Selective Discharge Rules

The ease of discharge of an ion depends on several factors, including the nature of the electrode, and the nature of the electrolyte (molten/aqueous, concentrated/dilute).

1. Cations always discharge at the cathode
2. Anions always discharge at the anode
3. The ions of the more reactive metals are more difficult to discharge than those of less reactive metals
4. The sulphate and nitrate ions are never discharged (but not when altered)
5. Halide ions will be difficult to discharge
6. The more concentrated the solution, the more chance the ions will be discharged

Note: if chloride ion is present it is most likely always going to be the anode product. The following equation represents the discharging of Hydroxide ions:

4 OH–  ->  O2 + 2 H2O + 4 e–