Copper and Nickel Deposition 3/3 – Electrogravimetry and the Electrochemical Series

Deposition of metals

The deposition of metal is not only used to modify a surface for its optical appearance, haptics, or corrosion protection; this technique can also be used for analytical purposes.

An electrode with a known mass is immersed in a solution containing the metal ions for quantification, the analyte. A potential is applied and the metal ions are deposited as metal or metal oxide films. Which potential is applied and how the metal is deposited depends on the metal.

Copper, for example, can be reduced at the electrode and deposited as elemental metal. It forms a properly attached film on platinum electrodes. This film can be washed, dried, and scaled. Lead ions are oxidized for deposition. A film of lead oxide is formed that sticks well to a platinum electrode. This film can be washed, dried, and scaled as well. The composition of lead oxide (PbO2) is known and so the amount of lead in the film can be calculated.

This technique for quantification demands clean and careful working. Damage of the film, remains of water or other solvents during scaling, unfinished deposition of the metal, or a wrong mass of the electrode without the film will lead to wrong results. To reduce the time needed for deposition the metal solution needs to be heated and stirred during the deposition. 

It seems easier to measure the charge that passes through the electrode during the deposition, but often side reactions and capacitive current appear during these processes and these will contribute to the charge passing through the electrode. 

Electrochemical series

The electrochemical series show that different processes happen at different potentials. For example, copper is deposited at less negative potentials than zinc, and cadmium is easier oxidized than lead. 

Of course these insights are also true for other redox active species than metals: Chloride will be oxidized to chlorine at lower potentials than water will be oxidized to oxygen. These differences can be used to determine different molecules from the same solution, if the difference is large enough for these processes to be regarded as separate.

A classic example is the determination of copper and nickel from a mixed solution of copper and nickel salts. First the copper is deposited at the platinum electrode and after that it is scaled. On the copper film the nickel film is deposited by applying more negative (cathodic) potential as used for the copper deposition. After deposition this film can be scaled too. Since an error from the copper quantification will create errors for the nickel quantification, these quantifications have to be performed carefully.