Electroplating
Electroplating is a practical application of electrolysis used to coat the surface of a metal object with a thin, uniform layer of another metal. In the electroplating circuit, the object to be plated is always made the cathode (the negative electrode), because metal ions in the electrolyte are attracted to it and undergo reduction, depositing as a solid metal coating on its surface. The anode (the positive electrode) is made of the plating metal itself, which means that as the electrolysis proceeds, the anode gradually dissolves, releasing fresh metal ions into the electrolyte and maintaining a constant supply of the plating metal ions. The electrolyte must be a solution of a soluble salt of the plating metal — for example, when silver-plating, the electrolyte is a solution containing Ag⁺ ions such as silver nitrate. Electroplating is widely used in industry for two main reasons: to improve the appearance of objects (making them look more attractive or valuable) and to improve resistance to corrosion by providing a protective outer layer. Unlike electroplating with inert electrodes (such as graphite or platinum), where the electrolyte decomposes and eventually becomes depleted, the dissolving anode in electroplating continuously replenishes the metal ions, allowing the process to run for extended periods without the electrolyte losing its effectiveness.
How Electroplating Works
Electroplating relies on the same principles as any electrolytic process: when a direct current is passed through an electrolyte, positively charged cations migrate towards the cathode and negatively charged anions migrate towards the anode. At the cathode, reduction takes place. The metal ions from the electrolyte gain electrons and are deposited as neutral metal atoms onto the surface of the cathode:
At the cathode (reduction): Mⁿ⁺ + ne⁻ → M (s)
For example, in silver plating:
Ag⁺ + e⁻ → Ag (s)
At the anode, oxidation takes place. Because the anode is made of the plating metal, the metal atoms lose electrons and enter the solution as ions:
At the anode (oxidation): M (s) → Mⁿ⁺ + ne⁻
For example:
Ag (s) → Ag⁺ + e⁻
The concentration of metal ions in the electrolyte therefore remains approximately constant throughout the process. The anode shrinks as it dissolves, while the cathode gains mass as the metal coating builds up. The thickness of the plated layer can be controlled by adjusting the current and the duration of the electrolysis — a larger current or a longer time produces a thicker coating.
Setup and Circuit
The electroplating setup consists of the following components:
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A DC power supply — a battery or power pack that provides a steady direct current. Alternating current cannot be used because the direction of ion movement would constantly reverse, preventing a coherent coating from forming.
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The cathode (negative electrode) — this is the object that is to be electroplated. It is connected to the negative terminal of the power supply. The object must be thoroughly cleaned before plating to ensure good adhesion; any grease, dirt, or oxide layer will cause the plating to peel or flake off.
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The anode (positive electrode) — this is a bar or sheet of the pure plating metal. It is connected to the positive terminal of the power supply. Over time, the anode visibly corrodes and decreases in mass.
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The electrolyte — a solution containing ions of the plating metal. Common electrolytes include:
- Silver nitrate (AgNO₃) for silver plating
- Copper(II) sulfate (CuSO₄) for copper plating
- Nickel(II) sulfate (NiSO₄) for nickel plating
- Chromium(III) sulfate (Cr₂(SO₄)₃) for chromium plating
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Connecting wires and possibly a variable resistor (rheostat) to control the current.
The circuit is essentially the same as any electrolytic cell: electrons flow from the negative terminal of the power supply to the cathode, through the external circuit, and back to the positive terminal from the anode. Within the electrolyte, the current is carried by the movement of ions.
Common Examples at IGCSE
| Plating Metal | Object Plated | Electrolyte Used | Purpose |
|---|---|---|---|
| Silver (Ag) | Jewellery, cutlery, trophies | Silver nitrate solution (AgNO₃) | Improves appearance — gives a shiny, attractive finish; silver is relatively unreactive so it resists tarnishing longer than base metals |
| Chromium (Cr) | Car bumpers, bicycle handlebars, bathroom fittings, kitchen taps | Chromium(III) sulfate solution (Cr₂(SO₄)₃) | Improves appearance (bright, shiny finish) AND provides excellent corrosion resistance; chromium is very hard and does not tarnish |
| Tin (Sn) | Steel food cans | Tin(II) sulfate solution (SnSO₄) | Prevents corrosion — tin is non-toxic and resists attack by food acids, protecting the steel can from rusting; this is why “tin cans” are actually steel coated with tin |
| Zinc (Zn) — galvanising | Iron and steel structures (roofing sheets, buckets, gates, lamp posts) | Zinc sulfate solution (ZnSO₄) | Provides sacrificial protection — zinc is more reactive than iron, so even if the coating is scratched, the zinc corrodes preferentially and the iron underneath is protected |
Note that galvanising can also be done by dipping the object in molten zinc (hot-dip galvanising) rather than by electroplating, but the principle of coating iron with a protective layer of zinc is the same.
