Tiff; OXIDES OF IRON. With oxygen, iron forms three distinct compounds: Ferrous oxide, Fe(); ferric oxide, Fe,0,; and ferric anhydride, Felt, When oxidized at high tempera tures. iron yields the substance but this is really a clinbination of ferrous and ferric oxides. Perron.: o.ride, or rather ferrous hydroxide, Fe (Off may he obtained by adding caustic soda to a solution of green vitriol (ferrous sulphate). It is a white compound readily absorbing oxygen from the air. even if kept under water, and thus changing into ferric oxide: the oxidation causes its color to change gradually from white to green. gray, and brown. It is but spar ingly soluble in water. the solution having an alkaline reaction. if boiled with a solution of caustic potash. ferrous hydroxide attacks the water of the solution, setting free its hydrogen and combining with it: oxygen to form ferric hydrox ide. 'Toe oxide, in its anhydrous form. is found extensively as hematite. it is prepared arti ficially, by heating green vitriol, for use as an oil 11111111„ for wood, being known as rofrothor. Ordinary rouge, which is used for polishing glass and metals, is artificial ferric oxide refined to 41 tine powder. Ferric oxide may be obtained in the form of crystals having a dark-violet color. by heating green vitriol with common salt. If strongly heated, ferric oxide loses its property of readily dissolving in acids. and can then be dis solved only in strong acids 111111 only at a high temperature. If heated to a white heat, it loses part of its oxygen and becomes converted into NN 111(1 CX11111its marked magnetic proper ties. The hydrate of ferric oxide, as ordinarily obtained by adding alkalies to solutions of ferric salts. has the composition •Fe,11,.:311,(1, and is noddy soluble in acids. Another hydrate of fer ric oxide has the eomposition and, like the anhydrous oxide, sloes not, if heated. dissolve acids. By exactly netitral• icing a solution of ferric chloride with alkali. and subjecting the resulting liquid Loa process of hydrolysis, pure ferric. oxide may be obtained in aqueous solutions. But, like oiler colloidal so lutions (see Col.i.olus that of ferric oxide is unstable. and the oxide readily passes from the soluble to the insoluble form. The hydrated forms of the oxide may be emiverted into the ordinary anhydrous form by the application of a moderate heat ; at a certain point of this process, the substance suddenly be•onn•s incandeseent- showing that a peculiar moles-1118• change is tak ing place in it—and after that it is found to have lost its property of readily dying in acids. The exact nature of the change is unknown. F(rrie anhydride is unknown in the free state. It is the hypothetical anhydride of ferric acid. 11,,,Fe0,. which is likewise unknown in the free state, but certain of whose salts may he readily prepared. Thu:. potassium ferrate, K,Fe0,. may be obtained by heating small pieces of iron with the chlorite of potassium. From the existence of such salts it is Glvident that, much unlike fer rous oxide, Which is distinctly alkaline in re action, the peroxide of iron acts as a feeble acid. The tendency of peroxidiz•d iron to pass into the stable ferric state is even greater than the tendency of ferrous iron to pass into that state, and hence the ferrates act, as powerful oxidizers. readily burning such substances as oxalic arid, readily changing manganous oxide into man ganese dioxide, etc.
Turf SALTS OF IRON. Corresponding to ferrous oxide and ferric oxide, respectively, are two of iron salts, ferrous salts and ferric salts.
The action of acids on metallic iron, in the abserr.e of oxidizing agents. causes the forma tion of ferrous salts. among which deserve men tion the sulphate, the sulphide, the chloride, and the oxalate. Ferrous sulphate, known as green vitriol, or iron vitriol, is the substance from which the compounds of iron are generally pre pared. The sulphate itself is obtained as a by product in certain industrial processes. and may be prepared by the action of sulphuric acid on metallic iron. In its ordinary, hydrated form its composition corresponds to the formula FeS0,711f). It has a greenish color that is scarcely perceptible when the salt is dissolved in water; the solution readily takes up oxygen, which causes the formation of ferric sulphate, and hence must be kept in sealed vessels out of contact with air, if it is to be preserved un changed. Ferrous sulphate is used for a variety of purposes in the arts; it is employed in making fuming sulphuric acid, in dyeing, as a disin fectant, in making coleotImr and rouge. etc. Ferrous sulphide, FeS, which has been found in many meteoric stones, may be made by heating iron filings with flowers of sulphur. It is large ly used in chemical laboratories for the prepara tion of sulphureted hydrogen, which it yields on coming into contact with dilute sulphuric or hydrochloric acid. Ferrous chloride. or rather its hydrated form, may be prepared by the action of hydrochloric acid on metallic iron. The crystalline anhydrous chloride, FeC1,,, may be prepared by the action of gaseous hydrochloric acid on red-hot iron. Ferrous ox alate, which is a powerful reducing agent, is used as a developer in photography, potassium-ferrous oxalate being used for the same purpose. An other important compound containing iron in the ferrous state is the well-known potassium ferroeyanide, or yellow prussiate of potash, which may be found described under HYDROFERROCYANIC ACID.
Among the ferric salts deserve mention the chloride, the sulphide. the nitrate, and the phos phate. Ferric chloride, is a volatile and extremely hygroscopic salt prepared by the ac tion of chlorine upon red-hot metallic iron. Its solutions in water have a brown color which may possibly be due not to the ferric chloride itself, but to the formation of basic chlorides of iron, which are eventually precipitated out of solutions of ferric salts. Commercial ferric chloride contains a considerable percentage of water, and hence contains basic chlorides, prob ably some free ferric hydroxide, etc. It is pre pared by dissolving ordinary ferric hydroxide in hydrochloric acid. Ferric- sulphide, oc curs in nature abundantly as iron pyrite; it is used for the preparation of sulphurous anhydride in manufacturing, sulphuric acid and in bleach ing. Ferric nitrate, is obtained by dissolving metallic iron in an excess of cold nitric acid and allowing the solution to evaporate in a vacuum; the crystals thus obtained cor respond to the formula and melt at 35° C. In aqueous solution, the nitrate gradu ally decomposes unless an excess of free nitric acid is present. Ferric phosphate, is an insoluble white substance formed when acid sodium phosphate is added to solutions of ferric acetate. Another compound containing iron in the ferric state. viz. potassium ferricyanide, may be found described under HYDROFERRICYANIC ACID.