Standard hydrogen electrode
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A standard hydrogen electrode (abbreviated SHE) is a redox electrode which is placed in the basis of the thermodynamic scale of oxidation-reduction potentials. Its standard electrode potential (E0) is declared to be zero. Potentials of any other electrodes are compared with the standard hydrogen electrode.
Standard_hydrogen_electrode.jpg
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 square. The more electrode square, the faster electrode kinetics.
- necessity to use electrode material which can adsorb hydrogen at its interface. This factor also improve electrode kinetics.
Nernst equation should be written as:
<math>E={RT \over F}\ln {a_{H^+} \over (p_{H2})^{1/2}}<math>
or
<math>E=-{RT \over F}pH - {RT \over 2F}\ln {p_{H2}}<math>
where
- aH+ is the activity of the hydrogen ions, mol/l
- pH2 is the partial pressure of the hydrogen gas, atm
- R is the universal gas constant
- T is the temperature in kelvins
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.
Construction
The scheme of the standard hydrogen electrode:
1 - platinized platinum electrode
2 - hydrogen blow
3 - solution of the acid with activity of H+ = 1 mol/l
4 - hydroseal for prevention of the oxygen inteference
5 - reservoir through the which the second half-element of the galvanic cell should be attached