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(from the Greek kalos, “beautiful,” and melas, “black”), mercurous chloride, Hg2Cl2; a colorless powder. When treated with caustic alkalies or ammonia, it turns black (hence the name). The darkening occurs as result of the precipitation of finely powdered mercury according to the reactions
Hg2Cl2 + 2NaOH - Hg + HgO + 2NaCl + H2O
Hg2Cl2 + 2NH 4OH = HgNH2Cl + Hg + NH4C1 + 2H2O
Calomel has a density of 7, 160 kg/m3 and evaporates without fusion. Its sublimation point is 383.7°C. It is poorly soluble in water.
Calomel is used in the preparation of calomel electrodes and as a catalyst in organic reactions. In medicine it serves as an antibacterial agent, applied externally in ointment form in cases of wart infections and gonorrheal conjunctivitis and as a safeguard against venereal diseases (locally). Calomel is sometimes used internally as a cholagogue.
In nature calomel exists as a rare mineral of the halide group.It crystallizes in the tetragonal system and has an adamantineluster. Its hardness on the mineralogical scale measures 1.5.Native calomel exhibits very high double refraction. It is formedin the oxidation zone of mercury deposits.