612

PROTOSULPHATE OF IRON.

tals on cooling, with 7 H2O. They effloresce in a dry air, and form a white crust, which soon becomes of a rusty-brown colour, owing to the absorption of oxygen and formation of a basic ferric sulphate. If crystallized at a temperature of 176° the ferrous sulphate forms right rhombic prisms, which contain only 4 H ̧. It may also be obtained crystallized with 3 and with 2 atoms of water. For commercial purposes sulphate of iron is formed by the decomposition of iron pyrites, or of aluminous schists containing pyrites, as already described when speaking of the manufacture of alum (667). The sulphate of iron thus obtained has a grass-green colour, owing to the presence of ferric sulphate. Sulphate of iron is largely used in combination with astringent vegetable matters as a black dye; ordinary writing-ink is a compound of this kind.

Owing

This salt is insoluble in alcohol, and requires twice its weight of cold water for its solution. Its solubility is greater at 194° than at 2120, 100 parts of water dissolving 370 parts of the crys tals at 194° and only 333 at the boiling-point. This anomaly is probably dependent upon causes similar to those observed in the case of the sulphate and the carbonate of sodium. If exposed to the air, the solution absorbs oxygen, and a rusty precipitate occurs, which is a basic ferric sulphate composed of 2 Fc,,SO ̧ . 3 H2, while normal ferric sulphate remains in solution. to its strong attraction for oxygen, ferrous sulphate is occasionally used as a reducing agent: it is thus employed to precipitate gold and palladium in the metallic form from their solutions, and indigo is by its means brought into the soluble condition. If heated gradually, each atom of the crystallized sulphate loses 6 atoms of water and forms a white powder; I atom of water being retained at all temperatures below 500°. At a red heat the sulphate is decomposed; sulphuric anhydride is liberated, but one portion of the anhydride yields part of its oxygen to the iron, which is converted into the sesquioxide, whilst sulphurous anhydride escapes (2 FeSO=Fe2O3+SO2+SO3); but as in practice the salt cannot be rendered anhydrous in large quantities, a little water distils with the sulphuric anhydride, which is condensed as a brown fuming liquid, the Nordhausen Sulphuric Acid' (415). The residual sesquioxide of iron is sold under the name of colcothar (757).

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The aqueous solution of ferrous sulphate in common with that of all the ferrous salts, absorbs a large quantity of nitric oxide; forming a deep brown solution which has a powerful attraction for oxygen: if this solution be heated in closed vessels, the gas is for

PERSULPHATE OF IRON.

613

the most part expelled unchanged; if heated in air, nitric acid is formed in the liquid, and this converts the iron into a ferric salt.

With the sulphates of potassium and ammonium, green vitriol yields double salts precisely analogous in form and composition to those which are formed by these sulphates with sulphate of copper. The formula of the potassium salt is (FeSOK2SO4. 6 H ̧Ð).

(768) Ferric Sulphate; Persulphate or Sesquisulphate of Iron (Fe""23S0=400, or Fe2O3, 3 SO3=200) is made either by treating brown hæmatite with an excess of strong sulphuric acid, allowing it to digest for some time, and then expelling the excess of acid at a heat short of redness; or by adding to the solution of 1 equivalent of ferrous sulphate, half an equivalent of oil of vitriol, boiling, and peroxidizing the iron by adding to the solution nitric acid in small quantities as long as any red fumes are given off. A yellowish-white deliquescent mass is obtained on evaporation, from which the acid is expelled by a red heat; at a more moderate heat the salt is rendered anhydrous: water dissolves it but slowly. It is found native in large quantities in Chili in the form of a white powder (Fe""2 3 SO4.9 H2O). Several hydrated basic ferric sulphates may be obtained.

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With sulphate of potassium and the sulphates of the other alkali-metals ferric sulphate forms double salts, resembling common alum, in form and composition as well as in taste. The potassium salt (KFe'” 2 SO ̧ . 12 H2O) is astringent, very soluble in water, but insoluble in alcohol: it is very prone to spontaneous decomposition, and becomes converted from a colourless transparent crystal into a brown, gummy, deliquescent mass: this change is also produced by heating the salt to a temperature below 212°. The mixed solutions of the two sulphates should therefore be allowed to evaporate spontaneously during the preparation of this salt. The double salt with ammonium (H ̧NFe'” 2 SO4. 12 H2‡, sp. gr. 1'718) is much more permanent, and crystallizes readily in beautiful octohedra.

