COMPOUNDS OF IRON Oxides and Hydroxides.—Iron forms three oxides : ferrous oxide, FeO; ferric oxide, and ferroso-ferric oxide, The first two give rise to well-defined series of salts, the ferrous salts, wherein the metal is bivalent, and the ferric salts wherein the metal is trivalent ; the former readily pass into the latter on oxidation, and the latter into the former on reduction.
Ferrous oxide is obtained as a black pyrophoric powder when ferric oxide is reduced in hydrogen at 300°. Sabatier and Sender ens obtained it by acting with nitrous oxide on metallic iron at 200°, and Tissandier by heating the metal to 900° in car bon dioxide; Donau, on the other hand, obtained a magnetic and crystalline ferroso-ferric oxide at 1200°. It may also be prepared as a black velvety powder which readily takes up oxygen from the air by adding ferrous oxalate to boiling caustic potash. Ferrous hydrate, Fe(OH),, when prepared from a pure ferrous salt and caustic soda or potash free from air, is a white powder which may be preserved in an atmosphere of hydrogen. Usually, how ever, it forms a greenish mass, owing to partial oxidation. It oxidizes on exposure with considerable evolution of heat ; it rapidly absorbs carbon dioxide, and readily dissolves in acids to form fer rous salts, which are usually white when anhydrous, but greenish when hydrated.
Ferric oxide or iron sesquioxide, constitutes the valuable ores red haematite and specular iron; the minerals brown haema tite or limonite, and gothite and also iron rust are hydrated forms.
It is obtained as a steel-grey crystalline powder by igniting any ferric salt containing a volatile acid or by passing ferric chloride vapour over heated lime. When finely ground it forms a brownish red powder which dissolves slowly in acids, the most effective solvent being a boiling mixture of eight parts of sulphuric acid and three of water. Ferric oxide is employed as a pigment, as jeweller's rouge and for polishing metals. Several ill-defined hydrates are known, corresponding to various minerals in compo sition, and "colloidal" solutions have also been prepared. (See COLLOIDS.) Thomas Graham obtained a soluble hydrate by dis solving the freshly prepared hydrate in ferric chloride and dialys ing the solution, the soluble hydrate being left in the dialyser. All the chlorine, however, does not appear to be removed by this process, the residue having the composition but it may be by electrolysing in a porous cell. On standing, the solu tion usually gelatinizes, a process accelerated by the addition of an electrolyte. It is employed in medicine under the name Liquor ferri dialysati, or "dialysed iron." Red ferric hydroxide dissolves in acids to form a well-defined series of salts, the ferric salts, also obtained by oxidizing ferrous salts; they are usually colourless when anhydrous, but yellow or brown when hydrated. It has also feebly acidic properties, forming ferrites with strong bases.
Magnetite, is regarded as ferrous ferrite, Most "mill scale," i.e., the oxidized surface formed on red hot iron and steel, is This important ore of iron is most celebrated for its magnetic properties, but the mineral is not al ways magnetic, although invariably attracted by a magnet. It may be obtained artificially by passing steam over red-hot iron. It dissolves in acids to form a mixture of a ferrous and ferric salt, and if an alkali is added to the solution a black precipitate is obtained which dries •to a dark brown mass of the composition this substance is attracted by a magnet, and thus may be separated from the admixed ferric oxide. Calcium, mag nesium and zinc ferrites, Mg, Zn), are obtained by intensely heating mixtures of the oxides ; magnesium ferrite occurs in nature as the mineral magnoferrite, and zinc ferrite as franklinite, both forming black octahedra.
Ferric Acid, H2Fe04.—By fusing iron with saltpetre and ex tracting the melt with water, or by adding a solution of ferric nitrate in nitric acid to strong potash, an amethyst or purple-red solution is obtained which contains potassium ferrate. The same solution results when chlorine is passed into strong potash solu tion containing ferric hydrate in suspension. Haber and Pick pre pared potassium ferrate by electrolysing concentrated potash so lution at 7o degrees Centigrade, between two cast iron electrodes, the current being reversed every few minutes. When concentrated, the solution is nearly black, and on heating it yields a yellow solution of potassium ferrite, oxygen being evolved. Barium fer rate, obtained as a dark red powder by adding barium chloride to a solution of potassium ferrate, is fairly stable. It dissolves in acetic acid to form a red solution, is not decomposed by cold sulphuric acid, but with hydrochloric or nitric acid it yields barium and ferric salts, with evolution of chlorine or oxygen.
