indicator electrode → forms an electrochemical half cell with the ions of interest in the test solution
For Ag+: Ag+|Ag couple → change in [Ag+] → affects reaction quotient, Q → changes cell potential, Ecell
For Halides (X-): Ag+ + X- → AgX (s)
Ksp expression for AgX (s) → Ag+ + X-
Why can a silver electrode be used as an indicator electrode for Ag+ and halides?
• The silver electrode contains two membranes. One membrane contains a substance that reacts with Ag+ and halides to form an ionic silver-halide complex. The potential difference created at the second, ion-selective membrane changes as the [silver-halide complex] changes.
• Because Ag+ and halides equilibrate with ion-exchange sites at the outer surface of the ion-selective membrane. Diffusion of Ag+ and halides out of the membrane creates a charge imbalance or potential difference. Changes in [Ag+] and [halides] alter the magnitude of the potential difference.
• Because the reaction at the silver electrode involves the Ag+ | Ag couple, changes in the [Ag+] effect the potential at the electrode. Halides react with Ag+ to form solid silver halides. At equilibrium, the [Ag+] equals Ksp/[halide], so changes in the [halide] effect the potential at the electrode.
• The reaction at the silver electrode forms a silver-halide complex. Formation of the silver-halide complex reduces the amount of metallic silver in the electrode. Changes in the mass of the silver electrode alter the potential measured at the electrode.