Page "Electrochemical cell" Paragraph 3

An electrochemical cell consists of two half-cells.

Each half-cell consists of an electrode, and an electrolyte.

The two half-cells may use the same electrolyte, or they may use different electrolytes.

The chemical reactions in the cell may involve the electrolyte, the electrodes or an external substance ( as in fuel cells which may use hydrogen gas as a reactant ).

In a full electrochemical cell, species from one half-cell lose electrons ( oxidation ) to their electrode while species from the other half-cell gain electrons ( reduction ) from their electrode.

A salt bridge ( e. g. filter paper soaked in KNO < sub > 3 </ sub >) is often employed to provide ionic contact between two half-cells with different electrolytes, to prevent the solutions from mixing and causing unwanted side reactions.

As electrons flow from one half-cell to the other, a difference in charge is established.

If no salt bridge were used, this charge difference would prevent further flow of electrons.

A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the contents of each separate.

Other devices for achieving separation of solutions are porous pots and gelled solutions.

A porous pot is used in the Bunsen cell ( right ).