Since NaF is the salt of a strong base and a weak acid it should not be surprising that the bromthymol blue shows it is slightly basic (blue-green) before anything happens. The power supply acts as an "electron pump" with the electrons leaving the negative terminal and returning to the positive terminal. That means in an electrolytic cell reduction occurs where the negative terminal of the power supply is attached. As in a galvanic cell, this electrode is called the cathode. Where the positive terminal of the power supply is attached, oxidation occurs. That electrode is the anode.

What is reduced in the NaF cell? A gas forms at each electrode (more at the cathode than the anode) and the solution becomes very basic at the cathode and acidic at the anode. The mixture contains Na⁺, F^- and H₂O. To decide what substance is reduced and what is oxidized we need to look at the SRP table.