Question

A solid-state fluoride ion-selective electrode responds to F-but not to HF. It also responds to hydroxide ion at high concentration when. 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.

Answer 1

At higher concentration of NaF (10-4 M) the electrode shows lower potential value but at lower concentration of NaF (10-5 M) it is higher. We know that HF is a weak acid and at lower pH values concentration of H+ ion will be higher. So this higher amount of hydrogen ion will be associated with Fluoride ion and the concentration of HF is higher. But the concentration of F- will be lower. So at lower pH values obtained potential will be lower compare to higher pH values.

 

Qualitative sketch:

At higher concentration of NaF (10-4 M) the electrode shows lower potential value but at lower concentration of NaF