Mercury(I) chloride

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	Template:R22, Template:R36/37/38, Template:R50/53
S-Phrases	Template:S2, Template:S13, Template:S24/25 Template:S46, Template:S60, Template: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) Infobox disclaimer and references

Mercury(I) chloride (chemical formula Hg₂Cl₂, often still known by its ancient name of calomel) is a chloride of mercury, a dense white or yellowish-white, ordorless powder. It is usually called mercurous chloride.

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 the mineral's brown or dark brown color in its native state (presumably due to the presence of impurities). The name may also refer to its characteristic disproportionation reaction with ammonia, which gives a black coloration due to the metallic mercury formed. It is still referred to as the mineral horn quicksilver or horn mercury.

From the early 1830’s throughout the civil war, this compound was used as a laxative. This previous use in medicine as a diuretic and purgative was discontinued because of the toxic risks of mercury poisoning. It has also found uses in cosmetics as soaps and skin lightening creams, but the same risks applied.

Properties

The lack of solubility of the mercurous chloride makes it less dangerous than its mercuric chloride counterpart. The metal-metal bond , which creates a binuclear compound, present in the mercurous chloride compound creates an oxidation state of +1. This makes it more stable, and less soluble than the Hg²⁺1. Mercury is unique among the group 12 metals for its ability to form the [Hg₂]²⁺ ion which is found in mercurous chloride. Mercurous Chloride is a linear molecule and contains an inversion center. This means that is has D_∞h symmetry. The crystal lattice structure is shown below:

Below is a theoretical mass spectrum calculated using known isotopic ratio’s2.

By using the known percentages of each isotope in the cmplex, the theoretical mass spectrum can be created. In this case, the most abundant compound of Hg₂Cl₂ has a mass of 480.88 amu. with an absorbance percent of 95%.

Preparation and Reactions

When ammonia is added to solid Mercurous chloride, the mercury is oxidized and reduced. This ability to undergo oxidation and reduction is called a disproportionation reaction. The Chemical formula shown below describes chemically this process:

Hg₂Cl₂ + 2NH₃ → Hg + Hg(NH₂)Cl + NH₄⁺ + Cl⁻

Mercurous Chloride can also be created via metathesis reaction involving pure salt and aqueous mercury nitrate. In this reaction, the ions exchange metal centers resulting in mercurous chloride and sodium nitrate.

2NaCl + Hg₂(NO₃)₂ → Hg₂Cl₂ + 2NaNO₃

A third mechanism for creating mercurous chloride can be achieved similarly by a reaction of mercurous nitrate and hydrochloric acid.

2HCl + Hg₂(NO₃)₂ → Hg₂Cl₂ + 2HNO₃

Mercurous chloride can also be created by a simple titration of elemental mercury and Mercuric chloride.

Hg+ HgCl₂ → Hg₂Cl₂

Mercurous chloride has also been extensively in electrochemistry. Since, as discussed previously, this compound can be easily oxidized or reduced, it is used as a reference electrode. Calomel-hydrogen electrodes are referenced in many older publications involving electrochemical cell potentials. Over the past fifty years, it has been overshadowed by the silver/silver chloride (Ag/AgCl) electrodes. 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.

Hg₂Cl₂ → Hg₂²⁺ + 2Cl⁻

H₂O → H⁺ + OH⁻

Hg(l)|Hg₂Cl₂|Cl⁻||H⁺+|H_2(g)|Pt

Light reactions

Mercurous chloride is also involved in several reactions involving light. The simplest of these is decomposition into mercury(II) chloride and elementary mercury.

Light + Hg₂Cl₂ → HgCl₂ + Hg⁺

The formation of mercurous chloride by reaction with a known wavelength of light can be used to determine the amount of calories in that light beam. This will allow provide the opportunity to calculate the number of photons in the light beam. This is done by using a machine called an actinometer. By utilizing a light reaction in the presence of mercury(II) chloride and ammonium oxalate mercurous chloride is produced. The amount of mercurous chloride formed is proportional to the intensity of light.

2HgCl₂ + (NH₄)₂C₂0₄ + Light → Hg₂Cl_2(s) + 2[NH₄⁺][C1⁻] + 2C0₂

External links

• International Chemical Safety Card 0984
• National Pollutant Inventory - Mercury and compounds Fact Sheet
• NIOSH Pocket Guide to Chemical Hazards
• NIST Standard Reference Database
• European Chemicals Bureau