Ferric chloride, may be obtained anhydrous by the action of dry chlorine on the metal at a moderate red-heat, or by passing hydrochloric acid gas over heated ferric oxide. It forms iron black plates or tablets which appear red by transmitted and a metallic green by reflected light. It is very deliquescent, and readily dissolves in water, forming a brown or yellow solution, from which several hydrates may be separated. The solution is best prepared by dissolving the hydroxide in hydrochloric acid and removing the excess of acid by evaporation, or by passing chlo rine into the solution obtained by dissolving the metal in hydro chloric acid and removing the excess of chlorine by a current of carbon dioxide. It also dissolves in alcohol and ether; boiling point determinations of the molecular weight in these solutions point to the formula Vapour density determinations at indicate a partial dissociation of the double molecule on stronger heating it splits into ferrous chloride and chlorine. It forms red crystalline double salts with the chlorides of the metals of the alkalis and of the magnesium group. An aqueous solution of ferric chloride is used in pharmacy under the name Liquor ferri perchloridi.
Ferrous bromide, is obtained as yellowish crystals by the union of bromine and iron at a dull red-heat, or as bluish-green rhombic tables of the composition by crystallizing a solution of iron in hydrobromic acid. Ferric bromide, is obtained as dark red crystals by heating iron in an excess of bromine vapour. It closely resembles the chloride in being del iquescent, dissolving ferric hydrate and in yielding basic salts. Ferrous iodide, is obtained as a grey crystalline mass by the direct union of its components. Ferric iodide does not appear to exist.
Ferric-sulphide, is obtained by gently heating a mixture of its constituent elements, or by the action of sulphuretted hy drogen on ferric oxide at temperatures below io0°. It is also prepared by precipitating a ferric salt with ammonium sulphide ; unless the alkali be in excess a mixture of ferrous sulphide and sulphur is obtained. Magnetic pyrites or pyrrhotite has a com position varying between and It has a somewhat brassy colour, and occurs massive or as hexagonal plates; it is attracted by a magnet and is sometimes itself mag netic. The mineral is abundant in Canada, where the presence of about 5% of nickel makes it a valuable ore of this metal. Iron disulphide, constitutes the minerals pyrite, which is im portant as a source of sulphuric acid, and marcasite (q.v.) ; cop per pyrites is (Cu, Pyrite may be prepared artificially by gently heating ferrous sulphide with sulphur, or as brassy octa hedra and cubes by slowly heating an intimate mixture of ferric oxide, sulphur and sal-ammoniac. It is insoluble in dilute acids, but dissolves in nitric acid with separation of sulphur.
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 these, or by adding strong sulphuric acid to a solution. It is sparingly soluble in water, and on heating it yields ferric oxide and sulphur dioxide. Many basic ferric sulphates are known, some of which occur as minerals. The iron alums are obtained by crystallizing solutions of equivalent quantities of ferric and an alkaline sulphate. Ferric potassium sulphate, common iron alum, K2SO4.Fe2(SO4)3.24H20, forms bright violet octahedra.
tained a nitride by acting upon anhydrous ferrous chloride or bromide, finely divided reduced iron, or iron amalgam with am monia at 420° ; and also, in a compact form, by the action of am monia 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, 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, 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 or monoclinic crystals of 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 un stable compounds of the ferrous salt and nitric oxide, probably of the type the reaction, however, is reversible, the composition varying with temperature, concentration and nature of the salt. The reaction is utilized in the "brown ring" test for nitrates. Reddish brown amorphous powders of the formulae and are obtained by passing the gas over an hydrous 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 are obtained, which at 6o° form the yellow Complicated compounds discov ered by Roussin (Ann. Chim. Phys., 1858, 52, p. 285) are obtained by the interaction of ferrous sulphate and alkaline nitrates and sul phides. Two classes may be distinguished :—( ) the ferrodinitroso salts; e.g., potassium ferrodinitrososulphide, and (2) the ferroheptanitroso salts ; e.g., K potassium ferroheptanitrososulphide. These salts yield the corresponding acids with sulphuric acid. For further information the above paper by Roussin, and also one by Pavel (Ber., 1882, 15, p. 2600), should be consulted.
Ferrous carbonate, FeCO,, or spathic iron ore, may be obtained as microscopic rhombohedra by adding sodium bicarbonate to ferrous sulphate and heating to 15o° for 36 hours. Ferrous sul phate and sodium carbonate in the cold give a flocculent precipi tate, 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),, obtained by L. Mond, Quincke and Langer, by treating iron from ferrous oxalate with carbon monoxide, and heating at 5o°, is a pale yellow liquid which freezes at about —2o°, and boils at 102.5°. Air and moisture decompose it. The halogens give ferrous and ferric halides and carbon monoxide ; hydrochloric and hydrobromic acids have no action, but hydriodic decomposes it. On exposure to sunlight, either alone or dissolved in ether or ligroin, it gives lustrous orange plates of diferrononacarbonyl, If this substance be heated in ethereal solution to 5o°, it deposits lustrous dark green tablets of ferrotetracarbonyl, very stable at ordinary temperatures, but decomposing at 14e-15o° into iron and carbon monoxide. 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 blood-red coloration with a thiocyanate. Iron is estimated either by reduction to the ferrous condition and titration by potassium permanganate or dichromate, or by titration in the ferric con dition by means of titanous chloride. (See also CHEMISTRY : Analytical.) A. Werner prepared salts of ferrous tri-dipyridyl, which were resolved into dextro- and laevo rotatory forms (see STEREOCHEMISTRY), and W. Thomas sim ilarly resolved potassium ferrioxalate, (C204)3].
