Detection of pH and oxidation reduction potential (ORP) in aqueous solutions typically requires a reference electrode that supplies a constant and known voltage regardless of solution conditions. These electrodes are challenging to construct for benchtop operation and are even more difficult to microfabricate. Current microfabricated reference electrodes and ion-selective electrodes use relatively difficult processing steps and have relatively short user lifetimes. For instance, silver/silver-chloride devices require construction of a constant concentration silver-chloride reservoir, and ISFET fabrication requires silicon doping.
We have constructed a three-electrode platinum sensor that can detect the conductivity, pH, and ORP of an aqueous solution. The three-electrodes system measures conductivity and ORP sequentially using an AC and a DC signal, respectively, passed through two electrodes. To measure pH, a controlled current signal generates both a constant voltage and a pH-varying voltage on the three-electrode system. This novel method, unlike other micro-fabricated reference electrodes, uses platinum electrodes and, hence, has an almost unlimited lifetime. Further, the fabrication procedure is comparatively easy and cheap: only single layer of platinum deposition on a glass wafer was necessary. The novel pH and ORP sensor can measure conductivity, pH, and ORP in non-oxidizing (below 1 ppm chlorite) solutions. Measurements in oxidizing solutions (above 12 ppm chlorite) are also possible although modification of one electrode with a surface coating is necessary.