In a potentiometric titration, what determines the end-point potential?

• A. The end point corresponds to the equivalence point where the redox species are present in equal or significant concentrations, causing a large change in measured potential.
• B. The potential is determined by the solution’s color change.
• C. The end point is when the current reaches a maximum.
• D. The end point is always at zero potential.

Answer: (A)

In potentiometric titration, the end-point is set by a sharp change in the measured potential as the redox balance in the solution passes through the equivalence point. The electrode potential follows the Nernst relationship, which depends on the ratio of oxidized to reduced forms of the redox couple present. Before equivalence, one form dominates and the potential moves gradually with added titrant. At equivalence, the two forms are present in comparable amounts, so even a small amount of titrant causes a large change in that ratio and a noticeable jump in potential. After equivalence, the solution is dominated by the titrant’s redox couple, and the potential levels off to a value characteristic of that couple.

Color change is the signal used in visual indicators, not the electrical signal measured in potentiometry. The end-point here is not defined by a maximum current, which is something seen in amperometric methods. And it is not constrained to zero potential—the end-point potential depends on the specific redox system and electrode in use.