Absolute electrode potential of the SHE

To create a scale of electrode potentials based on eq. 1 A. 12, we need to know the contact potential difference between the solution phase of a suitable reference electrode system and the electrode itself. The most accurate measurements of this type have been made by using the standard hydrogen electrode as the reference electrode and mercury, the work function of which is known to good accuracy (although it is not normally used in the SHE), as the reference metal. Equation 1A.12 then gives 

The 20 mV error derives from the nncertainty in the work function of mercury. Eqnation 1 A. 14 indicates that the work required to transfer an electron from the interior of the eqnilibrated SHE electrode to a point just outside the solution phase is 4.44 + 0.02 eV. 

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