SURFACE STUDIES OF QUINHYDRONE PH SENSORS

A polymer-modified non-glass electrode was fabricated using quinhydrone (QH) in a poly(vinyl chloride) (PVC) matrix supported by a low porosity carbon rod. The electrode has a Nernstian response with a slope of 34.1+/-7.3 mV/pH (25 degrees C), a linear working range of pH 3 to 10 and an average response time of 5 to 7 min. Electrode stability was maintained over a period of 36 days without the need of pre-treatment prior to use or immersion in solution when not in use. X-ray photoelectron spectroscopy (XPS) was used to probe the composition and surface characteristics of the electrode in attempt to understand the chemical basis of electrode performance. Angle resolved XPS showed that electrodes with a poor performance had an oxygen enhanced surface attributed to increased surface concentration of oxygen containing QH. This electroactive species is readily oxidised when in contact with solution. Electrodes with a high performance had a chlorine enhanced surface, indicating a PVC-rich surface. The PVC serves both as a matrix and a protective layer for the embedded QH. The comparison of electrochemical and XPS data indicates that electrode composition and performance are closely related to fabrication techniques.