Electrochemical series

The electrochemical series is an ordered list of ions used to predict which ion is discharged first during electrolysis. Ions of less reactive elements sit lower in the series and are discharged more readily. Malay: siri elektrokimia · Chinese: 电化学序.

The series ranks cations and anions by their tendency to be discharged, that is, to gain or lose electrons at an electrode. Among cations, those of less reactive metals such as copper and silver, and hydrogen, are discharged in preference to those of reactive metals such as sodium and potassium. Among anions, the order commonly used places halides and simple ions above the very stable sulfate and nitrate ions. Position in the series is only the first of three factors: concentration of the solution and the type of electrode can override the ordering in specific cases.

Example. In the electrolysis of dilute sodium chloride solution with carbon electrodes, both Na⁺ and H⁺ are present at the cathode. Because H⁺ is below Na⁺ in the series, hydrogen is discharged, and hydrogen gas is released rather than sodium metal.

Confusion to avoid. Do not mix up the electrochemical series with the reactivity series. The reactivity series ranks metals by how readily their atoms lose electrons, whereas the electrochemical series ranks ions by how readily they are discharged, and it includes anions and hydrogen as well as metals. The two orders are related but are used for different predictions.

Reading the series. In practice you use the series like a ladder. At the cathode the cation nearer the bottom is discharged first, and at the anode the anion nearer the bottom of the anion list is discharged first, unless concentration or an active electrode changes the outcome. For a concentrated halide solution, for instance, the halide ion may be discharged in preference to the hydroxide ion even though position on its own would not predict it, so the series must always be read alongside the other two factors.

The electrochemical series is the backbone of selective discharge, so it links directly to electrolysis, voltaic cells and the choice of products at each electrode, all of which are examined in SPM Chemistry.