602

PROTOXIDE AND SESQUIOXIDE OF IRON.

The mode of preparing zinc-plate, or galvanized iron has been already described (707). Tin-plate is prepared by an analogous process; it consists of iron superficially alloyed with tin.

(756) OXIDES OF IRON.-Iron yields four definite compounds with oxygen: 1. The protoxide (FeO), which is the base of the green, or ferrous salts of iron: 2. The sesquioxide (Fe‚Ð ̧), which is the base of the red, or ferric salts: 3. The black, or magnetic oxide (Fe), which may be viewed as a compound of the two preceding oxides, or (FeO, Fe,0); it does not form any definite salts: 4. Ferric acid, which is a weak and unstable metallic acid, and as such it reacts with the alkalies, forming salts like ferrate of potassium K,F ̧з

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Protoxide of Iron; Ferrous Oxide (Fe✪=72, or FeO=36): Composition in 100 parts, Fe, 77'78; →, 22.22.—It is obtained in the form of a white hydrate by dissolving a pure ferrous salt in water recently boiled, and precipitating by an alkali the solution of which has been similarly treated, both being allowed to cool out of contact with air, and being mixed in vessels from which air is excluded. If this precipitate be boiled in a vessel from which oxygen is excluded, it loses its water of hydration, like the oxide of copper under similar circumstances. The hydrated oxide absorbs oxygen greedily from the air, passing through various shades of light green, bluish green, and black, till finally it assumes an ochry hue, due to the formation of the hydrated sesquioxide. It is insoluble in water, but is somewhat soluble in ammonia; this solution quickly absorbs oxygen from the air, and a film of insoluble sesquioxide of iron is formed upon its surface. The protoxide is readily dissolved by acids, and forms with them salts which are known as the ferrous salts or protosalts of iron; they have a green colour, and an astringent, inky taste. The solutions of these salts, when exposed to the air, absorb oxygen, and are decomposed; in which case ferric salts are formed, one portion of which is retained in solution, whilst a basic ferric salt falls as a rusty insoluble precipitate. For example, in the case of the green sulphate of iron, the change may be represented as follows:

Ferrous sulphate.

Soluble ferric
sulphate.

Insoluble ferric
sulphate.

20 FeSO4 + 5 →1⁄2 + 6 H20 = 6 [(Fe), 3 §0] + 2 (2 Fe’"¿Ð삧¤3, 3 H2O).

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(757) Peroxide, Red Oxide, or Sesquioxide of Iron; Ferric Oxide (Fe""=160, or Fe2O,=80): Comp. in 100 parts, Fe, 70 ; →, 30.-The anhydrous sesquioxide is obtained for the arts by igniting the ferrous sulphate (767), and is known under the names.

SESQUIOXIDE OF IRON.

603

of colcothar, crocus of Mars, or rouge, according to the degree of levigation to which it has been submitted; it is extensively employed, amongst other uses, for polishing glass, and by jewellers for putting a finish to their goods. It is also employed as a red pigment.

The sesquioxide occurs native in great abundance: several of its varieties have been already mentioned as among the most valuable ores of iron. The specular ore of Elba (sp. gr. 5'22) often presents natural facets of the most perfect polish, and of remarkable size and lustre. It occurs crystallized in forms of the rhombohedral system, and is isomorphous with alumina in corundum. Red hæmatite, or bloodstone (sp. gr. from 4.8 to 5'0), another of its varieties, is extremely hard, and, when polished, is employed for burnishing gilt trinkets.

Hydrates of Sesquioxide of Iron.-There are several of these. Brown hæmatite is the hydrate (2 Fe,,, 3 H2O) Sp. Gr. 3'98. This mineral is readily dissolved by acids. It contains 59.89 per cent. of iron, with 25.67 of oxygen, and 14'44 of water. Another native hydrate göthite (H2O,Fе¿Ð ̧; sp. gr. 412 to 4'37) has been found crystallized in prisms. Brown hæmatite gives the red and yellow colour to the different varieties of clay.

