Reference Electrode Potentials

The Calomel Electrode

The calomel electrode is usually constructed from a platinum wire inserted into a mixture of calomel (mercurous chloride, Hg₂Cl₂) and liquid mercury, with an electrolyte solution of KCl or NaCl. The relevant half cell equation is: Hg₂Cl₂ + 2e^– → 2Hg_liq + 2Cl^–

As this equation implies, the electrode potential is dependent on chloride concentration, but independent of hydrogen ion (acid) concentration. Calomel electrode are unstable above 50°C owing to the disproportionation reaction: Hg₂Cl₂ → Hg_liq + Hg</sub> Cl₂

Commercial calomel electrodes are available from:

Koslow Scientific (USA)

Ionode Pty Ltd (Australia)

In Europe the use of calomel electrodes is increasingly problematic because many countries no longer permit the use of mercury-containing devices.

Table 1. Potential (V) of the Calomel Electrode at 25°C

Conditions	vs NHE	vs SCE	LJ	Reference
Hg/Hg2Cl2, KCl (0.1 M)	0.3337	0.0925	-	1, 3
Hg/Hg2Cl2, KCl (0.1 M)	0.336	0.092	Yes	2
NCE	0.2801	0.0389	-	1, 3
NCE	0.283	0.039	Yes	2
Hg/Hg2Cl2, KCl (3. 5M)	0.250	0.006	Yes	2
SCE	0.2412	0	-	1, 3
SCE	0.244	0	Yes	2
SSCE	0.2360	-0.0052	-	1

Notes

LJ, liquid junction. Value obtained using a cell which included a liquid junction potential.

NCE, normal calomel electrode: Hg/Hg₂Cl₂, KCl (1 M)

NHE, normal hydrogen electrode

SCE, saturated calomel electrode: Hg/Hg₂Cl₂, KCl (sat'd)

SSCE, saturated salt calomel electrode: Hg/Hg₂Cl₂, NaCl (sat'd)

For values at other temperatures see a calculator here.

References

1. "Electrochemical Methods: Fundamentals and Applications", AJ Bard and LR Faulkner, John Wiley & Sons, NY (2000). See the table on inside back cover.

2. "Electrochemistry for Chemists, Second Edition", DT Sawyer, AJ Sobkowiak, J Roberts, Jr., John Wiley & Sons, NY (1995). See Section 5.2.

3. "Handbook of Analytical Chemistry", L Meites (ed.), McGraw Hill, NY (1963). See Section 5.