Zinc Sulphate, ZnSO₄

The heptahydrate, ZnSO₄.7H₂O, occurs naturally as goslarite, which crystallises in rhombic needles of hardness 2-2.5 and density 1.95-2.04. Shining rectangular tabular crystals, belonging to the rhombic system, of density 3.74 at 15° C., that are identical with natural zincosite, ZnSO₄, are obtained by evaporating down a solution of zinc sulphate in sulphuric acid and carefully heating off the excess of acid. They soon become cloudy on exposure to air. Octahedral crystals of anhydrous zinc sulphate were prepared by heating hydrated zinc sulphates with ammonium sulphate. They dissolved slowly in cold water and rapidly in hot.

White amorphous anhydrous zinc sulphate is easily obtained by heating the hydrated sulphates. It may be heated to a dull red heat without decomposition, though it is converted into zinc oxide at higher temperatures.

The specific heat of anhydrous zinc sulphate is 0.174, and its density has been given variously from 3.400-3.681.

Zinc sulphide is formed by heating zinc sulphate either with an excess of sulphur or rapidly (to a white heat) with carbon. Zinc oxide is produced when a mixture of zinc sulphate and carbon is raised more slowly to dull redness, and metallic zinc if the temperature is raised to a bright red. Ammonia gas reduces the heated sulphate to a residue with the composition ZnO.ZnS.

One hundred parts of methyl alcohol dissolve 0.65 parts of anhydrous zinc sulphate, and the salt dissolves readily in water with the evolution of heat.

[ZnSO₄]+400Aq. = ZnSO₄.Aq. + 18.43 Cal.

At 20° C. 100 parts of water dissolve over 50 parts of ZnSO₄.

Aqueous solutions of zinc sulphate are only slightly hydrolysed -0.03 per cent, in molar solution.

At temperatures below 39° C. the heptahydrate, ZnSO₄.7H₂O, usually crystallises from pure solutions of zinc sulphate made by dissolving the metal, sulphide, oxide, hydroxide, or carbonate in dilute sulphuric acid.

It is prepared industrially by carefully roasting zinc blende, extracting with water, and crystallising. Zinc sulphide ores are also treated with sulphuric acid, and the resulting sulphate, after treating with water and filtering, crystallised out. It is known commercially as white vitriol, and has a great variety of uses. It is used in dyeing and calico printing, for preparing pigments, varnishes, and various zinc compounds, and in disinfection, preserving wood or hides, flame proofing, and pharmacy.

It crystallises in rhombic prisms, isomorphous with Epsom salts.

Monoclinic crystals, isomorphous with FeSO₄.7H₂O, have been obtained by introducing a crystal of a monoclinic vitriol, like CoSO₄.7H₂O, into the supersaturated solution. They have been usually obtained as mixed crystals.

The density of white vitriol has been variously given from 1.931-1.976. The specific heat of the crystals is 0.328; they melt at about 50° C., effloresce slowly in the air and lose water more completely, eventually becoming anhydrous, as the temperature is raised.

[ZnSO₄.7H₂O] +400Aq. = ZnSO₄.Aq. -4.26 Cal.

The solubility of ZnSO₄.7H₂O is -

Temperature, ° C	0.1	15	30	35	39
Grm. ZnSO4 in 100 grm. Water	41.93	50.88	61.92	66.61	70.05

At 39° C. the heptahydrate is transformed into the hexahydrate. This is important for the use of the Clark standard cell, which consists of the system mercury - mercurous sulphate - zinc sulphate crystals - zinc sulphate solution - zinc sulphate crystals - zinc amalgam (the mercurous sulphate being made into a paste with some zinc sulphate).

The solubility of the hexahydrate is -

Temperature, ° C.	0.1	15	30	35	39
Grm. ZnSO4 in 100 grm. Water	49.48	57.15	65.82	67.99	70.08

Both the hepta- and hexa-hydrate can exist for some time in a labile state outside their normal range of existence.

Monoclinic crystals of the hexahydrate, isomorphous with the corresponding magnesium salt, separate from solutions of zinc sulphate at temperatures above 39° C. Tetragonal crystals have been obtained by introducing a tetragonal crystal of NiSO₄.6H₂O into the supersaturated solution. The density of the monoclinic crystals is 2.056.

