Standard hydrogen electrode

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The standard hydrogen electrode (abbreviated SHE), also called normal hydrogen electrode (NHE), is a redox electrode which is placed in the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be 4.4V to 4.6V, but to form a basis for comparison with all other electrode reactions, Hydrogen's standard electrode potential (E⁰) is declared to be zero. Potentials of any other electrodes are compared with the standard hydrogen electrode.

Hydrogen electrode is based on the redox half cell:

2H⁺(aq) + 2e^- → H₂(g)

This redox reaction occurs at platinized platinum electrode. Usage of such electrode is caused by several factors:

necessity to employ electrode with very large area. The greater the electrode area, the faster electrode kinetics.
necessity to use electrode material which can adsorb hydrogen at its interface. This factor also improve electrode kinetics.

The Nernst equation should be written as:

$E={RT \over F}\ln {a_{H^+} \over (p_{H2}/p^0)^{1/2}}$

$E=-{2.303RT \over F}pH - {RT \over 2F}\ln {p_{H2}/p^0}$

where:

a_H+ is the activity of the hydrogen ions, a_H⁺=f_H⁺ C_H⁺ /C⁰
p_H2 is the partial pressure of the hydrogen gas, in pascals, Pa
R is the universal gas constant
T is the temperature, in kelvins
f_H⁺ is the activity coefficient of H⁺
C⁰ is the standard concentration mols dm^-3
p⁰ is the standard pressure 10⁵ in Pa

[edit] Interference

Because of the high adsorption activity of the platinized platinum electrode, it's very important to protect electrode surface and solution for the presence of organic substances as well as oxygen of atmosphere.

[edit] Construction

The scheme of the standard hydrogen electrode:

platinized platinum electrode
hydrogen blow
solution of the acid with activity of H⁺ = 1 mol dm^-3
hydroseal for prevention of the oxygen interference
reservoir through which the second half-element of the galvanic cell should be attached. This creates an ionically conductive path to the working electrode of interest.