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It

The sesquioxide is best obtained in a state of purity, by precipitating the sesquichloride of iron by ammonia in excess. falls as a bulky light-brown flocculent hydrate, which shrinks remarkably as it dries: if precipitated in a cold solution, and dried without heat over sulphuric acid, it contains 2 Fe‚Ð ̧, 3 H2O (St. Gilles), but it is apt to retain a little ammonia, which is easily expelled by heat. The same hydrate is also formed when moist iron is allowed to become oxidized by exposure to air. If the hydrate be not dried, but allowed to remain for some months under water, it becomes crystalline, and, according to Wittstein, is converted into an allotropic hydrate (2 Fe,,, 3 H2O?), but if dried at 212° it retains 10'11 per cent. of water, corresponding in composition to (Fe,,,H,). Hydrated peroxide of iron slowly parts with its water at a prolonged heat of 600°, and if subsequently heated to dull redness, it suddenly contracts in bulk, and glows brightly for a few moments whilst undergoing molecular change; after this it is dissolved by acids with difficulty, but is readily attacked by a solution of ferrous chloride in hydrochloric acid at a very high temperature the sesquioxide loses one-ninth of its oxygen, and is converted into the magnetic oxide of iron.

Hydrated sesquioxide of iron, when recently precipitated from cold solutions, is easily soluble in acids, forming the persalts of

604

HYDRATED SESQUIOXIDE OF IRON.

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iron, or ferric salts; they have a strongly acid reaction, and do not crystallize: many of them are deliquescent. Their concentrated solutions have the property of dissolving a considerable excess of the oxide, in which case they assume a deep red colour. If these basic solutions be diluted and boiled, the iron is entirely separated in the form of an insoluble ferric subsalt. If well washed and freshly precipitated hydrated peroxide of iron, obtained by the action of ammonia upon the sesquichloride in the cold, be boiled in water for a few minutes it becomes converted into the hydrate, Fe,,, H2O; but if the ebullition be continued for 8 or 10 hours, its colour becomes changed from ochry brown to brick red, and it is converted into an allotropic modification of the same hydrate, and by prolonged boiling a portion of it even loses all its water. This modified oxide is insoluble in strong boiling nitric acid, and only slowly soluble in hot hydrochloric acid. Cold acetic acid, and cold diluted hydrochloric and nitric acids dissolve it, forming a red liquid which appears to be turbid by reflected light; concentrated nitric or hydrochloric acid occasions a red precipitate in this solution, but it becomes redissolved on the addition of water. The solution is also precipitated by the addition of any sulphate, or of any salts of the alkali-metals. If the ordinary hydrated sesquioxide of iron be kept long in water, especially if at the same time it be exposed to a low temperature, it experiences a similar modification in composition and properties. An acetic solution of this oxide, if kept for some time in a closed vessel at 212°, becomes of a brighter red colour. It appears to be turbid when viewed by reflected light, but clear by transmitted light. It has lost its astringent metallic taste. The addition of a soluble sulphate causes an immediate precipitate, and so do the strong acids: it is no longer reddened by the addition of a sulphocyanide, and does not give a precipitate of Prussian blue with ferrocyanide of potassium (Péan de St. Gilles, Ann. de Chimie, III. xlvi. 47). Graham found that if a solution of ferric chloride, in which a large quantity of hydrated ferric oxide had been dissolved by prolonged digestion in the cold, were submitted to dialysis, a solution was eventually obtained which contained a proportion of 98.5 of the oxide and 15 of hydrochloric acid. This solution, however, in a few weeks became gelatinous spontaneously in the bottle to which it was transferred.

Hydrated sesquioxide of iron is now used to some extent for the purpose of purifying coal-gas from sulphuretted hydrogen, which is always produced during the distillation of coal. For this purpose the oxide is mixed with sawdust, and placed, in layers of

BLACK OR MAGNETIC OXIDE OF IRON.

605

10 or 12 inches in thickness, upon the perforated shelves of a dry lime purifier: hydrated sesquisulphide of iron and water are formed; Fe, ≈ H2Ð+ 3 H2S=Fe2S ̧, x H2O+ 3 H2Ð. After the mixture has ceased to absorb any more sulphuretted hydrogen, it is oxidized by exposure to a current of air; hydrated sesquioxide of iron is thus reproduced, and sulphur is set free; 2 Fe2S3, x H ̧Ð+ 3 →2= 2 Fe‚Ð ̧, x H2O+ 3 S2. The mixture may again be used for the same purpose as at first, and this process may be repeated several times in succession, until the accumulation of sulphur mechanically impairs the absorbent powers of the mixture. Considerable elevation of temperature attends the act of reoxidation, which must therefore be prevented from taking place with too much rapidity.

