New metal-oxide-type pH sensors

Abstract

Two new types of metal-oxide pH sensors with a direct solid-state contact have been developed and tested. Their performance was demonstrated near ambient temperatures with a samarium substituted ceria ceramic membrane and single crystals of several molybdenum oxide bronzes. The pH sensors with Na-molybdenum-oxide bronzes show near-ideal Nernstian behaviour in the pH range 3–9. The response was not affected by the direction of the pH change. In contrast, the pH sensor with the Na-tungsten bronze sensitive electrode (SE) shows a deviation from linearity, as well as non-Nernstian behaviour with a slope − 76.3 mV/pH at 25 °C. The response time of most molybdenum bronze pH sensors is less than 5 s for 90% response. The pH sensors with stabilized ceria membrane show close to ambient temperature non-Nernstian behaviour in the pH range 3–10 with a slope − 23.8 mV/pH at 25 °C. With increasing temperatures the behaviour of ceria membrane pH sensors approaches more and more a Nernstian response with slope − 39.9 mV/pH at 60 °C. Both pH sensors also can be used to follow acid-base titrations.