Mercury(I) chloride

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.

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 Over the past 50 years, it has been superseded by the silver/silver chloride (Ag/AgCl) electrode.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.

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 → Hg2Cl2(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

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
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.