Pierre obtained a pentahydrate by evaporating a solution of zinc sulphate at 40°-50° C. till it began to crystallise, and cooling slowly to 35° C., and a salt of density 2.206 was obtained by boiling the finely powdered heptahydrate with alcohol of density 0.825.

The existences of a tetrahydrate and a trihydrate were deduced from the heats of hydration of zinc sulphates.

A dihydrate has been prepared by pouring a saturated solution of zinc sulphate into concentrated sulphuric acid and washing the precipitate with absolute alcohol. This salt had a density of 2.958. The heptahydrate is said to become the dihydrate in dry air, and, according to Etard, the same salt is obtained by evaporating a zinc sulphate solution at 120° C., or by heating a solution saturated at 15° C. in a closed vessel at 180° C.

When a solution of zinc sulphate is evaporated on the water-bath the residue is the monohydrate ZnSO₄.H₂O. It also deposits from a boiling saturated solution and crystallises from a solution of the heptahydrate in methyl alcohol. It has been prepared by heating heptahydrated zinc sulphate to 100° C. or 110° C.

The density of ZnSO₄.H₂O has been given as 3.076 and 3.289. Its specific heat is 0.202.

[ZnSO₄.H₂O]+400Aq. = ZnSO₄.Aq.+9.95 Cal.

Rhombohedral crystals of ZnSO₄.3½H₂O have been reported. Lescoeur suggested that the number of hydrates had been overestimated.

The following basic sulphates have been reported: 8ZnO.SO₃.16H₂O; 8ZnO.SO₃.7H₂O; 8ZnO.SO₃.6H₂O; 8ZnO.SO₃.2H₂O; 8ZnO.SO₃.H₂O; 6ZnO.SO₃.10H₂O; 5ZnO.SO₃; 4ZnO.SO₃.7H₂O; 4ZnO.SO₃.6H₂O; 4ZnO.SO₃.5H₂O; 4ZnO.SO₃. 3H₂O; (ZnOH)₄SO₄.

According to Lubkowskaia only 4ZnO.SO₃.3H₂O exists as a definite compound.

Zinc sulphate absorbs ammonia gas in proportions depending upon the temperature. Zinc sulphate pentammoniate, ZnSO₄.5NH₄, decomposes at 19° C., the tetrammoniate at 98.5° C., and the triammoniate at 185° C. A hexammoniate exists at temperatures less than this range and a diammoniate at higher.

From a solution obtained by passing ammonia gas into a warm concentrated solution of zinc sulphate until it just cleared, the hydrated compounds ZnSO₄.4NH₄.4H₂O, ZnSO₄.4NH₄.2H₂O, and ZnSO₄.2NH₄.H₂O were obtained. They lose ammonia and water progressively when heated - anhydrous zinc sulphate being ultimately left.

Deliquescent needles or tabular crystals of ZnSO₄.4NH₄.3H₂O were obtained by treating a cooled solution of zinc sulphate in aqueous ammonia with ammonia gas.

In ammoniacal solutions of zinc sulphate there are probably complex ions containing NH₄.

The acid sulphate SO₄.ZnSO₄.8H₂O deposits in monoclinic crystals from solutions of zinc sulphate containing sulphuric acid. It dissolves sparingly in cold water, readily in hot, and loses water and sulphuric acid when heated.

Crystals of 3SO₄.ZnSO₄ have been separated by cooling the acid in the first chambers during the manufacture of sulphuric acid from zinc blende. When heated strongly they decompose into zinc sulphate and sulphuric acid.

There is a large group of salts represented by the general formula M₂SO₄.RSO₄.6H₂O, where R is a divalent metal and M is NH₄, thallium, or any alkali metal except lithium or sodium. The lithium and sodium salts either do not exist or have a different proportion of water of crystallisation. The salts with R = zinc can be prepared by dissolving the constituent simple salts in molecular proportions and crystallising.

Zinc sulphate also forms a series of double salts with the general formula ZnSO₄.MSO₄.14H₂O, where M is magnesium, copper, manganese, iron (ferrous), or nickel.

The following salts have also been described: Na₂SO₄.ZnSO₄.4H₂O; KSO₄.ZnSO₄; KSO₄.2ZnSO₄; CuSO₄.3ZnSO₄.28H₂O; (Zn, Fe)SO₄.H₂O, and (Zn, Cr)SO₄.H₂O.

Zinc sulphate is also a constituent of numerous complex salts.