Reference Electrode Potentials

The inspiration and source for much of the information on this page was the web site Research Solution and Sources created by the late Dr Bob Rodgers.

Do you need to convert potentials obtained with one reference electrode to the equivalent values versus another reference electrode? Then try this calculator

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 (see reference 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 (~0.2 mg/100 mL), 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 Silver/Silver Sulfate (Ag/Ag₂SO₄) Electrode

The Ag/Ag₂SO₄ electrode is enjoying increasing popularity as a replacement for the Hg/HgSO₄ electrode where a mercury and chloride free electrode is required. It is especially used in experiments to do with lead–acid batteries (reference 8).

It is constructed from a silver wire, part of which is covered with finely divided silver sulfate (usually prepared electrochemically using the silver wire as an anode in a sulfuric acid electrolyte solution, see reference 7). If making your own electrode be careful to exclude even trace amounts of halide in the starting reagents.

The relevant half cell equation is: Ag/Ag₂SO₄ + 2e^– → 2Ag_(s) + SO₄^2–

Silver sulfate is sparingly soluble in water (~0.83 g/100 mL). Thus the electrolyte must be high in sulfate ion concentration to ensure that it is saturated in silver ion.

A commercial calomel electrodes is available from:

Koslow Scientific (USA)

Table 1. Potential (V) of the Ag/Ag₂SO₄ Electrode at 25°C

Electrolyte Solution	vs NHE	vs SCE	LJ	Reference
H2SO4 (0.5 M)	0.72	0.048	-	1, 3, 5
H2SO4 (1.0 M)	0.71	0.47	Yes	4, 5
K2SO4 (sat'd)	0.68	0.44	-	2, 5

Notes

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

NHE, normal hydrogen electrode

SCE, saturated calomel electrode

See more information about how these numbers were calculated.

Reference 7 suggests that the electrolyte filling solution should have sulfate concentrations higher than 0.01 M and silver ion concentrations between 0.001 – 1 mM, for most stable results.

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. Electrochemical Deposition of Co under the Influence of High Magnetic Fields, M. Uhlemann, A. Krause, J. P. Chopart, and A. Gebert, Journal of the Electrochemical Society, 152, C817-C826, 2005. DOI: 10.1149/1.2073167

5. Calculated by adding 40 mV to the corresponding mercury-mercurous sulfate electrode. More info here.

7. On the stability of the silver/silver sulfate reference electrode, Matěj Velický, Kin Y. Tamb, and Robert A. W. Dryfe, Analytical Methods, 4, 1207-1211, 2012.

DOI: 10.1039/C2AY00011C

8. Silver–silver sulfate reference electrodes for lead-acid batteries, Paul Ruetschi, Journal of Power Sources, 113, 363–370, 2003. DOI: 10.1016/S0378-7753(02)00549-9

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 insoluble in water (~0.4 mg/100 mL) to about the same extent as silver chloride, see reference 5. 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)

and some other companies that make pH electrodes (though many such companies have stopped production because of falling demand). 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

5. "Compilation and Evaluation of Solubility Data in the Mercury(I) Chloride–Water System", Y Marcus, Journal of Physical Chemistry Reference Data, 9, 1307–1329, 1980. www.nist.gov/data/PDFfiles/jpcrd173.pdf

The Mercury/Mercury Oxide (Hg/HgO) Electrode

Many conventional reference electrodes (Ag/AgCl, Ag/Ag₂SO₄, calomel) will have limited life times in very alkaline solutions, requiring their filling solutions to be replaced frequently to prevent hydroxide diffusing into the electrode will cause shifts in the electrode potential. This can be ameliorated, to some extent, by the use of a double junction, but an electrode that is stable under alkaline conditions is often a better answer.

The Hg/HgO electrode is ideal for use in alkaline solutions. The relevant half cell equation is: HgO_(s) + 2e^– + H₂O → Hg_(liq) + 2OH^–

Mercuric oxide is relatively insoluble in water (~5.3 mg/100 mL), and even less soluble in alkaline solutions.

As the half cell equation suggests the potential is dependent on the hydroxide ion concentration used in the electrolyte solution. It is usual to match the hydroxide concentration of the filling solution to that of the sample to minimise the junction potential.

A commercial electrode is available (see reference 2).

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

Electrolyte Solution	vs NHE	vs SCE	LJ	Reference
NaOH (0.1 M)	0.165	-0.076	-	1
NaOH (1 M)	0.140	-0.101	-	1
KOH (20%)	0.098	-	?	2

Notes

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

NHE, normal hydrogen electrode

SCE, saturated calomel electrode

References

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

2. Koslow Scientific Testing Instruments

Disclaimer

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