calomel

 

or calomel, chemical compound, Hg₂Cl₂, a white crystalline powder, very slightly soluble in water. It was once used medicinally as a purgative, cathartic, liver stimulant, and to eliminate parasitic worms, but is rarely so used today because it is readily decomposed into metallic mercury and the very poisonous mercuric chloride on exposure to sunlight or if heated in the presence of moisture. Mercurous chloride is a less dangerous poison than mercuric chloride chiefly because it is much less soluble; it is highly toxic if retained in the body. Mercurous chloride is prepared by sublimation from a mixture of mercury and mercuric chloride or by precipitation from a mercurous chloride solution on adding chloride ion. It is also found in nature as horn quicksilver. The calomel electrode, often used as a reference in determining electric potentials and for measuring the pH of solutions, contains mercurous chloride, mercury metal, and potassium chloride solution.

Mercury(I) chloride
IUPAC name	Dimercury dichloride
Other names	Mercurous chloride; calomel
Identifiers
CAS number	10112-91-1
RTECS number	OV8750000
Properties
Molecular formula	Hg2Cl2
Molar mass	472.09 g/mol
Appearance	White solid
Density	7.150 g/cm3, solid
Melting point	383 °C (sublimes)
Solubility in water	0.2 mg/100 mL
Hazards
Main hazards	Harmful, dangerous for the environment
R-phrases	R22, R36/37/38, R50/53
S-phrases	(S2), S13, S24/25 S46, S60, S61
Flash point	Non-flammable
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

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.^[1][2]

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 .

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:

Preparation and reactions

Mercurous chloride forms by the reaction of elemental mercury and mercuric chloride:

Hg + HgCl₂ → Hg₂Cl₂

It can be prepared via metathesis reaction involving aqueous mercury(I) nitrate using various chloride sources including NaCl or HCl.

2HCl + Hg₂(NO₃)₂ → Hg₂Cl₂ + 2HNO₃

Ammonia causes Hg₂Cl₂ to disproportionate:

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 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]}

Photochemistry

Mercurous chloride decomposes into mercury(II) chloride and elemental mercury upon exposure to UV light.

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.

2HgCl₂ + (NH₄)₂C₂O₄ + Light → Hg₂Cl_2(s) + 2[NH₄⁺][Cl⁻] + 2CO₂

This particular reaction was invented by J.M. Eder (hence the name Eder reaction) in 1880 and reinvestigated by W. E. Rosevaere in 1929 ^[3]

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.

Safety considerations

Main article: Mercury poisoning

Mercurous chloride is toxic, although due to its low solubility in water it is generally less dangerous than its mercuric chloride counterpart. It was used in medicine as a diuretic and purgative (laxative), e.g. from the early 1830s through the 1860s as a purgative in the U.S. These medicinal uses were discontinued.

It has also found uses in cosmetics as soaps and skin lightening creams, but these preparations are now illegal to manufacture or import in many countries including U.S., Canada, Japan and Europe. A study of workers involved in the production of these preparations, showed that the sodium salt of 2,3-dimercapto-1-propanesulfonic acid (DMPS) was effective in lowering the body burden of mercury and in decreasing the urinary mercury concentration to normal levels.^[4]

References

1. ^ Housecroft, Catherine E., Sharpe, Alan G.: Inorganic Chemistry 2nd edition. Pearson/Prentice Hall, NY 2001, pp 696-697
2. ^ Skoog, Douglas A., F. James Holler and Timothy A. Nieman; Principles of Instrumental Analysis; 5th Edition;Saunders College Pub., PE. 1998, pp 253-271
3. ^ THE X-RAY PHOTOCHEMICAL REACTION BETWEEN POTASSIUM OXALATE AND MERCURIC CHLORIDE W. E. Roseveare J. Am. Chem. Soc.; 1930; 52(7) pp 2612 - 2619; doi:10.1021/ja01370a005
4. ^ D. Gonzalez-Ramirez, M. Zuniga-Charles, A. Narro-Juarez, Y. Molina-Recio, K. M. Hurlbut, R. C. Dart and H. V. Aposhian (1998). "DMPS (2,3-Dimercaptopropane-1-sulfonate, Dimaval) Decreases the Body Burden of Mercury in Humans Exposed to Mercurous Chloride" (free full text). Journal of Pharmacology and Experimental Therapy 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

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