The selectivity coefficient Pot F ,OH K - - for a fluoride ISE is 0.1.
a. In general, what must an ion-sensing membrane in an ion-selective electrode do in order to generate the measured response (a potential difference across the membrane)? How does the inorganic crystal membrane used in the fluoride ISE accomplish this? Why does this electrode also respond to hydroxide?
b. Is it better to have a large or a small selectivity coefficient? Assuming constant ionic strength, what OH- concentration in a solution without F- would give the same response as a solution with 1.0 × 10-5 M F- and negligible OH-?
c. Why must fluoride-selective electrodes be used at moderate pH values (neither too high or too low)? Besides controlling pH in this region, why are citrate buffers often used for fluoride measurements? What is α(F-) at pH 5.5?
d. What is [OH-] at pH 5.5 and pH 10.0? If a fluoride ISE gives a response of +64.7 mV versus SCE when placed in 1.5 × 10-5 M F- at pH 5.5, what would the measured response be at pH 10.0 (assuming no change in ionic strength)? Would you over- or underestimate the fluoride concentration if you tried to use it at pH 10, and by about how much?
e. What potentials would you have measured in part d if you had used a Ag/AgCl reference electrode instead of a SCE?