In order to determine the standard electrode potential of an electrode, the electrode in standard conditions is connected to standard hydrogen electrode (SHE) to constitute a cell. If the electrode forms the negative terminal of the cell it is allotted negative value of electrode potential and if it forms the positive terminal of the cell, it is allotted a positive value of electrode potential. The potential difference between the electrodes is determined with the help of potentiometer or vacuum tube voltmeter. At the same time, the direction of flow of conventional current in the external circuit is also noticed with the help of galvanometer. This helps us to know the positive and negative terminals of the cell because conventional current in the external circuit flows from +ve terminal to -ve terminal. Now, the direction of the flow of electrons is opposite to that of conventional current. Since electron flow in external circuit occurs from anode to cathode it helps us to mark anode and cathode electrodes of the cell.
Knowing the E cell and electrode potential of one of the electrodes (SHE), that of the other can be calculated
For example, in order to find out standard electrode potential of zinc electrode, zinc electrode containing 1 M concentration of Zn2+ ions is connected to SHE as shown in Fig. 33.6. The voltmeter reading shows the potential difference of 0.76 volts. The electron flow is found to be from zinc electrode to SHE. From this it follows that Zn acts as anode while SHE acts as cathode. The net cell reaction is
Similarly, when standard copper electrode is coupled with SHE the voltmeter reading shows a potential difference (E cell) of 0.34 volts. The electron flow is found to occur from SHE towards copper electrode as shown in Fig. 33.7. Therefore, SHE in this cell acts as anode and copper electrode acts as cathode. The cell reaction is