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Electrodes, Anode, Cathode

 

Electrodes are materials that conduct electricity, which are used to establish contact with the non metallic part of a circuit such as electrolyte, vacuum, or semiconductor. They enable electrical current to be passed from one point to another, such as, from a power source to a device like lamp.

Electrodes are usually made of metals, such as silver, lead, copper, and zinc. They are also made of certain non metals that conduct electricity, such as graphite and mercury. They are made in different forms and shapes, including, rod, pole, wire, and plate.

In an electrochemical cell, two kinds of electrodes are used. These are Anode, and Cathode.

The anode is the electrode where negative ions or anions in the cell migrate to. The negative ions lose electrons here and oxidation takes place. The anode is therefore defined as the electrode where electrons or current leave the cell.

The cathode is the electrode where cations or positive ions in solution migrate to. Here, they acquire electrons and become reduced (reduction reaction takes place here). Therefore, the cathode can be defined as the electrode where electrons or current enter the cell.

Any of the electrodes can be the anode or cathode in an electrochemical cell depending on the direction of flow of current.

In an electrolytic cell, where an external source of power is applied to the cell, the cathode is the negative electrode where electrons or current enter the cell. The negatively charged cathode donates electrons to the cations which get reduced. The anode on the other hand becomes the positive electrode.

However, in a Voltaic or Galvanic cell, or shaft battery, where electrical energy is generated from chemical substances, the anode becomes the negative electrode while the cathode is positive.  

Nature of the electrode used:

Sometimes, the nature of the electrode used may determine the ions discharged in an electrolytic process.

Example, considering the electrolysis of sodium chloride solution using separately, platinum and mercury cathode. Using platinum cathode, H+ is discharged in preference to Na+ according to the positions of their ions in the electrochemical series – thus hydrogen gas is produced at the cathode.

But if mercury cathode is used, Na+ will be discharged instead of H+, to form a mixture of mercury and sodium, called sodium amalgam, Na/Hg. The preferential discharge of Na+ is due to the fact that less energy is used, compared with the discharge of H+ when mercury cathode is used.

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