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.
Frequently Asked Questions
What scientific concept do you need to know in order to solve this problem?
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