Difference between revisions of "Reference Electrode Potentials"
(→The Silver/Silver Chloride (Ag/AgCl) Electrode) |
(→The Silver/Silver Chloride (Ag/AgCl) Electrode) |
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AgCl + e<sup>–</sup> → Ag<sub><i>(s)</i></sub> + Cl<sup>–</sup> | AgCl + e<sup>–</sup> → Ag<sub><i>(s)</i></sub> + Cl<sup>–</sup> | ||
− | As this equation implies, the electrode potential is dependent on silver ion and chloride concentration, but as the silver ion concentration is limited by the low solubility | + | As this equation implies, the electrode potential is dependent on silver ion and chloride concentration, but as the silver ion concentration is limited by the low solubility of AgCl 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 80°C or more, and are commercially available from many companies. | Ag/AgCl electrodes can be used up to 80°C or more, and are commercially available from many companies. |
Revision as of 15:01, 4 August 2014
The Silver/Silver Chloride (Ag/AgCl) Electrode
The Ag/AgCl electrode is by far the most popular type of reference electrode in use.
It is constructed from a silver wire, partly covered with finely divided silver chloride inserted into a solution of KCl or NaCl. The relevant half cell equation is: AgCl + e– → Ag(s) + Cl–
As this equation implies, the electrode potential is dependent on silver ion and chloride concentration, but as the silver ion concentration is limited by the low solubility of AgCl 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 80°C or more, and are commercially available from many companies.
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", 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
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– → 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)
In Europe the use of calomel electrodes is increasingly problematic because many countries no longer permit the use of mercury-containing devices.
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", 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