). Under controlled conditions such a heterogeneous r edox reaction resulted in a highly selective and reproducible response to reducing species, especially to sulfite/hydrogensulfite. The respo nse of the all-solid-state contact electrode was linear with a slope o f -70 +/-5 mV between 0.1 and 5 x 10(-6) mol dm(-3) sulfite/hydrogensu lfite concentration and the useful pH range from 0.1 to 1 x 10(-6) mol dm(-3) sulfite/hydrogensulfite was between 3.0 and 9.0. Further, the selectivity of sulfite over iodide, thiocyanate, thiosulfate and bromi de was much improved compared with that for a Liquid membrane sulfite electrode.