Ferrous solutions absorb nitric oxide, forming dark green to black solutions. The coloration is due to the production of unstable compounds of the ferrous salt and nitric oxide, and it seems that in neutral solutions the compound is made up of one molecule of salt to one of gas; the reaction, however, is reversible, the composition varying with temperature, concentration and nature of the salt. Ferrous chloride dissolved in strong hydrochloric acid absorbs two molecules of the gas (Kohlschütter and Kutscheroff, Ber., 1907, 40, p. 873). Ferric chloride also absorbs the gas. Reddish brown amorphous powders of the formulae 2FeCl3·NO and 4FeCl3·NO are obtained by passing the gas over anhydrous ferric chloride. By passing the gas into an ethereal solution of the salt, nitrosyl chloride is produced, and on evaporating over sulphuric acid, black needles of FeCl2·NO·2H2O are obtained, which at 60° form the yellow FeCl2·NO. Complicated compounds, discovered by Roussin in 1858, are obtained by the interaction of ferrous sulphate and alkaline nitrites and sulphides. Two classes may be distinguished:—(1) the ferrodinitroso salts, e.g. K[Fe(NO)2S], potassium ferrodinitrososulphide, and (2) the ferroheptanitroso salts, e.g. K[Fe4(NO)7S8], potassium ferroheptanitrososulphide. These salts yield the corresponding acids with sulphuric acid. The dinitroso acid slowly decomposes into sulphuretted hydrogen, nitrogen, nitrous oxide, and the heptanitroso acid. The heptanitroso acid is precipitated as a brown amorphous mass by dilute sulphuric acid, but if the salt be heated with strong acid it yields nitrogen, nitric oxide, sulphur, sulphuretted hydrogen, and ferric, ammonium and potassium sulphates.
Phosphides, Phosphates.—H. Le Chatelier and S. Wologdine (Compt. rend., 1909, 149, p. 709) have obtained Fe3P, Fe2P, FeP, Fe2P3, but failed to prepare five other phosphides previously described. Fe3P occurs as crystals in the product of fusing iron with phosphorus; it dissolves in strong hydrochloric acid. Fe2P forms crystalline needles insoluble in acids except aqua regia; it is obtained by fusing copper phosphide with iron. FeP is obtained by passing phosphorus vapour over Fe2P at a red-heat. Fe2P3 is prepared by the action of phosphorus iodide vapour on reduced iron. Ferrous phosphate, Fe3(PO4)2·8H2O, occurs in nature as the mineral vivianite. It may be obtained artificially as a white precipitate, which rapidly turns blue or green on exposure, by mixing solutions of ferrous sulphate and sodium phosphate. It is employed in medicine. Normal ferric phosphate, FePO4·2H2O, occurs as the mineral strengite, and is obtained as a yellowish-white precipitate by mixing solutions of ferric chloride and sodium phosphate. It is insoluble in dilute acetic acid, but dissolves in mineral acids. The acid salts Fe(H2PO4)3 and 2FeH3(PO4)2·5H2O have been described. Basic salts have been prepared, and several occur in the mineral kingdom; dufrenite is Fe2(OH)3PO4.
Arsenides, Arsenites, &c.—Several iron arsenides occur as minerals; lölingite, FeAs2, forms silvery rhombic prisms; mispickel or arsenical pyrites, Fe2AsS2, is an important commercial source of arsenic. A basic ferric arsenite, 4Fe2O3·As2O3·5H2O, is obtained as a flocculent brown precipitate by adding an arsenite to ferric acetate, or by shaking freshly prepared ferric hydrate with a solution of arsenious oxide. The last reaction is the basis of the application of ferric hydrate as an antidote in arsenical poisoning. Normal ferric arsenate, FeAsO4·2H2O, constitutes the mineral scorodite; pharmacosiderite is the basic arsenate 2FeAsO4·Fe(OK)3·5H2O. An acid arsenate, 2Fe2(HAsO4)3·9H2O, is obtained as a white precipitate by mixing solutions of ferric chloride and ordinary sodium phosphate. It readily dissolves in hydrochloric acid.
Carbides, Carbonates.—The carbides of iron play an important part in determining the properties of the different modifications of the commercial metal, and are discussed under [Iron and Steel].
Ferrous carbonate, FeCO3, or spathic iron ore, may be obtained as microscopic rhombohedra by adding sodium bicarbonate to ferrous sulphate and heating to 150° for 36 hours. Ferrous sulphate and sodium carbonate in the cold give a flocculent precipitate, at first white but rapidly turning green owing to oxidation. A soluble carbonate and a ferric salt give a precipitate which loses carbon dioxide on drying. Of great interest are the carbonyl compounds. Ferropentacarbonyl, Fe(CO)5, obtained by L. Mond, Quincke and Langer (Jour. Chem. Soc., 1891; see also ibid. 1910, p. 798) by treating iron from ferrous oxalate with carbon monoxide, and heating at 150°, is a pale yellow liquid which freezes at about −20°, and boils at 102.5°. Air and moisture decompose it. The halogens give ferrous and ferric haloids and carbon monoxide; hydrochloric and hydrobromic acids have no action, but hydriodic decomposes it. By exposure to sunlight, either alone or dissolved in ether or ligroin, it gives lustrous orange plates of diferrononacarbonyl, Fe2(CO)9. If this substance be heated in ethereal solution to 50°, it deposits lustrous dark-green tablets of ferrotetracarbonyl, Fe(CO)4, very stable at ordinary temperatures, but decomposing at 140°-150° into iron and carbon monoxide (J. Dewar and H. O. Jones, Abst. J.C.S., 1907, ii. 266). For the cyanides see [Prussic Acid].
Ferrous salts give a greenish precipitate with an alkali, whilst ferric give a characteristic red one. Ferrous salts also give a bluish white precipitate with ferrocyanide, which on exposure turns to a dark blue; ferric salts are characterized by the intense purple coloration with a thiocyanate. (See also [Chemistry], § Analytical). For the quantitative estimation see [Assaying].
A recent atomic weight determination by Richards and Baxter (Zeit. anorg. Chem., 1900, 23, p. 245; 1904, 38, p. 232), who found the amount of silver bromide given by ferrous bromide, gave the value 55.44 [O = 16].
Pharmacology.
All the official salts and preparations of iron are made directly or indirectly from the metal. The pharmacopoeial forms of iron are as follow:—
1. Ferrum, annealed iron wire No. 35 or wrought iron nails free from oxide; from which we have the preparation Vinum ferri, iron wine, iron digested in sherry wine for thirty days. (Strength, 1 in 20.)