On evaporation of the solution, the salt is obtained in clear bluish-green rhomboidal crystals, containing seven atoms of water. This salt is commercially known as copperas or green vitriol, and its various applications in technology are noticed in the article VITRIOLS, BLUE AND GREEN.
Phosphate of iron is obtained by precipitating a solution of a protosalt of iron with phosphate of soda, when a white precipitate of phosphate of iron is thrown down.
All these salts, especially the carbonate and sulphate, are extensively used in medi cine.
Silicate and phosphate of iron occur naturally in several minerals.
The peroxide of iron, termed also sesonioxide; red oxide, or ferric oxide, is obtained in an anhydrous form by igniting the protosulphate, anthis known in the arts under the names colcothar, crocus of itars,,or rouge, according to the, degree of levigation to which it has been submitted. It is employed for polishing glass, jewelry, etc., and is also used as a pigment. It occurs both in the anhydrous and in the hydrated form in vari ous minerals.
The hydrated peroxide is obtained by precipitating a solution of a persalt of iron, or of a ferric salt, with an excess of potash, ammonia, or alkaline carbo nate. It falls as a yellowish-brown flocculent precipitate, which when dried forms a dense brown mass. This hydrated peroxide of iron, when freshly prepared and sus pended in water, is regarded as an antidote, in arsenical poisoning. Rust, as has been already mentioned, is a hydrated peroxide, combined with a little ammonia.
The most important of the persalts of iron, or ferric salts, are the neutral and the basic sulphate, whese formulae are and respectively, the nitrate the phosphate, and the silicate. Of these, the neutral sulphate, the phosphate, and the silicate occur in various minerals. The nitrate, which is obtained by the solution of iron in nitric acid, is a useful medicinal agent.
The back or magnetic oxide and ferric acid, which has not been obtained in a free state, and is only known as a constituent of certain salts, must be passed over without com ment.
b. Haloid salts of iron—the chlorides, bromides, and iodides—next require notice. There are two chlorides—viz., a -protochloride (FeCI) and a perchloride or sesquichloride The latter may be obtained by dissolving peroxide of iron in hydrochloric acid. The tincture of the sesquiehloride of iron is perhaps more generally employed in
medicine than any other preparation of this metal. The protiodide is an extremely valuable therapeutic agent.
c. There are probably several sulphides or sulphurets of iron. The ordinary sulphide is a protosnlphide (FeS). It occurs in small quantity in meteoric, iron. It may be obtained artificially by the direct union of the two elements at a high temperature, or by the precipitation of a protosalt of iron by sulphide of ammonium. It exists in glistening masses, varying in color from a grayish yellow to a reddish brown. It is insoluble in water, but in moist air becomes rapidly oxidized into protosulphate of iron. With acids, it develops sulphureted hydrogen. The bisulphide of iron is the iron pyrites of mineralogists, and the mundic of commerce. Under the latter name, it is used exten sively in the preparation of oil of vitriol. There are also other sulphides of less impor tance.
The protosalts and the persalts, or the ferrous and the ferric salts, give totally different reactions with the ordinary tests. The solutions of the former have a greenish color and a peculiar metallic taste, while those of the latter are generally of a brownish-yellow color, and are very acid. Stilphureted hydrogen gives no precipitate with an acid solu tion of a ferrous salt, while it gives a milky precipitate of sulphur with a solution of a ferric salt. Potash, soda, and ammonia throw down a white hydrated oxide from the former, and a brown hydrated peroxide from the latter. Ferrocyanide of potassium gives with ferrous salts a white precipitate, which soon becomes blue, while with ferric salts it at once produces a blue precipitate, even in a very dilute solution. Tincture of galls (tannic acid) produces no immediate change of color with the ferrous, but a deep blackish-blue color (ink) with the ferric salts. Sulphocyanide of potassium produces no change with the ferrous, but gives a deep blood-red tint with the ferric salts. Sue cinate and benzoate of ammonia produce no precipitate or change of color with the former, while with the latter, if the solution is not too acid, they throw down pale reddish-brown precipitates.