Zinc chloride

Zinc chloride is an inorganic chemical compound with the formula ZnCl₂·nH₂O, with n ranging from 0 to 4.5, forming hydrates. Zinc chloride, anhydrous and its hydrates, are colorless or white crystalline solids, and are highly soluble in water. Five hydrates of zinc chloride are known, as well as four polymorphs of anhydrous zinc chloride.

All forms of zinc chloride are deliquescent. They can usually be produced by the reaction of zinc or its compounds with some form of hydrogen chloride. Anhydrous zinc compound is a Lewis acid, readily forming complexes with a variety of Lewis bases. Zinc chloride finds wide application in textile processing, metallurgical fluxes, chemical synthesis of organic compounds, such as benzaldehyde, and processes to produce other compounds of zinc.

Zinc chloride has long been known but currently practiced industrial applications all evolved in the latter half of 20th century.

An amorphous cement formed from aqueous zinc chloride and zinc oxide was first investigated in 1855 by Stanislas Sorel. Sorel later went on to investigate the related magnesium oxychloride cement, which bears his name.

Dilute aqueous zinc chloride was used as a disinfectant under the name "Burnett's Disinfecting Fluid". From 1839 Sir William Burnett promoted its use as a disinfectant as well as a wood preservative. The Royal Navy conducted trials into its use as a disinfectant in the late 1840s, including during the cholera epidemic of 1849; and at the same time experiments were conducted into its preservative properties as applicable to the shipbuilding and railway industries. Burnett had some commercial success with his eponymous fluid. Following his death however, its use was largely superseded by that of carbolic acid and other proprietary products.

Unlike other metal dichlorides, zinc dichloride adopts several crystalline forms (polymorphs). Four polymorph are known: α, β, γ, and δ. Each features Zn²⁺ centers surrounded in a tetrahedral manner by four chloride ligands.

Here a, b, and c are lattice constants, Z is the number of structure units per unit cell, and ρ is the density calculated from the structure parameters.

The orthorhombic form (δ) rapidly changes to another polymorph upon exposure to the atmosphere. A possible explanation is that the OH⁻ ions originating from the absorbed water facilitate the rearrangement. Rapid cooling of molten ZnCl₂ gives a glass.