Question 14.E.C: A solid-state fluoride ion-selective electrode responds to F...
A solid-state fluoride ion-selective electrode responds to F^{−} but not to HF. It also responds to hydroxide ion at high concentration when [OH^{−}] ≈ [F^{−}]/10. Suppose that such an electrode gave a potential of +100 mV (versus S.C.E.) in 10^{−5} M NaF and +41 mV in 10^{−4} M NaF. Sketch qualitatively how the potential would vary if the electrode were immersed in 10^{−5} M NaF and the pH ranged from 1 to 13.
At intermediate pH, the voltage will be constant at 100 mV. When [OH^{−}] ≈ [F^{−}]/10 = 10^{−6} M (pH = 8), the electrode begins to respond to OH^{−} and the voltage will decrease (i.e., the electrode potential will change in the same direction as if more F^{−} were being added). Near pH = 3.17 (= pK_{a} for HF), F^{−} reacts with H^{+} and the concentration of free F^{−} decreases. At pH = 1.17, [F^{−}] ≈ 1% of 10^{−5} M = 10^{−7} M, and E ≈ 100 + 2(59) = 218 mV. A qualitative sketch of this behavior is shown here.
