is assigned a zero potential at all temperatures corresponding to the reaction H + (aq) + e – ® H (g) The standard hydrogen electrode consists of a platinum electrode coated with platinum black. The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled through it. The concentration of both the reduced and oxidised forms of hydrogen is maintained at unity (Fig. .
). This implies that the pressure of hydrogen gas is one bar and the concentration of hydrogen ion in the solution is one molar. . .
Measurement of Electrode Potential Fig. . : Standard Hydrogen Electrode (SHE). At K the emf of the cell, standard hydrogen electrode çç second half-cell constructed by taking standard hydrogen electrode as anode (reference half-cell) and the other half-cell as cathode, gives the reduction potential of the other half-cell.
If the concentrations of the oxidised and the reduced forms of the species in the right hand half-cell are unity, then the cell potential is equal to standard electrode potential, E o R of the given half-cell. E o = E o R – E o L As E o L for standard hydrogen electrode is zero. E o = E o R – = E o R The measured emf of the cell: Pt(s) ç H (g, bar) ç H + (aq, M) çç Cu + (aq, M) ú Cu is . V and it is also the value for the standard electrode potential of the half-cell corresponding to the reaction: + (aq, 1M) + e – ® Cu(s) Similarly, the measured emf of the cell: Pt(s) ç H (g, bar) ç H + (aq, M) çç Zn + (aq, 1M) ç Zn is - .
V corresponding to the standard electrode potential of the half-cell reaction: + (aq, M) + 2e – ® Zn(s) The positive value of the standard electrode potential in the first case indicates that Cu + ions get reduced more easily than H + ions. The reverse process cannot occur, that is,