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(769) FERROUS NITRATE (Fe 2 NO ̧ . 6 H ̧¤=180+108).— This salt may be obtained by dissolving protosulphide of iron in cold nitric acid diluted with 4 or 5 times its bulk of water. Sulphuretted hydrogen is evolved in abundance, and on evaporating the solution in vacuo over oil of vitriol, it crystallizes in pale-green rhombs, which when heated evolve nitric oxide, and yield a basic ferric nitrate; 6 (Fe 2 NO3)=2 NO+3 Fе‚Ð ̧, 5 N‚Ð ̧• This change sometimes occurs in warm weather spontaneously. The basic salt is then freely soluble in water, and is not decom

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614

FERROUS NITRATE AND CARBONATE.

posed by ebullition. Ferrous nitrate may also be procured by decomposing a solution of ferrous sulphate by an equivalent quantity of nitrate of barium. It cannot be obtained in a pure form by treating iron with diluted nitric acid: since in that case the metal is dissolved without evolution of gas, and ammonia is formed in the liquid :

10 HNO3+4 Fe=4 (Fe 2 NOg) + H1NNƏ ̧+3 H2O.

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Ferric Nitrate (Fe"" 3 NO, xH ̧→).—When nitric acid of sp. gr. 12 or 13 is digested upon metallic iron, a violent action occurs attended with the extrication of nitrous anhydride and of nitric oxide; the iron is at the same time converted into ferric nitrate, which is obtained with difficulty on evaporation in prismatic hydrated crystals. An insoluble basic nitrate is commonly

formed at the same time.

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(770) Ferrous carbonate or protocarbonate of iron (Fe€Ð ̧=116; or FeO, CO2=58); Comp. in 100 parts, Fe, 48·27, or FeÐ, 62:07 ; € 37'93.—This substance is found native in immense quantities, forming a valuable ore of iron. In its less usual condition, when crystallized, it constitutes spathic iron ore, and occurs in yellowish lenticular crystals, the primary form of which is a rhombohedron, isomorphous with calcareous spar. The native carbonate very often contains carbonate of manganese. The clay-iron ore, from which the greater part of the English iron is obtained, is, as already mentioned, an impure carbonate of iron. Clay iron-stone, besides the more usual form of bands or seams accompanying the coal strata, occurs also in detached nodules or lumps, sometimes of very large size, imbedded in the clay of the same formations. When carbonate of iron is heated strongly in vessels from which air is excluded, carbonic anhydride and carbonic oxide are expelled, and magnetic oxide of iron is left, the decomposition being as follows: 3 FeCO, Fe,1+2 CO2+CO. Carbonate of iron is the salt contained in most ferruginous springs, in which it is held in solution by free carbonic acid: it is rarely present in a larger quantity than I grain per pint. Mere exposure to air causes its separation; the acid escapes, oxygen is absorbed, and hydrated peroxide of iron, mixed with a small quantity of organic matter, subsides, forming the ochry deposits so usual around chalybeate springs. The ferrous carbonate may be produced artificially by mixing a ferrous salt with a carbonate of one of the alkali-metals, when it falls as a pale green voluminous hydrate which is speedily altered by exposure to air; it absorbs oxygen, rapidly losing its carbonic acid, and is converted into the red hydrated sesquioxide;

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OXALATES AND PHOSPHATES OF IRON.

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during the process of drying it is therefore almost completely decomposed. No stable ferric carbonate exists, but the acid-carbonate of potassium as well as that of sodium dissolves the hydrated peroxide of iron; the red solution thus formed is very slowly decomposed by prolonged ebullition.

(771) OXALATES OF IRON.-Ferrous oxalate (2 Fe¤‚Ðμ 3 H2O) Ꮎ is occasionally formed in the brown-coal strata, in yellow fibrous or granular masses, known as Humboldtite or iron-resin. It may be prepared artificially by precipitating the solution of green sulphate of iron by oxalate of ammonium; or by exposing a solution of ferric oxalate in oxalic acid to the sun. Carbonic acid is then evolved, and ferrous oxalate with 2 H, is thrown down as a yellow crystalline and nearly insoluble powder.

