Electrolysis of ionic solutions

Electrolysing aqueous solutions of ionic compounds can be more complicated than electrolysing molten compounds, because the water molecules can provide hydrogen ions (H+) and hydroxide ions (OH), in addition to the ions from the ionic compounds.

At the negative electrode

Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series:

  • the metal will be produced if it is less reactive than hydrogen
  • hydrogen will be produced if the metal is more reactive than hydrogen
The relative reactivity of selected elements from most to least: potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, tin, lead, hydrogen, copper, silver, gold, platinum.The reactivity series of metal – carbon and hydrogen are not metals, but they are shown for comparison

For example, the electrolysis of copper(II) chloride solution produces copper at the negative electrode. However, the electrolysis of sodium chloride solution produces hydrogen. Note that the electrolysis of sodium chloride solution produces chlorine gas and hydrogen gas but also leaves a solution of sodium hydroxide as well.

At the positive electrode

If the negative ion from the ionic compound is simple (eg Cl or Br), then that element is produced. If the negative ion is a compound ion (eg NO3, SO42–, CO32–), then oxygen is produced from the hydroxide ion present instead.

The table summarises some of the elements you should expect to get during electrolysis.

Negative ion in solutionElement given off at positive electrode
Chloride, ClChlorine, Cl2
Bromide, BrBromine, Br2
Iodide, IIodine, I2

At the negative and positive electrodes

This table shows some common ionic compounds (in solution), and the elements released when their solutions are electrolysed using inert electrodes, eg carbon electrodes:

Ionic substanceElement at –Element at +
Copper(II) chloride, CuCl2Copper, CuChlorine, Cl2
Sodium chloride, NaClHydrogen, H2Chlorine, Cl2
Hydrochloric acid, HClHydrogen, H2Chlorine, Cl2

Electrolysis of sodium chloride solution (brine)

As shown above, the electrolysis of sodium chloride solution will produce chlorine gas at the anode and hydrogen gas at the cathode.

  • at the anode, 2Cl → Cl2 + 2e (oxidation)
  • at the cathode, 2H+ + 2e → H2 (reduction)

During the electrolysis, hydrogen and chloride ions are removed from solution whereas sodium and hydroxide ions are left behind in solution. This means that sodium hydroxide is also formed during the electrolysis of sodium chloride solution. This process is carried out on an industrial scale using sea water to produce hydrogen gas, chlorine gas and sodium hydroxide solution.