GALVANIC CELL

GALVANIC OR VOLTAIC CELLS

The names of cells, galvanic and voltaic are given in honor of Italian scientists Luigi Galvani (1737-1798) and Alessandro Volta (1745-1827) for their work in electrochemistry. 

In this cell a spontaneous chemical reaction occurring inside it produces electricity. 

Each galvanic cell is made up of two half cells. 

Each half cell consists of a metal strip immersed in a solution of its own ions of known concentration. For example, 

A strip of zinc metal immersed in a 1M aqueous solution of Zn²⁺ ions forms a half cell. 

It follows, therefore, that two metal plates and the solutions of the ions of metals of known concentration are required for the construction of a galvanic cell. 

First, two half cells are constructed by immersing the two metal plates in the solutions of their ions placed in separate containers. 

The two half cells so constructed are then combined together to form the galvanic cell. 

The metal strips called electrodes are connected by an electrically conducting wire. 

Two solutions are connected by a salt bridge. 

In a galvanic cell, the potential of the cathode is higher than that of the anode. 

This is because the species that undergoes reduction at cathode removes electrons from cathode leaving a positive charge on it which corresponds to a high potential. 

At the anode, the oxidation results in the transfer of electrons to the electrode. 

The anode, therefore, acquires a negative charge that corresponds to a low potential. 

The anode is thus a negative electrode and the cathode a positive electrode. 

Salt bridge 

In a galvanic cell, the solutions are connected by a salt bridge. It is a U-shaped glass tube containing a saturated solution of an electrolyte such as KCl or NH4NO3, and a 5% agar solution. The ions of the electrolyte neither react with the ions of the electrode solutions neither oxidized nor reduced at the electrodes. The salt bridge is prepared by filling the glass tube with a hot saturated solution of the salt and agar solution and allowing it to cool. The cooled mixture sets to solid so that the solution does not come out on inverting the tube.

Functions of the salt bridge

 The salt bridge serves the following functions:

(i) It prevents the mixing of electrode solutions. 

(ii) It maintains electrical neutrality in both the solutions by a flow of ions. 

(iii) It provides electrical contact between the two solutions and thereby completes the electrical circuit.