Difference between revisions of "Reference Electrode Potentials"
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Revision as of 14:46, 4 August 2014
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