Electroplating vs Using Inert Electrodes
It is important at IGCSE to distinguish between electrolysis using inert electrodes and electroplating using a reactive (dissolving) anode:
| Feature | Inert Electrodes (e.g. graphite, platinum) | Electroplating (dissolving anode) |
|---|---|---|
| Anode material | Does not react; does not dissolve | Made of the plating metal; dissolves during electrolysis |
| What happens at anode | Anions are discharged (e.g. 4OH⁻ → O₂ + 2H₂O + 4e⁻) | Metal atoms lose electrons and enter solution (e.g. Cu → Cu²⁺ + 2e⁻) |
| Electrolyte composition | Changes over time — ions are discharged and not replenished, so the solution may change colour and concentration decreases | Remains approximately constant — as metal ions are deposited at the cathode, new metal ions dissolve from the anode |
| Electrode mass change | Anode does not lose mass (inert); cathode gains mass from deposited metal | Anode loses mass; cathode gains mass |
| Colour of electrolyte | May change or fade (e.g. blue CuSO₄ fades to pale blue or colourless) | Stays the same colour throughout |
Copper Purification as Electroplating
The electrolytic purification of copper is a special case of electroplating and is an important IGCSE topic. Impure copper (about 98-99% pure) from smelting is made the anode, and a thin sheet of pure copper is made the cathode. The electrolyte is copper(II) sulfate solution.
During electrolysis:
- At the anode: the impure copper dissolves: Cu → Cu²⁺ + 2e⁻. Impurities such as gold, silver, and platinum are less reactive than copper and do not dissolve — instead they fall to the bottom of the cell as anode sludge (which is valuable and can be recovered).
- At the cathode: pure copper ions are discharged and deposited: Cu²⁺ + 2e⁻ → Cu. Only copper is deposited because it is less reactive than impurities like zinc and iron, which remain as ions in solution.
The result is a cathode of 99.99% pure copper, which is suitable for use in electrical wiring, where high conductivity is essential. This process is essentially electroplating copper onto a copper cathode, with a dissolving anode made of impure copper.
Sources, Common Misconceptions
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus, Topic 4: Electrochemistry
- Edexcel IGCSE Chemistry Specification, Section 1: Principles of Chemistry (Electrolysis)
- Roger Norris, Cambridge IGCSE Chemistry Revision Guide
- Richard Harwood and Ian Lodge, Cambridge IGCSE Chemistry Coursebook
Common Misconceptions
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“The anode in electroplating is always graphite.” This is incorrect. In electroplating specifically, the anode must be made of the plating metal so that it dissolves and replenishes the electrolyte. Graphite or platinum anodes are used only when the electrolysis is carried out to decompose a compound.
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“Electroplating and galvanising are the same process.” While both involve coating a metal, galvanising specifically refers to coating iron or steel with zinc for sacrificial protection. Galvanising can be done by electroplating, but it is more commonly done by dipping in molten zinc.
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“The object to be plated goes on the positive terminal.” This is a very common error. The object must be connected to the negative terminal (the cathode) so that positively charged metal ions are attracted to it and deposit on its surface.
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“The colour of the electrolyte always stays the same.” This is true for electroplating with a dissolving anode because metal ions are replenished. However, with inert electrodes the electrolyte does change colour and concentration over time. Students must be careful to distinguish the two cases.
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“Electroplating works with an alternating current (AC).” It does not. AC would cause ions to move back and forth, and no coherent metal coating would form. A direct current (DC) is always required.
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“The anode gains mass during electroplating.” In fact, the opposite is true: the anode dissolves and therefore loses mass, while the cathode (the object being plated) gains mass.