Reference Electrode Potentials

The calomel electrode is usually constructed from a platinum wire inserted into a a mixture of calomel (mercurous chloride, Hg₂Cl₂) and mercury with an electrolyte solution of KCl or NaCl. The relevant half equation is:

Hg₂Cl₂ + 2e^– == 2Hg_liq + 2Cl^–

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

Conditions	vs NHE	vs SCE	LJ	Reference
Hg/Hg2Cl2, KCl (0.1M)	0.3337	0.0925	-	1,3
Hg/Hg2Cl2, KCl (0.1M)	0.336	0.092	Yes	2
Hg/Hg2Cl2, KCl (1M) NCE (Normal Calomel)	0.2801	0.0389	-	1,3
Hg/Hg2Cl2, KCl (1M) NCE (Normal Calomel)	0.283	0.039	Yes	2
Hg/Hg2Cl2, KCl (3.5M)	0.250	0.006	Yes	2
Hg/Hg2Cl2, KCl (sat'd) SCE (Sat'd Calomel)	0.2412	0	-	1,3
Hg/Hg2Cl2, KCl (sat'd) SCE (Sat'd Calomel)	0.244	0	Yes	2
Hg/Hg2Cl2, NaCl (sat'd)	0.2360	-0.0052	-	1

NOTES