All the official salts and preparations of iron are made directly or indirectly from the metal. The pharmacopoeia) forms of iron are as follow:— Ferri sulphas exsiccatus, which has two subpreparations: (a) Pilula ferri, "Blaud's pill," (b) Pilula aloes et ferri.
Ferri phosphas, a slate-blue powder of ferrous and ferric phos phates with some oxide. Its preparations are : (a) Syrupus ferri phosphatis ; (b) Syrupus ferri phosphatis cum quinine et strych nina, "Easton's syrup," in which each fluid drachm represents igr. of ferrous phosphate, /gr. of quinine sulphate, and -igr. of strychnine.
Syrupus ferri iodidi (strength 5.5gr. of ferrous iodide in one fl.dr.).
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.
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 : (a) Ferrum tartaratum, dark red scales, soluble in water.
(b) Ferri et quininae citratis, greenish yellow scales soluble in water.
(c) Ferri et ammonii citratis, red scales soluble in water, from which is prepared Vinum ferri citratis (ferri et ammonii citratis igr., orange wine I fl.dr.).
Substances containing tannic or gallic acid turn black when compounded with a ferric salt, so it cannot be used in combination with vegetable astringents except with the infusion of quassia or calumba. Iron may, however, be prescribed in combination with digitalis by the addition of dilute phosphoric acid. Alkalis and their carbonates, lime water, carbonate of calcium, magnesia and its carbonate give green precipitates with ferrous and brown with ferric salts.
There are numerous organic preparations of iron. Ferratin is a reddish brown substance which claims to be identical with the iron substance found in pig's liver. Carniferrin is a tasteless powder containing iron in combination with the phosphocarnic acid of muscle preparations, and contains 35% of iron. Ferra togen is prepared from ferric nuclein. Triferrin is a paranucleinate of iron, and contains 22% of iron and 21% of organically com bined phosphorus, prepared from the casein of cow's milk.
Haemoglobin is extracted from the blood of an ox and may be administered in bolus form. Dieterich's solution of peptonated iron contains about 2gr. of iron per oz. Vachetta used the albu minate of iron with striking success in grave cases of anaemia.
Succinate of iron has been prepared by Hausmann. Haematogen, introduced by Hommel, claims to contain the albuminous con stituents of the blood serum and all the blood salts as well as pure haemoglobin. Sicco, the name given to dry haematogen, is a tasteless powder. Haemalbumen, introduced by Dahmen, is soluble in warm water.
Internally, a large proportion of ordinary diet contains iron.
When given medicinally preparations of iron have an astringent taste, and the teeth and tongue are blackened owing to the forma tion of sulphide of iron. It is therefore advisable to take liquid iron preparations through a glass tube or a quill.
In the stomach all salts of iron are converted into ferric chloride. If given in excess, or if the hydrochloric acid in the gastric juice be deficient, iron acts directly as an astringent upon the mucous membrane of the stomach wall. Acid preparations are more likely to do this, and the acid set free after the formation of the chloride may act as an irritant. Iron, therefore, must not be given to subjects in whom the gastric functions are disturbed, and it should always be given after meals. Preparations which are not acid, or are only slightly acid, such as reduced iron, dialysed iron, the carbonate and scale preparations, do not disturb the digestion. In the intestine the ferric chloride becomes changed into an oxide of iron; the sub-chloride is converted into a ferrous carbonate, which is soluble. Lower in the bowel these are con verted into ferrous sulphide and tannate, and are eliminated with the faeces, turning them black. Iron in the intestine is astringent or constipating and is therefore useful occasionally to check diarrhoea and dysentery. Salts of iron being constipating, they are best used in combination with a purgative. The pill of iron and aloes (B.P.) is designed for this purpose. Iron is certainly absorbed from the intestinal canal. As the iron in the food sup plies all the iron in the body of a healthy person, there is no doubt that it is absorbed in the organic form. Whether inorganic salts are directly absorbed has been a matter of much discussion; it has, however, been directly proved by the experiments of Kunkel (Archiv fiir die gesarnte Pliysiologie des Menschen and der Tiere, lxi.) and Gaule. The amount of iron in the human blood is only 38gr. ; therefore, when an excess of iron is absorbed, part is ex creted immediately by the bowel and kidneys, and part is stored in the liver and spleen.
Iron is prescribed when the amount of haemoglobin in the blood is lowered or the red corpuscles are diminished. In certain farms of anaemia its administration rapidly improves the blood in both respects, though the exact method in which it acts is a matter of dispute. In cases with toxaemia from absorption of intestinal products, not only laxatives in combination with iron but intes tinal antiseptics are necessary. That form of neuralgia which is associated with anaemia usually yields to iron.