A SOLID COMPOSITE PH SENSOR-BASED ON QUINHYDRONE

A solid composite pH sensor can be made from quinhydrone, solid paraff in and surface-modified graphite powder. It exhibits an almost theoret ical slope of the potential-pH dependence (-57.7mV at 25 degrees C), a formal potential E(0)' = 0.6922V (vs. NHE), low standard deviations f or repeated measurements (0.04 pH units), fast potential response (15s ) and a working range up to pH 9.5. The sensor can be used in moderate ly concentrated mineral acids and, unlike the conventional quinhydrone electrode, it is also applicable for measurements in milk. The sensor shows the same salt effect and sensitivity towards strong oxidants an d reductants as the conventional quinhydrone electrode. The surface mo dified graphite was prepared by boiling graphite powder with concentra ted nitric acid. By voltammetry it was shown that the surface of the m odified graphite contains surface-cofined redox centers similar to tho se in quinhydrone, which can catalyze the electron transfer of the qui nhydrone on the surface of the sensor.