Electrochemistry

Electroplating is an electrodeposition process for producing a dense, uniform, and adherent coating, usually of metal or alloys, upon a surface by the act of electric current.The workpiece to be plated is the cathode (negative terminal). The anode, however, can be one of the two types: sacrificial anode (dissolvable anode) and permanent anode (inert anode).The sacrificial anodes are made of the metal that is to be deposited. The permanent anodes can only complete the electrical circuit, but cannot provide a source of fresh metal to replace what has been removed from the solution by deposition at the cathode. Platinum and carbon are usually used as inert anodes.

Electrolyte is the electrical conductor in which current is carried by ions rather than by free electrons (as in a metal). Electrolyte completes an electric circuit between two electrodes. Upon application of electric current, the positive ions in the electrolyte will move toward the cathode and the negatively charged ions toward the anode. This migration of ions through the electrolyte constitutes the electric current in that part of the circuit. The migration of electrons into the anode through the wiring and an electric generator and then back to the cathode constitutes the current in the external circuit. The metallic ions of the salt in the electrolyte carry a positive charge and are thus attracted to the cathode. When they reach the negatively charged workpiece, it provides electrons to reduce those positively charged ions to metallic form, and then the metal atoms will be deposited onto the surface of the negatively charged workpiece.

For plating copper from a solution of the metal salt copper sulfate (CuSO₄). The cathode, which is the workpiece to be plated, is charged negatively. Some of the electrons from the cathode bar transfer to the positively charged copper ions (Cu²⁺), setting them free as atoms of copper metal. These copper atoms take their place on the cathode surface and copper plate it. Concurrently, the same number of sulfate ions SO4^2-is discharged on the copper anodes, thereby completing the electrical circuit.

The application of an electrical potential between a copper anode and a copper cathode in a cell containing H₂SO₄ and CuSO₄ solution causes the following reaction and process to take place:

Copper is electrochemically dissolved from the anode into the solution:

Cu⁰ -2e = Cu²⁺ E0 = +0.34 V Producing copper cations plus electrons

The electrons produced by the above reaction are conducted towards the cathode through the external circuit and power supply
The Cu²⁺ cations in the solution are conducted to the negative electrode (cathode).
The electrons and the Cu²⁺ ions recombine at the cathode surface to produce copper metal which plates on the cathode:

Cu²⁺ + 2e = Cu⁰ E0 = + 0.34 V

Faraday's first law of electrolysis and Faraday's second law of electrolysis state that the amount of a material deposited on an electrode is proportional to the amount of electricity used. The amount of different substances liberated by a given quantity of electricity is proportional to their electrochemical equivalent (or chemical equivalent weight m):

m = Kit =A/F It

The electrochemical equivalent of an element (A) is its atomic weight divided by the valence change involved in the reaction. the current (I) , and the time of deposition, t (sec),m is the amount of the metal deposited. The Faraday constant (F) is presents the amount of electric charge carried by 1 mol: or the Avogadro's number of electrons.

F = 26.8 Ah[val]-1 or 96520AS[val]-1

Current efficiency(S) in % =100 Q/1.864It.10^-6, where Q is the weight of metal deposited or dissolved in g