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DoITPoMS Teaching & Learning Packages The Nernst Equation and Pourbaix Diagrams Other reference electrodes
The Nernst Equation and Pourbaix Diagrams AimsBefore you startIntroductionBackgroundThe Nernst equationConstruction of a Pourbaix DiagramAnatomy of a Pourbaix DiagramExamples of a Pourbaix DiagramConstructing a 3D Pourbaix DiagramSummaryQuestionsGoing further
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Other reference electrodes

It is often impractical to use the standard hydrogen electrode owing to the clumsy nature of using hydrogen gas. In practice, a range of alternative, secondary electrodes are used. 

The potentials of these electrodes are precisely known with respect to the SHE, so a measured potential can be easily converted to an equivalent one relative to the SHE. Three of the most common secondary electrodes are:

  • The saturated calomel electrode (SCE),
  • The silver/silver chloride electrode
  • The copper-copper(II) sulphate electrode.

The saturated calomel electrode (SCE)

The reaction is based on the reaction between this elemental mercury (Hg) and mercury(I) chloride (Hg2Cl2, "calomel").

\[{\rm{H}}{{\rm{g}}_2}{\rm{C}}{{\rm{l}}_2} + 2{e^ - } \Leftrightarrow 2{\rm{Hg}} + 2{\rm{C}}{{\rm{l}}^ - }\]

A one molar solution of potassium chloride in water forms the aqueous phase in contact with the mercury and the mercury(I) chloride.

The Nernst equation for this electrode can be expressed as

\[{E_e} = {E^o} - \frac{{2.303RT}}{{2F}}\log {[C{l^ - }]^2} = {E^0} - 0.0591\log [C{l^ - }]\]

In cell notation the electrode is written as: Cl- (saturated) | Hg2Cl2(s) | Hg(l) | Pt

The measured potential, E, of the SCE is +0.241 V (SHE) for a saturated chloride ion solution at 298 K.

The silver/silver chloride electrode

This is based on the reaction is between the silver metal (Ag) and silver(I) chloride (AgCl). The half-cell reaction is

AgCl + e- = Ag + Cl-

which gives a Nernst equation of

\[{E_e} = {E^0} - \frac{{2.303RT}}{F}\log [C{l^ - }] = {E^0} - 0.0591\log [C{l^ - }]\]

Changing the electrolyte concentration with this electrode changes the equilibrium electrode potential, so fixed values of chloride concentration are required.

In cell notation, this is written as Ag | AgCl | KCl(1M).

The measured potential, E = +0.235 V (SHE) at 298 K.

The copper-copper(II) sulphate electrode

The copper-copper(II) sulphate electrode is based on the redox reaction between copper metal and its salt - copper(II) sulphate.
The corresponding equation can be presented as follows:

Cu2+ + 2e- = Cu

The Nernst equation below shows the dependence of the potential of the copper-copper(II) sulphate electrode on the concentration copper-ions:

\[E = {E^0} + \frac{{2.303RT}}{{2F}}\log [C{u^{2 + }}] = {E^0} + 0.0295\log [C{u^{2 + }}]\]

The equilibrium potential of a copper-copper sulphate electrode is -0.318 V with respect to the standard hydrogen electrode for a saturation concentration of copper ions at 298 K.

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