Standard hydrogen electrode

The standard hydrogen electrode (SHE) has the following specifications:

    • hydrogen gas at 1 bar or 100 kPa in equilibrium with a 1.00 mol dm-3 solution (strictly speaking, the parameter is not [H+] = 1 M but a_{H^+}=1 , where a_{H^+} is the activity of hydrogen. For simplicity, we have assumed [H^+]=a_{H^+}).
    • a platinum black (finely divided platinum) coated platinum electrode in contact with both gas and solution
    • electrode potential of SHE is defined as 0V.

We can measure and record the electrode potential of a half-cell of electrolyte activity of 1, or for simplicity, of electrolyte concentration of 1.00 mol dm-3, by connecting it to the SHE. To maintain consistency in data reporting and avoid confusion, we adhere to the IUPAC convention as follows:

    1. Place the SHE on the left and the half-cell of interest on the right.
    2. Connect the positive lead of a high impedance voltmeter to the right-hand electrode and the ground lead to the left-hand electrode. Note that with the voltmeter connected in series to the circuit, we are measuring the potential difference of an open circuit with almost no current flowing. This is an accurate way to measure the potential difference, as it allows the concentration of the electrolytes to remain at 1.00 mol dm-3. Alternatively, we can use a potentiometer.
    3. The potential difference of the electrochemical cell, Ecell, is defined as Ecell = Eright Eleft.
    4. Ecell, measured at 25oC, is called the standard electrode potential, and is defined as a reduction potential, i.e. the potential for the reaction:

M_{oxidised}+ne^-\rightleftharpoons M_{reduced}

By following this convention, the value of Ecell , which is equal to Eright, measures the spontaneity of the half-cell reaction from left to right. For example, the diagram below shows that His oxidised to H+ in the left half-cell, with the electron migrating to the right cell where Cu2+ is reduced to Cu. 

The voltmeter records a value of +0.34V at 25oC. Since,


+0.34\; V=E_{right}-0\; V

the electrode potential of Eright is +0.34 V by convention, and represents the reduction half-cell reaction of

Cu^{2+}(aq)+2e^-\rightleftharpoons Cu(s)

We call this electrode potential the standard electrode potential of Cu2+/Cu and denote it with the symbol, Eo.

Cu^{2+}(aq)+2e^-\rightleftharpoons Cu(s)\; \; \; \; \; \; \; \; E^o=+0.34\; V

The electrochemical cell can be regarded as a battery with its positive end connected to the positive lead of the voltmeter. The current flows from the positive end of the battery (Cu electrode) to the negative end of the battery (H electrode). Along the way, it flows through the voltmeter and gives it a positive reading,

An electrochemical reaction is spontaneous when its Gibbs reaction energy, ΔrGo = –nFEo, is less than zero. Since the value of Ecell or Eo is positive, the Cu2+/Cu half-cell reaction is spontaneous from left to right and the overall electrochemical cell reaction is

H_2(g)+Cu^{2+}(aq)\rightleftharpoons 2H^+(aq)+Cu(s)

Let’s replace the half-cell with a Zn electrode in aqueous ZnSO4. The voltmeter now records a value of -0.76 V. Since,


-0.76\; V=E_{right}-0\; V

the electrode potential of Eright = -0.76 V, by convention, represents the reduction half-cell reaction of

Zn^{2+}(aq)+2e^-\rightleftharpoons Zn(s)\; \; \; \; \; \; \; \; E^o=-0.76\; V

Eo is now negative, meaning that the Zn2+/Zn half-cell reaction is not spontaneous from left to right. However, the fact that a current flows in the electrochemical cell is evidence that reactions are occurring in both half-cells. Furthermore, the current flow is reversed, which implies that the Zn2+/Zn half-cell reaction is spontaneous in the opposite direction, with Zn oxidising to Zn2+ because Zn is more electropositive than H.

Zn(s)\rightleftharpoons Zn^{2+}(aq)+2e^-

Therefore, in the SHE, H2 must be reduced to H+ and the overall electrochemical cell reaction is:

Zn(s)+2H^+(aq)\rightleftharpoons Zn^{2+}(aq)+H_2(g)

By replacing the right hand half-cell with different systems, we can measure their standard electrode potentials and tabulate the data to give the electrochemical series.


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