Mercury(I) chloride
Encyclopedia : M : ME : MER : Mercury(I) chloride
| Mercury(I) chloride | |
|---|---|
| |
| 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 subl. |
| Boiling point | 525°C (Pressure). |
| Structure | |
| Molecular shape | ? |
| Coordination geometry | Linear |
| Crystal structure | ? |
| Dipole moment | ? 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) fluoride 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) [Chemical infoboxInfobox disclaimer and references] | |
Mercury(I) chloride (chemical formula Hg2Cl2, also known as calomel or mercurous chloride) is a chloride of mercury, a dense white or yellowish-white, odorless powder.
Contents
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 desinfection before the 20th century .Properties
Mercury is unique among the group 12 metals for its ability to form the M-M bond so readily.1 Hg2Cl2 is a linear molecule. The crystal structure is shown below:
Preparation and reactions
Mercurous chloride forms by the direct reaction of elemental mercury and mercuric chloride:- :Hg+ HgCl2 → Hg2Cl2
- :2HCl + Hg2(NO3)2 → Hg2Cl2 + 2HNO3
- :Hg2Cl2 + 2NH3 → Hg + Hg(NH2)Cl + NH4Cl
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 fifty 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 seventy to one hundred millivolts.Photochemical reactions
Mercurous chloride decomposes into mercury(II) chloride and elemental mercury upon exposure to UV light.- :Hg2Cl2 → HgCl2 + Hg+
- :2HgCl2 + (NH4)2C2O4 + Light → Hg2Cl2(s) + 2[NH4+][Cl−] + 2CO2
Similar Compounds
Mercury(I) bromide, Hg2Br2, an light yellow substance, and mercury(I) iodide, Hg2I2, is greenish in colour. Both are poorly soluble. Mercury(I) flouride is unstable in the absence of a strong acid.Safety considerations
Due to its low solubility, mercurous chloride is less dangerous than its mercuric chloride counterpart. From the early 1830’s through the 1860's, 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.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.
External links
- [International Chemical Safety Card 0984]
- [National Pollutant Inventory - Mercury and compounds Fact Sheet]
- [NIOSH Pocket Guide to Chemical Hazards]
- For a full list of external links to MSDSs, spectroscopic data, commercial chemicals suppliers etc. for this compound, see [Chemical sources].
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