Bipolar pulse conductometric monitoring of ion-selective electrodes: Part 2. Studies with the fluoride-selective electrode

Abstract

Bipolar pulse conductometric monitoring of the fluoride ion-selective electrode (i.s.e.) is evaluated. It is shown that fluoride ion, in addition to affecting the electrode potential, also can have two effects upon the total resistance of the electrode. Fluoride ion can enter a gel layer on the surface of the doped lanthanum fluoride crystal; solution concentrations as low as 10 -9 M fluoride can significantly decrease the gel resistance. Fluoride concentrations in the potentiometric working range of the i.s.e. can also increase the electrode resistance. The conductometric detection limit is 2–4 decades lower than the potentiometric detection limit. However, because of the resistance factors, the conductometric curve is not monotonic, and shows a maximum at a fluoride concentration in the vicinity of the potentiometric detection limit. The two resistive changes possible have different time dependences; the decrease of the gel layer resistance becomes predominant at long measurement times, while the increase of the crystal resistance predominates within the first minute of exposure to solution. Hydroxide is shown to affect the i.s.e. potential and lower the gel layer resistance. Response time to changes in fluoride concentration are less than 15 s and do not show the strong concentration dependence observed potentiometrically. The i.s.e. is shown to have a slightly lower resistance to fluoride entering the crystal than to fluoride leaving it. The small resistive dependence on direction of ion migration may indicate a directional dependence of activation energy for ion transport across the membrane solution interfaces.