Zinc Chloride ZnCl2

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Zinc chloride is the name of chemical compounds with the formula ZnCl2 and its hydrates. Zinc chlorides, of which nine crystalline forms are known, are colorless or white, and are highly soluble in water.

ZnCl2 itself is hygroscopic and even deliquescent. Samples should therefore be protected from sources of moisture, including the water vapor present in ambient air. Zinc chloride finds wide application in textile processing, metallurgical fluxes, and chemical synthesis. No mineral with this chemical composition is known aside from the very rare mineral simonkolleite, Zn5(OH)8Cl2·H2O.

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Zinc Chloride Properties

Four crystalline forms (polymorphs) of ZnCl2 are known: α, β, γ, and δ, and in each case the Zn2+ ions are tetrahedrally coordinated to four chloride ions.[4]

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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 pure anhydrous orthorhombic form (δ) rapidly changes to one of the other forms on exposure to the atmosphere, and a possible explanation is that the OH− ions originating from the absorbed water facilitate the rearrangement. Rapid cooling of molten ZnCl2 gives a glass.

The covalent character of the anhydrous material is indicated by its relatively low melting point of 275 °C.[9] Further evidence for covalency is provided by the high solubility of the dichloride in ethereal solvents, where it forms adducts with the formula ZnCl2L2, where L = ligand such as O(C2H5)2. In the gas phase, ZnCl2 molecules are linear with a bond length of 205 pm.

Molten ZnCl2 has a high viscosity at its melting point and a comparatively low electrical conductivity, which increases markedly with temperature.[10][11] A Raman scattering study of the melt indicated the presence of polymeric structures,[12] and a neutron scattering study indicated the presence of tetrahedral {ZnCl4} complexes.

Industrial uses for Zinc Chloride

Concentrated aqueous solutions of zinc chloride (more than 64% weight/weight zinc chloride in water) have the interesting property of dissolving starch, silk, and cellulose. Thus, such solutions cannot be filtered through standard filter papers. Relevant to its affinity for these materials, ZnCl2 is used as a fireproofing agent and in fabric "refresheners" such as Febreze. Vulcanized fibre is made by soaking paper in concentrated zinc chloride.

Zinc chloride has the ability to react with metal oxides (MO) to give derivatives of the formula MZnOCl2. This reaction is relevant to the utility of ZnCl2 solution as a flux for soldering — it dissolves oxide coatings, exposing the clean metal surface. Fluxes with ZnCl2 as an active ingredient are sometimes called "tinner's fluid".

Typically this flux was prepared by dissolving zinc foil in dilute hydrochloric acid until the liquid ceased to evolve hydrogen; for this reason, such flux was once known as "killed spirits". Because of its corrosive nature, this flux is not suitable for situations where any residue cannot be cleaned away, such as electronic work.

This property also leads to its use in the manufacture of magnesia cements for dental fillings and certain mouthwashes as an active ingredient.

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