Cell Potential:

The potential difference between the two electrodes (half-cells) connected properly to form a galvanic cell and under conditions while no current flows through the system, is called the emf of the cell or cell potential. That is expressed in units of volts.

However, under the conditions when appreciable current flows through the cell, the methods of measurement of cell emf result the voltage values lower than the cell potential this is because the passage of current allows the appropriate reactions to occur, hence changing the concentrations of solution species and lowering the potential difference between the two electrodes. Also Ohmic drops due to the flow of current through internal resistance of the cell may lower the voltage and providing the measured potential which is less than the potential expected under conditions of no current flow.

Since no method of measurement of potential is possible for a no current flow through the cell it is apparent that what we measure is the emf (or voltage) of the cell at an insignificant current flow as a close approximate of the cell potential. Therefore, the cell potential is measured by a null point method with a potentiometer which involves balancing the cell output emf against an identical and measured externally applied voltage under an insignificant current (practically no current) flow through the cell.

The cell potential of a system can also be calculated on a theoretical basis using the Nernst equation. These computes could be completed in two ways. In one way, we calculate the potentials of the individual electrode using the Nernst equation that is each half-cell (electrode) involves a separate reaction which is considered in isolation. The higher value of reduction potential that is more positive value (or less negative) of the potential of the electrode indicates that it will act as cathode and will be written on the right side, whereas the less positive value (or more negative) that is lower value of potential indicates that it will act as anode and will be written on the left side. Now, the cell potential is the difference between the two electrode potentials (neglecting the liquid-junction potentials) such as:

E_cell = E_right - E_left                                                                           ... (1.37)

In the other way, we calculate Ecell in a consolidated manner by first calculating EO cell from the values of standard potentials of the individual electrodes:

E⁰_cell =  E^o _right  - E^o _left                                                                   ... (1.38)

and then we apply the Nernst equation for the cell reaction to calculate E_cell   (Equation 1.33).