Reference Electrode Potentials

The Silver/Silver Chloride (Ag/AgCl) Electrode

The Ag/AgCl electrode is by far the most popular type of laboratory reference electrode in use today.

It is constructed from a silver wire, part of which is 'chloridized' (covered with finely divided silver chloride). There are several ways to do this.⁷ The chloridized end of the wire is then inserted into an electrolyte solution of KCl or NaCl. The relevant half cell equation is: AgCl_(s) + e^– → Ag_(s) + Cl^–

Silver chloride is quite insoluble in water (~2 mg/100 g), but slightly more so in concentrated chloride solutions owing to the formation of the complex ion [AgCl₂]^–. Though electrode potential is actually dependent on silver ion concentration (as with any metal/metal ion electrode), this is limited by the low solubility of AgCl, and thus the actual potential is effectively controlled by the chloride concentration alone. Note also that the potential is independent of hydrogen ion (acid) concentration.

Ag/AgCl electrodes can be used up to 100°C (depending on the materials used to make the electrode), and are commercially available from many companies. The potential does vary with temperature, but between 10 – 40°C can be estimated by the equations (see reference 2):

E = 205 – 0.73 × (T – 25)     for an electrolyte of 3.5 M KCl

E = 199 – 1.01 × (T – 25)     for an electrolyte of saturated KCl

where T is the temperature (°C), and E is the electrode potential (mV).

Table 1. Potential (V) of the Ag/AgCl Electrode at 25°C

Electrolyte Solution	vs NHE	vs SCE	LJ	Reference
KCl (0.1 M)	0.2881	0.047	-	1, 3
KCl (3 M)	0.210	-0.032	-	4
KCl (3.5 M)	0.205	-0.039	Yes	2
KCl (sat'd)	0.197	-0.045	-	1
KCl (sat'd)	0.199	-0.045	Yes	2
KCl (sat'd)	0.1988	-0.042	-	2
NaCl (3 M)	0.209	-0.035	Yes	5
NaCl (sat'd)	0.197	-0.047	Yes	3
Seawater	0.25	0.01	Yes	6

Notes

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

NHE, normal hydrogen electrode

SCE, saturated calomel electrode

Seawater has approximately 0.47 M NaCl.

References

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

2. "Electrochemistry for Chemists, Second Edition", D T Sawyer, A J Sobkowiak, J Roberts, Jr., John Wiley & Sons, NY (1995). See Table 5.3

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

4. E P Friis, J E T Anderson, L L Madsen, N Bonander, Per Moller, J Ulstrup, Electrochimica Acta, 43, 1114-1122, 1998. DOI: 10.1016/S0013-4686(98)99006-5

5. www.bioanalytical.com/products/ec/faqele.html#Ref_Type

6. www.corrosion-doctors.org/Corrosion-Thermodynamics/Reference-Half-Cells.htm

7 "Reference Electrodes. Theory and Practice" David J G Ives, and George J Janz, (eds) Academic Press (1961). See pages 203 -213.

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. Calomel is reasonably insoluble in water (0.2 mg/100 mL). 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 electrodes are unstable much above 50°C owing to the disproportionation reaction: Hg₂Cl₂ → Hg_liq + HgCl₂

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

Electrolyte Solution	vs NHE	vs SCE	LJ	Reference
KCl (0.1 M)	0.3337	0.0925	-	1, 3
KCl (0.1 M)	0.336	0.092	Yes	2
NCE, KCl (1 M)	0.2801	0.0389	-	1, 3
NCE, KCl (1 M)	0.283	0.039	Yes	2
KCl (3.5M)	0.250	0.006	Yes	2
SCE, KCl (sat'd)	0.2412	0	-	1, 3
SCE, KCl (sat'd)	0.244	0	Yes	2
SSCE, NaCl (sat'd)	0.2360	-0.0052	-	1

Notes

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

NCE, normal calomel electrode

NHE, normal hydrogen electrode

SCE, saturated calomel electrode

SSCE, saturated salt calomel electrode

For values at other temperatures use this calculator.

References

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

2. "Electrochemistry for Chemists, Second Edition", D T Sawyer, A J 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.

4. "Standard E.m.f. of the hydrogen-calomel cell from 0 to 45°C ", S R Gupta, G J Hills and D J G Ives. Transactions of the Faraday Society, 59, 1874-1885, 1963. DOI: 10.1039/TF9635901874