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Cupric sulphate, CuSO4

The Cupric sulphate, CuSO4, is prepared by the action of dilute sulphuric acid on cupric oxide or carbonate, the salt crystallizing as pentahydrate on evaporation. It can also be obtained by dissolving the metal in nitric acid, and decomposing the nitrate by means of sulphuric acid. With access of air, the metal is also converted into the sulphate by sulphuric acid.

Several methods are applicable to the production of cupric sulphate on the manufacturing scale. Old copper plates are heated with excess of sulphur to bright redness in a reverberatory furnace with closed doors until combination is complete. The doors are then opened, and the mass is oxidized at dull-red heat. When oxidation is complete, the hot product is transferred into dilute sulphuric acid, and the clear solution concentrated after decantation. The crystals formed are of a moderate degree of purity. The process is also applicable to coarse copper, and to copper-glance and other sulphur ores of copper.

When the ores contain iron, it is impossible to separate the ferrous sulphate and cupric sulphate by crystallization. If the mixed sulphides are roasted at a suitable temperature, the ferrous sulphate formed is converted into oxide. Another method of separation depends on heating a solution of the two sulphates under pressure at 180° C., the ferrous salt crystallizing out. For agricultural purposes the removal of the iron is unnecessary.

Crude copper or one of its ores can also be roasted in air, and transformed into the sulphate by the action of sulphur dioxide.

The copper can first be converted into cupric chloride by the action of chlorine and water. With sulphuric acid the salt formed reacts to produce cupric sulphate and hydrochloric acid.

The formation of cupric sulphate can also be effected by dissolving the oxide in sulphuric acid. If the oxide has been produced from an ore containing silver or gold, addition to the acid of its own volume of water prevents solution of these metals. Another modification of the process involves roasting the argentiferous ore in a reverberatory furnace, digesting the mass with sulphuric acid, decanting the clear solution from the precipitated lead and gold, and running it into lead-lined vats containing copper plates. The silver, and part of the arsenic and antimony, are deposited on these plates, most of the bismuth being simultaneously precipitated as basic sulphate, and the iron reduced to ferrous sulphate. The cupric sulphate is isolated by crystallization.

An electrolytic method for the manufacture of cupric sulphate depends on the electrolysis of a solution of sodium sulphate, using copper electrodes, and passing a current of carbon dioxide through the liquid. The copper dissolves at the anode, and is precipitated as carbonate at the cathode. The carbonate is subsequently dissolved in sulphuric acid.

The pentahydrate forms azure-blue, triclinic crystals, converted by dehydration into the trihydraie, the monohydrate, and the white anhydrous salt. The respective densities of these forms at ordinary temperature are 2.282, 2.663, 3.289, 3.606, and their transition- temperatures on the absolute scale at 0.1 atm. are 325° C., 339° C., and 401° C.

A table giving the solubility of anhydrous cupric sulphate at various temperatures has been compiled by Meyerhoffer from the results obtained by several investigators.

The investigations of Lescoeur on the vapour-pressure of the hydrates confirm the assumption of the existence of the pentahydrate, trihydrate, and rnonohydrate. MacLeod-Brown has suggested two formulae for the pentahydrate to explain the step-by-step removal of water.

A detailed examination of the mechanism of the dehydration of the pentahydrate has been made by Guareschi. Over calcium chloride at 21° to 23° C., or in air at 45° to 50° C., it loses two molecules of water, forming the pale sky-blue trihydrate. In a thermostat at 60° C. the trihydrate gives up two more molecules of water, but exposure to air at the ordinary temperature reconverts it into the pentahydrate. The molecule of water present in the monohydrate is expelled at 206° C., and not at 114° C. as stated by Pierre, the elimination of the second half taking place slowly. Guareschi regards the monohydrate as having the formula

, and the semihydrate as .

The heat of formation of the anhydrous salt from its elements is given as 181.7 Cal. and 182.6 Cal.

With excess of cupric sulphate reduction with hypophosphorous acid yields metallic copper, but with excess of the acid cuprous hydride is precipitated. Cupric sulphate is also reduced by hydroxylamine.

