Determination of emf of a Cell

Determination of emf of a Cell

The emf of a cell

emf stands for electromotive force. A galvanic cell consists of two electrodes each having its own potential. The difference of potential between these two electrodes of a cell causes a current to flow from electrode of higher potential to the electrode of lower potential is called emf of the cell.
emf is expressed in volts. Greater the emf (i.e. potential difference between two electrode) greater the electricity flow and greater is the tendency of the cell redox to occur.
Potential of a cell assembled of two electrodes can be determined from the two individual electrode potentials using this formula-
ΔV_cell = E_red,cathode − E_red,anode
or, ΔV_cell = E_red,cathode + E_oxy,anode

can not be measured directly by a voltameter because on passing electricity, the emf of a cell is changed due to two reasons.

Firstly the internal resistance of the cell may consume some emf and secondly the chemical change can cause polarisation. Hence it is measured accurately by a potentiometer. It consists of a high resistance uniform wire AB, the terminal A & B of which are connected to a storage battery C having a constant and greater emf than that of the cell X under measurement. The positive terminal of the cell is connected with the positive terminal of the battery and the negative terminal of the cell is connected to the sliding contact O through the galvanometer G.
The sliding contact is then moved over the wire AB and a point O is noted at which no current passes through the galvanometer and the distance AO is measured. This distance AO is proportional to the emf of the cell X, E_x.
Similarly point O' is noted by replacing cell X by a standard cell (Weston cell having emf = 1.0183 at 20°C) and the distance AO' is measured. This distance AO' is proportional to the emf of the standard cell E_y.

Since the wire is of uniform thicness, the fall of potential per unit length must be same. If 'e' represents the fall of potential per units length, the above relation reduced to-

E_y, AO and AO' are known, So we can determine the emf of the cell X (E_x).
Here, we measure the external emf required to stop the current through the cell under measurement. As no current is passing through the cell at the time of measurement, two sources of above mentioned error are removed by measuring emf through potentiometer.