Bipolar pulse conductometric monitoring of ion-selective electrodes : Part 3. Studies with the calcium and fluoride electrodes in a continuous flow system

Abstract

Bipolar pulse conductometric monitoring of the calcium and fluoride ion-selective electrodes (i.s.e.'s) in a continuous flow system is evaluated. The location and composition of the counter electrode are critical; an electrochemically poised reference electrode is not necessary. The best scheme for conductometric measurements involves making a rapid series of current measurements at various pulse heights, and extrapolating the resulting linear current—voltage (i/ V) curve to V = 0. For ionic strengths higher than 10 −3 M, detection limits of about 10 −6 M were obtained with both the calcium and fluoride electrodes, monitored conductometrically and potentiometrically. Relative standard deviations were about 20% and 12% for calcium and fluoride, respectively, for both monitoring methods. Drift problems observed in previous work with conductometric monitoring of i.s.e.'s were reduced significantly. Flow rates of 20 ml min −1 were possible with the calcium electrode. The response time of the calcium electrode is influenced by both diffusion and migration processes. Interfering ions cause the current to overshoot the steady-state current on injection. The conductometric response time of the fluoride electrode is generally faster than the potentiometric response time; both methods have concentration-dependent response times.