Copper forms a number of basic sulphates, among them the mineral langite, CuSO4,3CuO,4H2O, prepared artificially by Sabatier by the interaction of cupric hydroxide and a solution of cupric sulphate. The mineral brochantite, 2CuSO4,5Cu(OH)2, has been prepared in the laboratory from cupric-sulphate solution by the action of limestone. Shenstone has described a crystalline sulphate, CuSO4,2Cu(OH)2. On mixing concentrated solutions of cupric sulphate and ammonium carbonate, and diluting the deep-purple solution, a voluminous, blue precipitate of the formula 15CuO,SO3 is produced. Other basic sulphates of this type are 3CuO,SO3; 4CuO,SO3; 5CuO,SO3;

and 10CuO,SO3. A basic sulphate of the approximate composition 7CuO,2SO3,6H2O has been described by Ost. With a saturated solution of cupric sulphate formaldehyde yields a green, crystalline precipitate of the formula CuSO4,CuO. The product is insoluble in water, but is transformed by moist air into a deep-green, crystalline substance of the composition CuSO4,Cu(OH)2.

Double sulphates of the type CuSO4,M2SO4,6H2O are produced by the interaction of cupric sulphate and the sulphates of the alkali-metals and ammonium. These double salts are isomorphous. The dissociation-pressures of those with M = K, or Rb, or Cs, or NH4 have been investigated by Caven and Ferguson. They could not prepare the hexahydrate of sodium cupric sulphate, CuSO4,Na2SO4,6H2O, but only the dihydrate. Potassium cupric sulphate, CuSO4,K2SO4,6H2O, is present in lava from Vesuvius, the mineral being known as cyanochroite. A double salt of the formula CuSO4,(NH4)2SO4 has also been prepared. The mineral kroehnkite, CuSO4,Na2SO4,2H2O, is found in the Atacama Desert. It was prepared by Graham by the interaction of solutions of cupric sulphate and sodium hydrogen sulphate, although no double salt was obtained by him from the normal sulphate. The solubility-relations of the hydrates CuSO4,5H2O and Na2SO4,10H2O have been investigated by Koppel, his results pointing to the possibility of the existence in the solution of a double salt of the formula


Cupric sulphate forms isomorphous double salts with zinc, ferrous, cobalt, nickel, manganese, cadmium, and magnesium sulphates. With zinc sulphate three crystalline types have been prepared: almost colourless, rhombic prisms with 7H2O; pale-blue, monoclinic crystals with 7H2O; dark-blue, triclinic crystals with 5H2O. With ferrous sulphate monoclinic and. triclinic double salts are formed. Cobalt sulphate behaves similarly, the red, monoclinic crystals containing a high percentage of cobalt, and the blue, triclinic crystals a high percentage of copper. Nickel sulphate yields three series of double salts analogous to those obtained from the zinc salt. For manganese sulphate the stable form is the triclinic, with 5H2O, but below 18° C. monoclinic crystals with 7H2O also exist. Cadmium sulphate forms two series of double salts: almost colourless, monoclinic crystals with approximately 3H2O; and blue, triclinic crystals with 5H2O. With magnesium sulphate three types are produced at ordinary temperatures: almost colourless, rhombic prisms with 7H2O; light-blue, monoclinic crystals with 7H2O; dark-blue, triclinic crystals with 5H2O.

Complex salts of cupric sulphate with cupric chloride, potassium sulphate, and potassium chloride have been described.

With ammonia, cupric sulphate combines to form complex derivatives. Thermochemical data indicate the existence of CuSO4,NH3; CuSO4,2NH3; CuSO4,4NH3; and CuSO4,5NH3. The existence of the complex CuSO4,4NH3 has also been postulated from physical measurements. The compound CuSO4,4NH3,H2O can be prepared by passing ammonia into a solution of cupric sulphate. It is stable in dry air. The compound CuSO4,5NH3 has been prepared by Rose and by Mendeleeff.

In alcoholic solution cupric sulphate combines with nitric oxide to form a double compound of the formula CuSO4,NO.

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