Difference between revisions of "Reference Electrode Potentials"

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'''Notes'''
 
'''Notes'''
 
: LJ, [http://en.wikipedia.org/wiki/Liquid_junction_potential liquid junction]. Value obtained using a cell which included a liquid junction potential.
 
: LJ, [http://en.wikipedia.org/wiki/Liquid_junction_potential liquid junction]. Value obtained using a cell which included a liquid junction potential.
−
:NHE, [http://en.wikipedia.org/wiki/Standard_hydrogen_electrode normal hydrogen electrode]
+
: NHE, [http://en.wikipedia.org/wiki/Standard_hydrogen_electrode normal hydrogen electrode]
−
:SCE, [http://en.wikipedia.org/wiki/Saturated_calomel_electrode saturated calomel electrode]
+
: SCE, [http://en.wikipedia.org/wiki/Saturated_calomel_electrode saturated calomel electrode]
 +
: See [http://www.consultrsr.net/resources/ref/ag2so4.htm  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 conditions.
 
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 conditions.

Revision as of 14:02, 6 August 2014

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 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).

The relevant half cell equation is: Ag/Ag2SO4 + 2e– → 2Ag(s) + SO42–

Silver sulfate is sparingly insoluble in water (~0.83 g/100 mL). The electrolyte is usually high in sulfate ion concentration to ensure that it is saturated in silver ion.


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 conditions.

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. M Uhlemann, A Krause, JP Chopart, A Gebert, J. Electrochem. Soc., 152 (2005), C817-C826.
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

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 (~0.4 mg/100 mL), see reference 5. The relevant half cell equation is: Hg2Cl2 + 2e– → 2Hgliq + 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 → Hgliq + HgCl2

Commercial calomel electrodes are available from:

Koslow Scientific (USA)
Ionode Pty Ltd (Australia)

and many other companies that provide pH electrodes (though many such companies have stooped making them 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
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