Mercury(I) chloride
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| Mercury(I) chloride | |
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| General | |
| Systematic name | Dimercury dichloride. |
| Other names | mercurous chloride calomel |
| Molecular formula | Hg2Cl2 |
| Molar mass | 472.09 g/mol |
| Appearance | White solid |
| CAS number | [10112-91-1] |
| EINECS number | 233-307-5 |
| Properties | |
| Density and phase | 7.150 g/cm3, solid |
| Solubility in water | 0.2 mg/100 mL (25 °C) |
| Melting point | 383 °C (sublimes) |
| Boiling point | 525°C (?? mm Hg) |
| Structure | |
| Molecular shape | ? |
| Coordination geometry |
Linear |
| Crystal structure | ? |
| Dipole moment | 0 D |
| Thermodynamic data | |
| Standard enthalpy of formation ΔfH°solid |
-264.93 kJ/mol |
| Standard molar entropy S°solid |
192.52 J.K−1.mol−1 |
| Safety data | |
| EU classification | Harmful Dangerous for the environment |
| R-Phrases | R22, R36/37/38, R50/53 |
| S-Phrases | S2, S13, S24/25 S46, S60, S61 |
| PEL-TWA (OSHA) | 0.1 mg/m3 (as Hg) |
| IDLH (NIOSH) | 10 mg/m3 (as Hg) |
| Flash point | Non-flammable. |
| RTECS number | OV8750000 |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Other anions | Mercury(I) bromide Mercury(I) iodide |
| Other cations | Mercury(II) chloride |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Mercury(I) chloride is the chemical compound with the formula Hg2Cl2. Also known as calomel or mercurous chloride, this dense white or yellowish-white, odorless solid is the principal example of a mercury(I) compound. It is a component of reference electrodes in electrochemistry.
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[edit] History
The name calomel is thought to come from the Greek καλος beautiful, and μελας black. This name (somewhat surprising for a white compound) is probably due to its characteristic disproportionation reaction with ammonia, which gives a spectacular black coloration due to the finely dispersed metallic mercury formed. It is also referred to as the mineral horn quicksilver or horn mercury. Calomel was used as a medicine internally for laxation and disinfection before the 20th century .
[edit] Properties
Mercury is unique among the group 12 metals for its ability to form the M-M bond so readily. Hg2Cl2 is a linear molecule. The crystal structure is shown below:
[edit] Preparation and reactions
Mercurous chloride forms by the reaction of elemental mercury and mercuric chloride:
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- Hg+ HgCl2 → Hg2Cl2
It can be prepared via metathesis reaction involving aqueous mercury(I) nitrate using various chloride sources including NaCl or HCl.
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- 2HCl + Hg2(NO3)2 → Hg2Cl2 + 2HNO3
Ammonia causes Hg2Cl2 to disproportionate:
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- Hg2Cl2 + 2NH3 → Hg + Hg(NH2)Cl + NH4Cl
[edit] Calomel electrode
Mercurous chloride is employed extensively in electrochemistry, taking advantage of the ease of its oxidation and reduction reactions. The calomel electrode is a reference electrode, especially in older publications. Over the past 50 years, it has been superseded by the silver/silver chloride (Ag/AgCl) electrode. Although the mercury electrodes have been widely abandoned due to the dangerous nature of mercury, many chemists believe they are still more accurate and are not dangerous as long as they are handled properly. The differences in experimental potentials vary little from literature values. Other electrodes can vary by 70 to 100 millivolts.[citation needed]
[edit] Photochemistry
Mercurous chloride decomposes into mercury(II) chloride and elemental mercury upon exposure to UV light.
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- Hg2Cl2 → HgCl2 + Hg
The formation of Hg can be used to calculate the number of photons in the light beam, by the technique of actinometry. By utilizing a light reaction in the presence of mercury(II) chloride and ammonium oxalate mercurous chloride is produced.
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- 2HgCl2 + (NH4)2C2O4 + Light → Hg2Cl2(s) + 2[NH4+][Cl−] + 2CO2
[edit] Related mercury(I) compounds
Mercury(I) bromide, Hg2Br2, a light yellow, whereas mercury(I) iodide, Hg2I2, is greenish in colour. Both are poorly soluble. Mercury(I) fluoride is unstable in the absence of a strong acid.
[edit] Safety considerations
Due to its low solubility, mercurous chloride is less dangerous than its mercuric chloride counterpart. From the early 1830s through the 1860s, this compound was used as a laxative in the U.S. This previous use in medicine as a diuretic and purgative was discontinued because of its toxicity. It has also found uses in cosmetics as soaps and skin lightening creams, but the same risks applied.
[edit] References
- Housecroft, Catherine E., Sharpe, Alan G.: Inorganic Chemistry 2nd edition. Pearson/Prentice Hall, NY 2001, pp 696-697
- Skoog, Douglas A., F. James Holler and Timothy A. Nieman; Principles of Instrumental Analysis; 5th Edition;Saunders College Pub., PE. 1998, pp 253-271
- Gonzalez-Ramirez D, Zuniga-Charles M, Narro-Juarez A, Molina-Recio Y, Hurlbut K.M, Dart R.C, Aposhian H.V.; DMPS (2,3-dimercaptopropane-1-sulfonate, dimaval) decreases the body burden of mercury in humans exposed to mercurous chloride.; J Pharmacol Exp Ther. 1998 Oct;287(1):8-12.