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Sesquioxide of iron combines with some of the more powerful bases, towards which it acts the part of a feeble acid. The compounds which it forms by heating it with the hydrates of potash and soda are easily decomposed by water, but the oxide retains traces of these bases with great obstinacy. According to Pelouze, when 4 atoms of lime and 1 atom of peroxide of iron are precipitated together and boiled, they unite and form a white compound (4 ЄaÐ,Fe ̧Ð ̧) which is readily decomposed by the feeblest acids. Sesquioxide of iron occurs native combined with oxide of zinc in crystals, mixed with oxide of manganese, constituting Franklinite. With protoxide of iron it forms the black or magnetic oxide of iron.

(758) Black or Magnetic Oxide of Iron (Fe); Sp. Gr. 5'09: Composition in 100 parts, Fe, 72°41; →, 27'59.—This oxide occurs as a well-known mineral, the loadstone, which acquires its magnetism from the inductive influence of the earth. It is found in primitive rocks, forming beds, or sometimes, as in Sweden, entire mountains. It furnishes a very pure and excellent iron, of which a large quantity is annually supplied from the Swedish and American mines. It has a black colour and metallic lustre; it crystallizes in cubes, octohedra, or rhombic dodecahedra. Magnetic oxide of iron is the principal constituent of the scales of oxide which are detached during the forging of wrought iron. It fuses at a high temperature, and is formed when iron is burned in oxygen,-the sesquioxide, which is the result of the combination, losing part of its oxygen, owing to the intensity of the heat developed during the combustion. The magnetic oxide is also formed by passing steam over heated iron turnings. A hydrate of this oxide (Fe,,, H2O) may be procured by dividing a freshly prepared solution of ferrous sulphate into three equal portions: two of these are acidulated with sulphuric acid and heated to the boiling-point; to the boiling

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liquid nitric acid is added gradually so long as its addition causes the evolution of nitric oxide: when this point is reached, the whole of the ferrous salt will have been converted into a ferric salt; 6 Fe"SO+3 H2SO4 + 2 HNO ̧ = 3 (Fe"1⁄2 3 SO1) + 2 NO + 4 H2O: the remaining portion of the solution of the ferrous sulphate is then poured into the hot liquid, and carbonate of sodium or caustic ammonia is added in slight excess: the solution and precipitate are boiled together, and the black oxide is formed as a heavy crystalline powder. The magnetic oxide is soluble without difficulty in hydrochloric acid, as well as in nitric acid and in aqua regia this oxide, however, does not form specific salts, but mixtures of ferrous and ferric salts; Fe+ 4 H2SO ̧=Fe” SO +(Fe'”)2 3 SO1+ 4 H2Ð.

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If recently precipitated hydrated sesquioxide of iron, obtained from the sesquichloride by ammonia, be well washed, and without being dried be boiled with water and iron turnings in large excess, hydrogen is evolved, and magnetic oxide of iron formed.

=

=

(759) Ferric Acid (H,Fe, 122, or HO,FeO 9 + 52).— If a mixture of 1 part of sesquioxide of iron and 4 parts of nitre be heated to full redness for some time, a brown mass is obtained, which with water gives a beautiful violet-coloured solution, due to the presence of ferrate of potassium. In this compound the iron is combined with a larger quantity of oxygen than in the sesquioxide, but the ferric anhydride has not been obtained in an isolated form. Ferrate of potassium may be more easily procured by suspending 1 part of recently precipitated hydrated sesquioxide of iron in a concentrated solution of potash, consisting of 30 parts of hydrate of potash and 50 of water, and then transmitting a current of chlorine gas: the ferrate of potassium is insoluble in a concentrated solution of potash, and is deposited as a black powder, which may be drained upon a tile (Fremy). This compound is very soluble in water, but is precipitated in black flocculi by a large excess of alkali. It is a very unstable salt: in dilute solutions the alkali becomes free, hydrated sesquioxide of iron subsides, and oxygen escapes. Organic matter decomposes it speedily, just as it does the permanganate of potassium: a temperature of 212° destroys it instantly if in solution, and the addition of an acid, even in quantity insufficient to neutralize the whole of the alkali, causes the immediate separation of oxygen, and precipitation of the sesquioxide of iron.

Ferrates of barium, strontium, and calcium may be obtained in the form of red insoluble precipitates, by admixture of solutions of the salts of the earths with a solution of the ferrate of potassium.