Electrode Potential

Standard Electrode Potential As we know that it is not possible for any half-cell to work independently and it is not possible to determine the absolute electrode potential of an electrode. We can only find the relative electrode potential. But this can be made possible by selecting one of the electrodes as a reference electrode and fixing its potential as zero. For this, reversible hydrogen electrode has been universally accepted as a reference electrode. It is standard hydrogen electrode (S.H.E.). STANDARD HYDROGEN ELECTRODE It consists of platinum wire sealed in a glass tube and has a platinum foil attached to it. The foil is coated with finely divided platinum and acts as platinum electrode. This electrode is dipped in an acidic solution containing H+ ion in 1M concentrations. Hydrogen gas at 1atmosphere pressure is bubbled into the solution at constant temperature.

Standard hydrogen electrode: The surface of the foil acts as a site for the reaction.Since this electrode behaves as both cathode as well as anode, so both oxidation and reduction reactions are occurring at a single electrode. The reactions occurring at the electrode are:

This standard electrode is regarded as reversible electrode.

The electrode potential of any half-cell can be determined by connecting it with this standard hydrogen electrode. The electrode potential of this standard electrode is taken as zero.

The electrode potential of a metal electrode, which is determined with respect to standard or normal hydrogen electrode, is called standard electrode potential (EO).

Measurement of standard electrode potential The standard electrode potential of a metal electrode can be measured by dipping electrode in a solution with 1 M concentration of an electrolyte. This constitutes the metal–metal ion electrode, M/MN+(aq). This half-cell is connected to S.H.E. through a salt bridge. Both the electrodes are connected to the voltmeter. From the measured e.m.f. of the cell the standard electrode potential of the half-cell is calculated.

Explanation: 1) Measurement of electrode potential of Zn2+/ Zn electrode: An electrode which consists of zinc rod immersed in 1M solution of ZnSO4 is combined with S.H.E.

Cell reaction can be obtained if we know which substance is going to oxidize or reduce at cathode and anode in the cell:

This can be determined by measuring the charges on the electrode because in a galvanic cell, cathode is positive electrode and anode is negative electrode. But when we connect the two electrodes with the voltmeter the proper measurements (e.m.f. of the cell) are obtained only if the positive terminal of the voltmeter is connected to positive electrode and negative terminal to the negative electrode. If the connections are not correct no reading will be obtained in the voltmeter.

Thus if the voltmeter gives the proper reading it indicates that the electrode connected to the positive terminal of the voltmeter acts as cathode and the electrode connected to the negative terminal of the voltmeter acts as anode and electrons flow from anode to cathode.

In the given cell electrons flow from zinc electrode to hydrogen electrode. Therefore the zinc electrode acts as anode and S.H.E. acts as cathode.

The cell can be represented as

The measured cell potential of the cell is 0.76 V. Then the e.m.f. of the cell

2) Measurement of electrode potential of Cu2+ Cu electrode:

The electrode potential of Copper – Copper ions can be measured by connecting the cell containing copper electrode in 1M CuSO4 solution with S.H.E. Here hydrogen has greater tendency to loose electron as compared to copper, so oxidation occurs at hydrogen electrode and reduction occurs at copper electrode. Consequently the hydrogen electrode acts as anode and copper electrode acts as cathode.

This cell can be represented as