of the two salts in water and crystallizing. It is very stable and is much used in volumetric analysis.
Ferric sulphate, Fe2(SO4)3, is obtained by adding nitric acid to a hot solution of ferrous sulphate containing sulphuric acid, colourless crystals being deposited on evaporating the solution. The anhydrous salt is obtained by heating, or by adding concentrated sulphuric acid to a solution. It is sparingly soluble in water, and on heating it yields ferric oxide and sulphur dioxide. The mineral coquimbite is Fe2(SO4)3·9H2O. Many basic ferric sulphates are known, some of which occur as minerals; carphosiderite is Fe(FeO)5(SO4)4·10H2O; amarantite is Fe(FeO)(SO4)2·7H2O; utahite is 3(FeO)2SO4·4H2O; copiapite is Fe3(FeO)(SO4)5·18H2O; castanite is Fe(FeO)(SO4)2·8H2O; römerite is FeSO4·Fe2(SO4)3·12H2O. The iron alums are obtained by crystallizing solutions of equivalent quantities of ferric and an alkaline sulphate. Ferric potassium sulphate, the common iron alum, K2SO4·Fe2(SO4)3·24H2O, forms bright violet octahedra.
Nitrides, Nitrates, &c.—Several nitrides are known. Guntz (Compt. rend., 1902, 135, p. 738) obtained ferrous nitride, Fe3N2, and ferric nitride, FeN, as black powders by heating lithium nitride with ferrous potassium chloride and ferric potassium chloride respectively. Fowler (Jour. Chem. Soc., 1901, p. 285) obtained a nitride Fe2N by acting upon anhydrous ferrous chloride or bromide, finely divided reduced iron, or iron amalgam with ammonia at 420°; and, also, in a compact form, by the action of ammonia on red-hot iron wire. It oxidizes on heating in air, and ignites in chlorine; on solution in mineral acids it yields ferrous and ammonium salts, hydrogen being liberated. A nitride appears to be formed when nitrogen is passed over heated iron, since the metal is rendered brittle. Ferrous nitrate, Fe(NO3)2·6H2O, is a very unstable salt, and is obtained by mixing solutions of ferrous sulphate and barium nitrate, filtering, and crystallizing in a vacuum over sulphuric acid. Ferric nitrate, Fe(NO3)3, is obtained by dissolving iron in nitric acid (the cold dilute acid leads to the formation of ferrous and ammonium nitrates) and crystallizing, when cubes of Fe(NO3)3·6H2O or monoclinic crystals of Fe(NO3)3·9H2O are obtained. It is used as a mordant.
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.)
2. Ferrum redactum, reduced iron, a powder containing at least 75% of metallic iron and a variable amount of oxide. A preparation of it is Trochiscus ferri redacti (strength, 1 grain of reduced iron in each).
3. Ferri sulphas, ferrous sulphate, from which is prepared Mistura ferri composita, “Griffiths’ mixture,” containing ferrous sulphate 25 gr., potassium carbonate 30 gr., myrrh 60 gr., sugar 60 gr., spirit of nutmeg 50 m., rose water 10 fl. oz.
4. Ferri sulphas exsiccatus, which has two subpreparations: (a) Pilula ferri, “Blaud’s pill” (exsiccated ferrous sulphate 150, exsiccated sodium carbonate 95, gum acacia 50, tragacanth 15, glycerin 10, syrup 150, water 20, each to contain about 1 grain of ferrous carbonate); (b) Pilula aloes et ferri (Barbadoes aloes 2, exsiccated ferrous sulphate 1, compound powder of cinnamon 3, syrup of glucose 3).
5. Ferri carbonas saccharatus, saccharated iron carbonate. The carbonate forms about one-third and is mixed with sugar into a greyish powder.
6. Ferri arsenas, iron arsenate, ferrous and ferric arsenates with some iron oxides, a greenish powder.
7. Ferri phosphas, a slate-blue powder of ferrous and ferric phosphates with some oxide. Its preparations are: (a) Syrupus ferri phosphatis (strength, 1 gr. of ferrous phosphate in each fluid drachm); (b) Syrupus ferri phosphatis cum quinina et strychnina, “Easton’s syrup” (iron wire 75 grs., concentrated phosphoric acid 10 fl. dr., powdered strychnine 5 gr., quinine sulphate 130 gr., syrup 14 fl. oz., water to make 20 fl. oz.), in which each fluid drachm represents 1 gr. of ferrous phosphate, 45 gr. of quinine sulphate, and 132 gr. of strychnine.
8. Syrupus ferri iodidi, iron wire, iodine, water and syrup (strength, 5.5 gr. of ferrous iodide in one fl. dr.).
9. Liquor ferri perchloridi fortis, strong solution of ferric chloride (strength, 22.5% of iron); its preparations only are prescribed, viz. Liquor ferri perchloridi and Tinctura ferri perchloridi.
10. Liquor ferri persulphatis, solution of ferric sulphate.
11. Liquor ferri pernitratus, solution of ferric nitrate (strength, 3.3% of iron).
12. Liquor ferri acetatis, solution of ferric acetate.
13. The scale preparations of iron, so called because they are dried to form scales, are three in number, the base of all being ferric hydrate:
