Potentiometric pH sensor based on an oil paste containing quinhydrone

An all-solid-state potentiometric pH sensor based on the dispersion of quinhydrone in carbon paste was developed. Quinhydrone was incorporated in a soft membrane that was in contact with a conducting wire via a conducting paste. This configuration overcomes the problem of fragility that is encountered in the glass electrode. This membrane is robust, reusable and renewable. The sensing membrane was composed of uniformly blended graphite powder, quinhydrone, and paraffin oil in a 6.7:1:2.9 weight to weight ratio. The sensor exhibited a sub-Nernstian response with a slope of 56.027 mV/[pH] and a linearity of 0.9991 at pH 2 to 10. The sensor response was highly repeatable with r.s.d.s of 1.84 % and 3.45 % at pH 4 and 9, respectively. The sensor showed stable responses for 3 hours at varying pHs (pH 5, 7 and 9) with r.s.d.s of less than 5%. It exhibited minimal hysteresis with a slight decrease in the slope (m= ± 1.424 mV/[pH]). The presence of possible interfering ions (Na⁺, K⁺, SO₄2^-, Cu²⁺ and Fe³⁺) were evaluated.