A SOLID-STATE PH SENSOR-BASED ON A NAFION-COATED IRIDIUM OXIDE INDICATOR ELECTRODE AND A POLYMER-BASED SILVER-CHLORIDE REFERENCE ELECTRODE

As an alternative to the glass pH electrode, an entirely solid-state pH sensor (pH-sensing and reference electrodes) has been developed based on an annealed permselective polymer(Nafion(TM))-coated iridium oxide pH indicator electrode and a polymer-modified silver-silver chloride reference electrode. When a solution of Nafion is coated onto the iridium oxide surface and annealed at 210 degrees C, it becomes permselective to cations. The membrane thus transports protons, but attenuates the effects of anionic oxidizing or reducing (redox) species that interfere with the response of an uncoated electrode. The reference-electrode design involves coating a silver-silver chloride surface with a chloride-ion-containing polymer (e.g., triethylamine quaternized polychloromethylstyrene). The chloride ion is trapped within this polymer layer by encapsulating it with a Nafion outer layer. After annealing, the Nafion membrane effectively blocks chloride-ion diffusion to the test solution and maintains a constant chloride-ion activity on the silver chloride surface; thus a constant electrode potential is maintained. Several sensor designs based on coated wires, cermets and alumina ceramics have been evaluated for pH response and stability. Distinctive features of the solid-state technology include glass-free construction, chemical resistance and high impact strength.