Chemistry · Ch 5 — Electrochemistry

Standard hydrogen electrode (SHE)

5.9.1

Standard hydrogen electrode (SHE)

Construction : SHE consists of a platinum plate coated with platinum black, used as the electrode. This plate is connected to the external circuit through a sealed narrow glass tube containing mercury, and is surrounded by an outer glass jacket. The platinum electrode is immersed in a 1 M $\mathrm{H^+}$ ion solution, which is kept saturated with dissolved $\mathrm{H_2}$ by bubbling hydrogen gas under 1 atm pressure through the side tube of the jacket, as shown in Fig. 5.6. Platinum does not take part in the electrode reaction — it is an inert electrode and serves as the site for electron transfer.

Figure 5.6Construction of the standard hydrogen electrode: a platinised platinum plate in 1 M hydrogen-ion solution inside a glass jacket, connected through a mercury contact and platinum wire to a copper wire, with pure dry hydrogen gas fed in at 1 atm.

Fig. 5.6 — Construction of the standard hydrogen electrode: a platinised platinum plate in 1 M hydrogen-ion solution inside a glass jacket, connected through a mercury contact and platinum wire to a copper wire, with pure dry hydrogen gas fed in at 1 atm.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

What this figure shows. The SHE assembly: the platinised platinum plate dips in the solution of H⊕ ions (1M) held in the vessel; pure and dry H₂ gas at 1 atm enters through the side tube of the glass jacket and bubbles over the plate; electrical contact runs from the plate up the Pt wire, through the mercury in the sealed narrow tube, to the external Cu wire. The two printed conditions — 1 M and 1 atm — are exactly the …

Formulation : The standard hydrogen electrode is represented as

\mathrm{H^{+}\,(1M)\ \vert\ H_2\,(g,\ 1atm)\ \vert\ Pt}

Electrode reaction : The platinum black, capable of adsorbing large quantities of $\mathrm{H_2}$ gas, allows the change from the gaseous to the ionic form and the reverse process to occur. The reduction half reaction at the electrode is

\mathrm{2H^{+}\,(1M) + 2e^- \longrightarrow H_2\,(g,\ 1atm)}, \qquad E^0_{H_2} = 0.000\ \mathrm{V}

Application of SHE

SHE is used as a primary reference electrode to determine the standard potentials of other electrodes.

To determine the standard potential of $\mathrm{Zn^{2+}\,(1M)\ \vert\ Zn\,(s)}$ , it is combined with SHE to form the cell

\mathrm{Zn\ \vert\ Zn^{2+}\,(1M)\ \Vert\ H^{+}\,(1M)\ \vert\ H_2\,(g,\ 1atm)\ \vert\ Pt}

This is shown in Fig. 5.7.

Figure 5.7Determination of a standard potential using the standard hydrogen electrode: a zinc anode half-cell and the SHE joined by a salt bridge, with electron flow through the external meter from zinc to hydrogen.

Fig. 5.7 — Determination of a standard potential using the standard hydrogen electrode: a zinc anode half-cell and the SHE joined by a salt bridge, with electron flow through the external meter from zinc to hydrogen.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

What this figure shows. The cell $\mathrm{Zn\,\vert\,Zn^{2+}(1M)\,\Vert\,H^{+}(1M)\,\vert\,H_2(g,\,1atm)\,\vert\,Pt}$ : the Zn anode (-) in 1 M ZnSO₄ solution on the left, the SHE (+) with H₂ (g, 1atm) in 1 M H⊕ ion solution on the right, joined by the salt bridge, with the e⊖ arrows showing electron flow through the meter from anode to cathode. The measured emf equals the standard potential of the zinc electrode because the SHE contributes zero. *( …

The standard cell potential $E^0_{cell}$ is measured. $E^0_{cell} = E^0_{H_2} - E^0_{Zn} = -E^0_{Zn}$ , because $E^0_{H_2}$ is zero. Thus the measured emf of the cell is equal to the standard potential of the $\mathrm{Zn^{2+}\,(1M)\ \vert\ Zn\,(s)}$ electrode (with reversed sign).

Difficulties in setting SHE

i. It is difficult to obtain pure and dry hydrogen gas.

ii. The pressure of hydrogen gas cannot be maintained exactly at 1 atm throughout the measurement.

iii. The concentration of the $\mathrm{H^+}$ ion solution cannot be exactly maintained at 1 M. Due to bubbling of gas into the solution, evaporation of water may take place; this results in changing the concentration of the solution.

Hydrogen gas electrode …

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