as, in addition to the two nearest neighbours, there are four other Cl atoms at 321 pm. Longer mercury polycations exist. 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. 2 HCl + Hg2(NO3)2 → Hg2Cl2 + 2 HNO3 Ammonia causes Hg2Cl2 to disproportionate: Hg2Cl2 + 2 NH3 → 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 Over the past 50 years, it has been superseded by the silver/silver chloride (Ag/AgCl) electrode.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, mercury(I) chloride, ammonium chloride and carbon dioxide are produced. 2 HgCl2 + (NH4)2C2O4 Hg2Cl2(s) + 2 [][Cl−] + 2 CO2 This particular reaction was discovered 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, is 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) in the United States from the late 1700s through the 1860s. Calomel was also a common ingredient in teething powders in Britain up until 1954, causing widespread mercury poisoning in the form of pink disease, which at the time had a mortality rate of 1 in 10. These medicinal uses