Selective discharge

Selective discharge is the process by which, when more than one type of ion could be discharged at an electrode, one ion is discharged in preference to the others. The choice depends on three factors: position in the electrochemical series, the concentration of the ions, and the type of electrode. Malay: penyahcasan terpilih · Chinese: 选择性放电.

Selective discharge matters most in aqueous solutions, because water provides H⁺ and OH⁻ ions in addition to the ions of the dissolved compound. At the cathode, then, both a metal cation and H⁺ may be present, and at the anode both a non-metal anion and OH⁻ may be present, so the cell must select which one to discharge. Position in the electrochemical series is the first guide, but a high concentration of an anion can promote its discharge, and an active electrode changes the anode reaction entirely.

Example. In concentrated sodium chloride solution with carbon electrodes, chloride and hydroxide ions both reach the anode. Although hydroxide is normally discharged more readily, the high concentration of chloride means chlorine is released instead, showing how concentration can override position.

Confusion to avoid. Do not rely on the electrochemical series alone. Many wrong answers come from ignoring concentration or the electrode material. Always state all three factors, then explain which one decides the case in front of you, and remember that at the cathode a reactive metal ion loses to H⁺, releasing hydrogen rather than the metal.

A worked routine. A dependable method is to list every ion present at each electrode, including H⁺ and OH⁻ from water, then apply the three factors in order: check the electrochemical series first, then ask whether a very high concentration favours a particular anion, and finally note whether the electrode is inert or active. Writing the chosen half-equation at each electrode, and saying why the other ion was not discharged, is exactly the structure that earns full marks.

Selective discharge is the reasoning that links the electrochemical series to the actual products of electrolysis, so it underlies electroplating, copper purification and many structured questions in SPM Chemistry.