Evaluation of the temperature influence on the response characteristics of poly(vinyl chloride) membrane calcium ion-selective electrodes with an internal reference or applied to an electrically conductive silver-epoxy support

Abstract

Results of a study of temperature effects on the response of calcium ion-selective electrodes with the calcium bis{di[4-(1,1,3,3-tetramethylbutyl)phenyl] phosphate}-dioctyl phenylphosphonate sensor system in poly(vinyl chloride) membranes are presented. Electrodes with an internal reference (“type A”) or with the membrane applied to a support made of electrically conductive silver-epoxy resin (“type B”) were studied in parallel for comparison purposes. Isothermal measurements were carried out in the temperature range 10–50 °C and included calibrations at different temperatures for evaluation of operational temperature ranges and slope and standard potential variations, and also hysteresis curves. Isopotential points were determined from calibrations or hysteresis curves. The influence of the chloride ion concentration in the inner solution of type A electrodes was studied. This type of electrode responded more closely to the thermodynamic behaviour than type B electrodes. Type A electrodes showed less hysteresis and the slope coefficients were close to the theoretical value. Moreover, when filled with 1 × 10^–2–1 × 10^–3M calcium chloride solutions, the isopotential point activity of type A electrodes (5 × 10^–4–10 × 10^–5 compared with 5 × 10⁶M for type B electrodes) is within their linear response range which makes them less sensitive to temperature fluctuations. Addition of caesium chloride to the inner solution allows further translation of the isopotential point concentration to minimise the effects of temperature variation on the potential.