Analytica Chimico Acta, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG 155 (1983) 11-20 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands zyxwvutsrqponmlkjih POLYMER-MEMBRANE pH ELECTRODES AS INTERNAL ELEMENTS FOR POTENTIOMETRIC GAS-SENSING SYSTEMS W. N. OPDYCKE, S. J. PARKS and M. E. MEYERHOFF* Department of Chemistry, University of Michigan, Ann Arbor, MI 48109 (U.S.A.) (Received 2nd June 1983) SUMMARY The use of polymer-membrane pH electrodes as internal sensing elements for the fab- rication of inexpensive ammonia and carbon dioxide gas sensing systems is reported. The pH-responsive polymer membranes are prepared by incorporating tridodecylamine as the neutral carrier in plasticized poly(viny1 chloride) membranes. Both static and con- tinuous-flow gas-sensing arrangements are described. For miniature static gas sensors, the internal polymer pH electrode can be made with or without an internal reference solution. In the latter case, the polymeric membrane is coated directly onto a graphite substrate. The polymer pH electrode can also be prepared in tubular form and used in conjunction with a gas dialysis chamber for automated continuous-flow determinations of carbon dioxide and ammonia. Slopes, response times, and reproducibility of these new gas-sensing systems are evaluated using optimized internal electrolytes, flow rates, and gas-permeable membrane materials. When appropriate reagents and materials are used, the static sensors exhibit slopes in the range 48-62 mV/decade with potentials repro- ducible to less than +1.5 mV at gas concentrations greater than lo3 M. The use of potentiometric gas sensors for the direct measurement of dissolved gases (NH3, COZ, NOz, etc.) or as final detectors for indirect bio- analytical methods involving enzymes and microbial cells has grown rapidly in recent years [l-3]. Commercial gas sensors commonly utilized in such applications have glass pH electrodes as internal elements [3] . Although these sensors often exhibit theoretically predicted response properties and Nemstian behavior [4], the commercial devices are rather large (tip dia- meter 12-15 mm) and expensive. Moreover, efforts to miniaturize these probes have been hampered by the fragile nature of the glass electrode and the flat bottom geometry required for functional static gas sensor designs. In view of the limitations imposed by the use of glass electrodes, several workers have prepared gas sensors with alternative internal pH-responsive transducers. Mascini and Cremisini [ 51 reported the use of thermo-molded antimony-type pH electrodes as inner elements while workers at General Electric have patented the concept of using electrodes based on palladium or iridium oxide for the fabrication of miniature CO, sensors [6]. Unfor- tunately, metal/metal oxide pH transducers may be subject to interferences from the redox environment of the sample (e.g., oxygen content) and this could limit their utility in the design of stable gas sensing systems. 0003-2670/83/$03.00 0 1983 Elsevier Science Publishers B.V.