) using a voltmeter. If we

) using a voltmeter. If we know the emf of any one of the electrodes which constitute the cell, we can calculate the emf of the other electrode from the measured emf of the cell using the expression ( . ). Hence, we need a reference electrode whose emf is known For that purpose, Standard Hydrogen Electrode ( SHE ) is used as the reference electrode. It has been assigned an arbitrary emf of exactly zero volt. It consists of a platinum electrode in contact with 1M HCl solution and atm hydrogen gas. The hydrogen gas is bubbled through the solution at  C as shown in the figure . . SHE can act as a cathode as well as an anode. The Half cell reactions are given below. If SHE is used as a cathode, the reduction reaction is 2H (aq,1M)+2e H (g, atm) E = volt o If SHE is used as an anode, the oxidation reaction is H (g, atm) 2H (aq, 1M) + 2e E volt o = Illustration Let us calculate the reduction potential of zinc electrode dipped in zinc sulphate solution using SHE. Step : The following galvanic cell is constructed using SHE Zn (s) Zn (aq, 1M) H (aq, 1M) H (g, 1atm) Pt (s) + Step : The emf of the above galvanic cell is measured using a volt meter. In this case, the measured emf of the above galvanic cell is .76V. Calculation We know that, E = E + E cell ox Zn Zn red SHE o o + o