During electrolysis, several ions may be present in the electrolyte, but only one type of cation and one type of anion are discharged (released as atoms or molecules) at their respective electrodes. This selective release is known as the preferential discharge of ions. The ions discharged depend on certain factors because ions compete for discharge at the electrodes.

The main factors affecting which ions get discharged during electrolysis include:

1. Position of ions in the Electrochemical (Activity) Series
2. Nature of the electrodes (inert or active)
3. Concentration of ions in the electrolyte

1. Position of Ions in the Electrochemical Series

Ions higher in the electrochemical (activity) series are more difficult to discharge, while ions lower in the series are discharged more easily. At the cathode, ions that are lower in the reactivity series gain electrons more readily. At the anode, ions that are easier to oxidize are preferentially discharged.

Example: In aqueous NaCl solution, Na⁺ and H⁺ compete at the cathode. H⁺ is lower in the electrochemical series, so H₂ is discharged:
Cathode Reaction: $$2H^+ + 2e^- \rightarrow H_2(g)$$

2. Nature of the Electrode

Electrodes can be:

1. Inert Electrodes (e.g., carbon/graphite, platinum) These do not take part in the reaction. Only the ions in solution are discharged.
2. Active Electrodes (e.g., Copper, Silver, Mercury) These dissolve into the solution or participate in reactions. They can influence which ions are discharged.

Example: In the electrolysis of CuSO₄ using copper electrodes. Instead of SO₄²⁻ being discharged, the anode dissolves leading to the discharge of copper ions on the anode:
Anode Reaction: $$Cu(s) \rightarrow Cu^{2+} + 2e^-$$ (Copper dissolution)

Cathode reaction: $$ Cu^{2+} + 2e^{-} → Cu $$ (Copper deposition)

Similarly, In the electrolysis of aqueous sodium chloride using mercury electrode, sodium ions are reduced at the mercury cathode due to sodium/mercury affinity. Sodium here is preferentially discharged over hydrogen. Sodium amalgam Na(Hg) is formed

Cathode reaction: $$ 2Na^{+} + 2e^{-} + Hg → Na(Hg) $$

Anode reaction: $$ 2Cl^{-} → Cl_{2} + 2e^{-} $$

Overall Reaction: $$2NaCl → 2Na(Hg) + Cl_2 $$

3. Concentration of Ions

When two ions have almost equal discharge tendencies (i.e close proximity in the activity series), the ion with the higher concentration is discharged preferentially.

Example: Concentrated vs dilute brine
- Dilute NaCl solution: OH⁻ is discharged at the anode.
- Concentrated NaCl solution: Cl⁻ is discharged because its concentration is higher.

Anode Reaction in concentrated brine: $$2Cl^- \rightarrow Cl_2(g) + 2e^-$$

The behaviour of ions during electrolysis is governed by how easily they gain or lose electrons. Ions that require less energy to be reduced (cations) or oxidized (anions) will be discharged first. When more than one competing ion is present, the system chooses the ion that offers the easiest path for electron transfer. Therefore, factors like the electrode material and ion concentration alter the energy required for these electron transfers.

These principles are essential in industrial electrolysis, metal purification, and electroplating.