ELECTROPLATING

Electroplating is a process of depositing a thin layer of a fine and superior metal (like chromium, zinc, nickel, gold etc.) over the article of a baser and cheaper metal (like iron, copper, brass), with the help of electric current.

Uses

Electroplating is very useful because of the following reasons:

1.   Surface protection e.g. nickel plating of iron to prevent corrosion.
2.   Makes the article attractive e.g., electroplating of silver or gold on brass etc.
3.   Repair of finer machine parts.

Process

The process of electroplating involves the following steps:

1.   Before electroplating the metal surface is cleaned thoroughly. Firstly, an alkaline solution is used to remove grease and then it is treated with acid to remove any oxide layer. It is then washed with water.
2.   The article to be electroplated is made cathode since metallic ions are positive and thus get deposited on the cathode.
3.   The anode is made of pure metal, which is to be coated on the article.
4.   The electrolyte is the salt of the metal to be coated on the article.
5.   A direct (D.C.) current is passed through the electrolyte.

The anode dissolves, depositing the metal ions from the solution on the article in the form of a metallic coating. The passage of low current is continued for a long time to ensure an even coating. Electrolysis of copper 11 sulphate solution using copper electrodes

The ions present in the solution are:

Cu²⁺, SO₄^2-,H⁺, OH^–

At the cathode

The positive ions are attracted to the negative cathode. There is competition between the copper ions and the hydrogen ions.

As the hydrogen ions are higher in the electrochemical series, the copper ions are preferentially reduced and copper metal is deposited at the electrode (a pink layer is observed)

Cu²⁺ + 2e Cu

At the anode

In this case the electrode is made of copper and it is easier for the copper to dissolve leaving its electrons behind on the anode than for any other ion to be released.

Cu Cu²⁺ + 2e

Ions remaining in solution

Copper is deposited at the cathode and is dissolved at the anode. Consequently the concentration of copper ions in solution remains constant. This can be used as a method of purification of copper as only pure copper is deposited at the cathode.

In this purification an anode made of impure copper is turned to pure copper at the cathode leaving the impurities behind (the sludge in the diagram).

Basic rules for electroplating an object metal M are as follows:

1. The object must be made the cathode
2. The electrolyte must be a solution of a salt of metal M,
3. The anode is made of a strip of metal M

Electrolysis of Lead Bromide.

Lead bromide must be heated until it is molten before it will conduct electricity. Electrolysis separates the molten ionic compound into its elements. The reactions at each electrode are called half equations.

The half equations are written so that the same number of electrons take part in each equation.

Pb²⁺ + 2e^– Pb (lead metal at the (-) cathode).

2Br^– Br₂ + 2e^– (bromine gas at the (+) anode).

Lead ions gain electrons (reduction) to form lead atoms. Bromide ions lose electrons (oxidation) to form bromine atoms. The bromine atoms combine to form molecules of bromine gas.

The overall reaction is;

PbBr₂(l) Pb(s) + Br₂(g)

Summary

Reactive metals (more reactive than hydrogen) are never deposited during electrolysis of aqueous solutions. If the metal ion comes from a metal more reactive than hydrogen then hydrogen gas is liberated at the cathode. Halide ions (chloride, bromide, and iodide) are released preferentially and if these are not present then the hydroxide ions from the water are released at the anode.