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Concept
Reference electrode
Summary
A reference electrode is an electrode that has a stable and well-known electrode potential. The overall chemical reaction taking place in a cell is made up of two independent half-reactions, which describe chemical changes at the two electrodes. To focus on the reaction at the working electrode, the reference electrode is standardized with constant (buffered or saturated) concentrations of each participant of the redox reaction.
There are many ways reference electrodes are used. The simplest is when the reference electrode is used as a half-cell to build an electrochemical cell. This allows the potential of the other half cell to be determined. An accurate and practical method to measure an electrode's potential in isolation (absolute electrode potential) has yet to be developed.
Common reference electrodes and potential with respect to the standard hydrogen electrode (SHE):
Standard hydrogen electrode (SHE) (E = 0.000 V) activity of H+ = 1 Molar
Normal hydrogen electrode (NHE) (E ≈ 0.000 V) concentration H+ = 1 Molar
Reversible hydrogen electrode (RHE) (E = 0.000 V - 0.0591 × pH) at 25°C
Saturated calomel electrode (SCE) (E = +0.241 V saturated)
Copper-copper(II) sulfate electrode (CSE) (E = +0.314 V)
Silver chloride electrode (E = +0.197 V in saturated KCl)
Silver chloride electrode (E = +0.210 V in 3.0 mol KCl/kg)
Silver chloride electrode (E = +0.22249 V in 3.0 mol KCl/L)
pH-electrode (in case of pH buffered solutions, see buffer solution)
Palladium-hydrogen electrode
Dynamic hydrogen electrode (DHE)
Mercury-mercurous sulfate electrode (E = +0.64 V in sat'd K2SO4, E = +0.68 V in 0.5 M H2SO4) (MSE)
While it is convenient to compare between solvents to qualitatively compare systems, this is not quantitatively meaningful. Much as pKa are related between solvents, but not the same, so is the case with E°. While the SHE might seem to be a reasonable reference for nonaqueous work as it turns out the platinum is rapidly poisoned by many solvents including acetonitrile causing uncontrolled drifts in potential.
Official source
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