(78b) Conductivity, pH, and Oxidation Reduction Potential Sensing with a Simple Three-Electrode Microfabricated System

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.