Mercury chloride is a name applied to two distinct inorganic compounds that contain mercury and chlorine. The term most commonly denotes mercuric chloride (chemical formula HgCl2) and mercurous chloride (chemical formula Hg2Cl2), which differ in mercury oxidation state, structure, and behaviour.
Characteristics and chemistry
Mercuric chloride (HgCl2) is a covalent molecular salt in which mercury is in the +2 oxidation state. It forms white crystals, is more soluble in water and organic solvents than the mercurous form, and can sublime on heating. Mercurous chloride (Hg2Cl2), also called calomel, contains the Hg2(2+) dimeric ion (formally mercury in the +1 state). Calomel is less soluble and appears as a white powder.
History and development
Both compounds have long histories of practical use. Calomel was widely used in medicine as a purgative and topical agent from the 18th through the 19th centuries. Mercuric chloride, historically known as corrosive sublimate, was used as a disinfectant, preservative, and in early photographic processes. Use declined as awareness of mercury's toxicity increased and safer alternatives were developed.
Uses and examples
- Historical medicinal uses (now largely discontinued).
- Mercuric chloride: reagent in organic synthesis, antiseptic and preservative in past applications.
- Mercurous chloride: reference material in electrochemistry (saturated calomel electrode) and as a laboratory reagent.
Toxicity, regulation and handling
Both forms of mercury chloride are toxic. Mercuric chloride is especially hazardous because it is readily absorbed and can cause severe systemic poisoning. Calomel, though less soluble, still presents chronic toxicity risks. Modern handling requires appropriate personal protective equipment, fume control, and disposal according to hazardous waste regulations. Many historic uses are now banned or severely restricted.
For chemical data, safety guidelines, and further reading see specialized chemical references.