Zinc Chloride, ZnCl₂

The pure anhydrous salt of Zinc Chloride, ZnCl₂, has been prepared by heating 3NH₄Cl.ZnCl₂ in a current of dry hydrogen chloride. The anhydrous salt has also been prepared by distilling anhydrous zinc sulphate with sodium or calcium chloride, by the action of heated zinc vapour and hydrogen on barium chloride, by distilling zinc with mercuric chloride, and by the action of chlorine on zinc oxide. Hydrous zinc chloride partially decomposes on heating, but the anhydrous salt may be obtained from it by treatment with concentrated hydrochloric acid and evaporation in an atmosphere of hydrogen chloride. It may also be prepared by adding zinc to molten lead chloride, passing dry chlorine into fused zinc, or by fusing zinc bromide in an atmosphere of chlorine.

Commercially, the salt is obtained anhydrous by distilling a mixture of zinc sulphate and sodium chloride, and in solution by dissolving zinc scraps, zinc blende, or zinc oxide in hydrochloric acid.

Anhydrous zinc chloride is white or greyish white, and derives its name, " butter of zinc," from its waxlike consistency. It is very hygroscopic, and the density of the fused salt at 25° C. is 2.907 (compared with water at 4° C.). It sublimes in chlorine to needle-shaped crystals that melt at 365° C, and boil at 730° C.

The solubility of zinc chloride has been given as 78.5 parts ZnCl₂ in 100 parts of solution, but the equilibrium curves between ZnCl₂ and water are difficult to interpret.

It is strongly dissociated in aqueous solution, though the anion ZnCr appears to be present in considerable concentration even in solutions more dilute than 0.1 molar. It is only hydrolysed slightly, and, according to Vogel, hydrous zinc chloride can be converted into the anhydrous by evaporation under about 20 inches of mercury, though water, especially at higher temperatures and concentrations, tends to form basic compounds.

Zinc chloride is soluble in many organic solvents, including alcohol, and combines with a number of organic compounds. It is indefinitely soluble in glycerine.

Nitrosyl chloride acts upon zinc to form pale yellow ZnCl₂.NOCl, which is unstable under ordinary conditions.

The formula ZnCl₂ is indicated by the vapour density, the freezing- points of solutions in urethane, and the boiling-points of quinoline solutions.

Molten zinc chloride conducts electricity well, and pure zinc separates at the cathode on electrolysis.

The specific heat of the fused salt is 0.13618.

Anhydrous zinc chloride is poisonous, and is used industrially for impregnating wood, for preserving animal stuffs, in the manufacture of parchment paper, for weighting cotton goods, in the dye industry, and as a disinfectant. It is also employed as a caustic in surgery and as a dehydrating reagent in many organic reactions. Its solution in water, since it destroys organic substances, is used for cleansing and as a flux in soldering.

[Zn] + (Cl₂) = [ZnCl₂]+97.140 Cal.

[ZnCl₂] + Aq. = ZnCl₂.Aq. + 15.220 Cal. (+9.767 Cal. in alcohol).

According to Mylius and Dietz, zinc chloride can crystallise out with 1 or l½ or 2½ or 3 or 4 molecules of water.

The trihydrate, ZnCl₂.3H₂O, separates in large crystals, when a solution containing 70.5 per cent. ZnCl₂ is cooled for twenty-four hours at 0° C. The crystals are hygroscopic rhombic tablets that begin to melt at 4° or 5° C., and form a homogeneous liquid at 9° C.

The needles of a dihydrate said to be obtained by cooling a saturated solution to -20° C. are doubtful.

The hydrate, ZnCl₂.l½H₂O, was obtained in bulky crystals by evaporating a solution of zinc chloride over phosphorus pentoxide.

A bluish-white mass of zincoso-zincic chloride, Zn₃Cl₅, results from the solution of metallic zinc in molten zinc chloride. It is very hygroscopic, rapidly hydrolysed by water, and is probably best formulated as ZnCl.2ZnCl₂.

Deliquescent rhombic crystals of the acid salt, 2ZnCl₂.HCl.2H₂O, separate from a solution of zinc chloride into which hydrogen chloride has been passed when the density of the liquid at 25° C. is 2. At 100° C. they lose HCl, and anhydrous ZnCl₂ is in contact with a concentrated solution of the salt and hydrochloric acid. If this mother-liquor is treated with hydrogen chloride more crystals separate, and finally, if the liquid is saturated with hydrogen chloride and cooled to 0° C., long needles of ZnCl₂.HCl.2H₂O crystallise. These readily lose hydrogen chloride in dry air and form 2ZnCl₂.HCl.2H₂O.

The diammoniate ZnCl₂.2NH₄ is formed in the active Leclanche cell. It can be prepared by passing ammonia gas into an alcoholic solution of zinc chloride, or by passing ammonia into a hot concentrated aqueous solution of zinc chloride, and, after cooling to separate the less soluble tetrammoniate, ZnCl₂.4NH₄.H₂O, crystallising the mother-liquor. The rhombic crystals are stable in air, insoluble in water or alcohol, and soluble in acids, ammonia, caustic alkalies, and ammonium chloride. Boiling water decomposes them. It can also be prepared by adding ammonia to cold dilute zinc chloride solution, by dissolving freshly precipitated zinc hydroxide in ammonium chloride, and by heating more highly ammoniated zinc chlorides. It distils when heated into glassy, hygroscopic monammoniate, ZnCl₂.NH₄.

The last named is very stable under heat, and is produced when ammonia acts on hot zinc chloride. Water decomposes it into ZnCl₂.2NH₄ and an oxychloride of zinc.

The more unstable tetrammoniate, ZnCl₂.4NH₄.H₂O, and pentammoniate, ZnCl₂.5NH₄.H₂O, have been prepared: the former in pearly leaflets by saturating an aqueous solution of zinc chloride with ammonia gas and cooling, the latter in octahedra by cooling a solution of zinc chloride in ammonia after passing ammonia gas. Isambert prepared ZnCl₂.6NH₄ and examined the heats of formation of the ammoni- ated zinc chlorides.

Various other compounds have been reported.

The hexammoniate ZnCl₂.6NH₄ is apparently formed when zinc chloride absorbs ammonia gas at -18° C. At ordinary temperatures less ammonia than corresponds to this formula is absorbed. It decomposes at 57.5° C. - at about 64.5° C. the tetrammoniate is formed, which decomposes at 92° C. and loses increasing quantities of ammonia as the temperature rises. At about 98° C. the composition of the residue approximates to ZnCl₂.2NH₄.

The following double chlorides of zinc have been described: LiCl.ZnCl₂.3H₂O and 2LiCl.ZnCl₂.2H₂O; 3LiCl.ZnCl₂.10H₂O may exist; 2NaCl.ZnCl₂.3H₂O; KCl.ZnCl₂.2H₂O and 2KCl.ZnCl₂; 2NH₄Cl.ZnCl₂; 3NH₄Cl.ZnCl₂; 2RbCl.ZnCl₂; 2CsCl.ZnCl₂ and 3CsCl.ZnCl₂Cl₂; CaCl₂.ZnCl₂. 5½H₂O and 2CaCl₂.ZnCl₂.6H₂O; SrCl₂.ZnCl₉.4H₂O and SrCl₂.ZnCl₂; BaCl₂.ZnCl₂.4H₂O; MgCl₂.ZnCl₂.6H₂O; ZnCl₂. PtCl₄.6H₂O.