Ferric oxalate is also a lemon-yellow powder, nearly insoluble in water, but soluble in excess of oxalic acid. It may be obtained by mixing a slight excess of a ferric salt with one of a soluble oxalate.

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Ferric oxalate forms several double salts of the formula (M ̧Fe ́ ́ 3 ¤ ̧Ð ̧), analogous to the blue double oxalates of chromium (794). Double oxalates of iron with potassium, sodium, ammonium, barium, strontium, and calcium have been obtained by digesting the acid oxalates of these metals upon hydrated oxide of iron. These double oxalates are all freely soluble. It is owing to the formation of the soluble potassium salt that acid oxalate of potassium (salt of sorrel) is useful for removing stains of ink and oxide of iron from linen.

(772) PHOSPHATES OF IRON.-Ferrous phosphate (Fe"HP, or HO, 2 FeO, PO,) falls as a white powder on adding triphosphate of sodium to a ferrous salt; by exposure to air it absorbs oxygen, and becomes blue. A hydrated blue phosphate of iron is found native;, it is known as vivianite. It is probably a mixture of ferrous and ferric phosphates (Fe"HPO, 2 Fe”” PO1), and contains in addition about 30 per cent. of water.

A ferric phosphate (Fe""PO,, 2 H2O)) is obtained as a white powder by decomposing perchloride of iron by an alkaline orthophosphate. Exposure of this salt to air produces no change. It is insoluble in acetic acid, but soluble in ferric acetate: phosphoric acid is sometimes precipitated in this form in the course of analysis. Its composition varies according as the phosphate of sodium or the salt of iron is in excess.

Several native silicates of iron are known, but they are not important. The 'finery slag' obtained in the conversion of cast into wrought iron consists chiefly of ferrous orthosilicate (Fe,Sie

616

CHARACTERS OF THE SALTS OF IRON.

or 2 FeO, SiO2). Oxide of iron rapidly attacks clay crucibles if fused in them.

(773) CHARACTERS OF THE SALTS OF IRON.-Iron forms two classes of salts, both of which are readily distinguished from each other and from those of other metals. The salts of iron are not poisonous, unless administered in excessive quantities; they form valuable tonics and astringents when taken internally. The solutions both of the ferrous and of the ferric salts have an inky, astringent taste.

1. Ferrous Salts, or Salts of the Protoxide.-These salts, when in solution, or when crystallized, have a pale-green colour; they redden litmus, but very feebly. With the alkalies a grey or green precipitate of hydrated protoxide is formed in their solutions: it passes quickly through various shades of green into brown by exposure to the air; this change of colour is due to the absorption of oxygen, in consequence of which the sesquioxide is eventually produced. If the precipitate be produced by ammonia, an excess of this reagent redissolves a part of the precipitate; and if the solution contains chloride of ammonium, the whole of the oxide will be redissolved: this solution absorbs oxygen rapidly from the air, and a film of sesquioxide of the metal is formed upon the surface. Ferrous salts of the mineral acids are not precipitated in slightly acid solutions by sulphuretted hydrogen; but they give a black precipitate of hydrated sulphide on adding a solution of sulphide of ammonium. Ferrocyanide of potassium gives a pale blue precipitate, which, on exposure to the air, deepens in tint, owing to the absorption of oxygen. Ferricyanide of potassium, when added to a neutral or acid solution, gives a bright-blue precipitate, which is one of the varieties of Prussian blue. If a solution of a ferrous salt be boiled with nitric acid, the metal is completely converted into a ferric salt: the same thing is effected by the action of chlorine or of bromine, or by boiling an acidulated solution of the salt with a small quantity of chlorate of potassium.

2. Ferric Salts, or Persalts, or Salts of the Peroxide.-In solution they have a yellow or reddish-brown colour. Hydrosulphuric acid reduces them to the state of ferrous salts, whilst a white deposit of sulphur occurs: for example, with ferric sulphate the following reaction takes place; 2 (Fe"1⁄2 3 SO1) +2 H2S=4 Fe′′SO + 2 H2SO1+S2. With sulphide of ammonium a black hydrated sesquisulphide of iron is precipitated. The hydrated alkalies give a reddish-brown voluminous precipitate of hydrated peroxide, in

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