Reference Electrode

 Reference Electrode Standard hydrogen electrode

The electrons released in oxidation reaction must have to go somewhere. Hence, each oxidation must be accompanied by reduction. Therefore, the occurrence of only oxidation or only reduction is not possible. Oxidation occurs at anode in galvanic cell. The electrons released flow through external circuit to cathode where they are used in reduction. The sum of oxidation and reduction reaction taking place inside the cell is known as redox reaction. This redox reaction is associated with a potential known as cell potential. The cell potential is thus algebraic sum of two electrode potential 

Thus, if the potential of one of the electrodes is zero then the measured emf of the cell will be equal to the potential of the other electrode. Hence, for measurement of single electrode potential, it must be coupled with another electrode for which potential is zero. Therefore it is necessary to choose an arbitrary standard electrode as a reference point.

The chemists have chosen standard hydrogen electrode as a reference electrode. The half cell reaction at this electrode may be oxidation or reduction associated with a potential of zero volt. This is called primary reference electrode.

Several other electrodes like calomel electrode, Ag-AgCl electrode, glass electrode are used as secondary reference electrodes since standard hydrogen electrode is not convenient to use. The potentials of these secondary reference electrodes are exactly measured by coupling with standard hydrogen electrode (SHE). These potentials of secondary reference electrode being known, they can be used as reference electrodes to determine the potentials of all other electrodes.