ACS Analytical Chemistry Practice Exam

A first-order indicator electrode works because the electrode potential is set directly by the redox equilibrium between the solid electrode material and its own ion in solution. In this case, the ion you’re trying to measure is the same element as the metal forming the electrode. The basic relation is E = E° + (RT/zF) ln a(M^z+), so the measured potential tracks the activity of that specific ion directly. A simple example is a zinc electrode measuring Zn2+ activity, where the Zn2+ in solution and the Zn metal on the electrode establish the potential through the Zn2+ + 2e− ⇌ Zn(s) couple. That is why this option is the best: the ion of interest is the same as the metal used in the electrode, giving a straightforward, Nernstian response. Other scenarios, where the ion is not the same as the electrode material or where the electrode isn’t a metal, involve different mechanisms and aren’t considered first-order indicator electrodes.