Mercury(I) chloride

formation]] Δ_fH°_solid

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	^*
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	-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	, ,
S-Phrases	, , , ,
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, ε_r, 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 infobox

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.

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

It can be prepared via metathesis reaction involving aqueous mercury(I) nitrate using various chloride sources including salt 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 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⁺

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^*^* + 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

Mercury compounds | Chlorides | Metal halides

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