Page images
PDF
EPUB

or by evaporating the alcohol solution of the nitrate. It cannot be produced by the action of the hydrates of potassium or ammonium on neutral solutions of uranylic salts, since the yellow precipitates which these salts occasion are always combinations of potassium or ammonium with uranyle and oxygen: they are sometimes called uranates. The composition of the potassium compound is K(U2O), O; that of the ammonium compound is unknown. They are insoluble in excess of their precipitants: the ammonium compound is slightly soluble in water, soluble in carbonate of ammonium, but insoluble in chloride of ammonium; the potassium compound dissolves in the acid carbonate of potassium.

The Sulphide is produced by the action of the sulphides of potassium or ammonium. It is a brownish-black precipitate, which subsides very slowly. Its composition is probably US3.

It is soluble in excess of the precipitants, and slightly soluble in water, yielding a brown solution: it dissolves in acids with decomposition.

THE SULPHATE is soluble, and it forms no insoluble DOUBLE SULPHATE. THE CARBONATE produced by the action of the carbonates of potassium or ammonium is a compound analogous to the hydrate just described. A similar compound salt is also produced by the action of carbonate of calcium. They are yellow precipitates.

The composition of the carbonate of potassium precipitate is K, CO., U2OCO; that of the ammonium compound is similar: both are readily soluble in excess of their precipitants.

THE OXALATE is produced by the action of oxalic acid: oxalate of potassium and oxalate of ammonium generally produce a compound salt. It occurs as a yellow powder or in yellow crystalline grains.

Its composition, when air-dried, is (U2O)2Ō+3aq; when dried at 100° C. it is (U, 0),O+aq.

It dissolves easily in warm solutions of oxalate of potassium or ammonium, and, on cooling, crystallizes out as the double salt. It dissolves in 125 parts of water at 14° C, and in 30 parts of boiling water: it is more soluble in strong acids.

The Ferrocyanide is produced by ferrocyanide of potassium. It is a dark brownish-red precipitate.

Its composition is probably (U2O), Cfy. It dissolves in the carbonates of sodium and ammonium.

THE FERRICYANIDE is produced by ferricyanide of potassium. It is a brownish-red precipitate. It dissolves in carbonate of ammonium.

THE PHOSPHATE is produced by phosphate of sodium. It is a light yellow precipitate.

Its composition is (U2O)2H PO̟+3aq.

It scarcely dissolves at all in water, but easily in carbonate of ammonium.

The special reagents of the first and second groups are not known to yield any characteristic reactions with uranic salts.

The chief means employed for the detection of uranic salts are these,the blowpipe reactions, and the formation of the so-called hydrate and ferrocyanide.

SALTS OF TITANIUM.

The body titanium forms three oxides; one only, however, is of sufficient importance to be considered analytically, and that is possessed of the properties of an acid-radical. This oxide, the so-called titanic acid, dissolves in hydrochloric and other acids; and from its being precipitated from that solution by alkaline hydrates or carbonates, in excess of which it is insoluble, its reactions are frequently considered in this group: we shall, however, treat of it in the third subdivision of the acid-radicals.

SALTS OF TANTALUM, NIOBIUM, AND PELOPIUM.

These three rare bodies are known to form but one oxide each, which plays the part of an acid-radical. They have been placed in this group for the same reason as has been adduced under TITANIUM; they, however, exhibit a difference in their behaviour with hydrate of potassium, being soluble in excess of that reagent. They will be considered in the third subdivision of acid-radicals.

SALTS OF IRON.

The metal iron resembles uranium and chromium in forming a large number of oxides; many of these are, however, believed to be combinations of oxides among themselves, since they yield those oxides as products of decomposition. Thus the oxide Fe2O may be decomposed into salts of the ferrous and ferric series, by the action of the ferrocyanide and the ferricyanide of potassium, whence it is argued by some that its real composition must be Fe, 0, (Fe),0,. The truly important oxides of iron are Fe, 0, (Fe),0,, and FeO,: the two former represent series of salts, the ferrous and ferric series respectively, while the latter has the properties of a compound acid-radical, and as such will be treated of in the next chapter. Ferrous salts are generally white if anhydrous, but when containing water of crystallization they are usually of a pale bluish-green. Ferric salts also are often white when anhydrous, and of a reddish-brown or reddishyellow colour when crystallized with water. The soluble salts of iron in general have a nauseous metallic taste.

All salts of iron when heated before the blowpipe are converted in the oxidizing flame into (Fe), O,, which is brown while hot,

6

and red when cold; and if this sesquioxide be heated carefully in the reducing flame, it is converted into the black oxide Fe ̧0: both of these compounds remain infusible. Iron salts give no characteristic reaction with nitrate of cobalt before the blowpipe, and impart no colour to the flame; but when fused with borax, they produce a bead which in the oxidizing flame is red while hot, and yellow after cooling, and in the reducing flame is slightly yellow while hot, and bottle-green or colourless when cold.

The production of a ferrate (M FeO2) does not form the same characteristic test for iron as the analogous chromium compound does for chromium. Ferrate of potassium (KFeO2) is obtained by the action of oxidizing agents on sesquioxide of iron. Its aqueous solution is of a magnificent wine-red colour; but the salt is very unstable, giving off oxygen and depositing sesquioxide of iron.

FERROUS SALTS, OR PROTO-SALTS OF IRON.

Solution for the reactions:-ferrous sulphate (Fe2SO) in

water.

The chief insoluble salts of this series are these,-the cyanide, the hydrate, the sulphide, the carbonate, the oxalate, the ferrocyanide, the ferricyanide, and the phosphate. The great tendency which these salts have to pass into ferric salts is a serious obstacle in observing their reactions, which are in consequence usually more or less mixed with the reactions due to ferric salts.

In making the solution for the reactions, the salt should be washed two or three times with water, to dissolve away the outer coating; and, when finally dissolved, access of air should be prevented by stopping the mouth of the test-tube with the thumb.

THE CYANIDE is produced by cyanide of potassium acting upon pure ferrous salts, as a light reddish-brown precipitate.

Its composition is thought to be FeCy.

It dissolves in acids. It is also very soluble in cyanide of potassium, uniting with it to form a compound which was formerly regarded as a double salt having the formula 2(KCy), FeCy:

this compound was, however, noticed to differ from other double salts, inasmuch as the iron existing in it could not be precipitated even by so powerful a reagent as an alkaline sulphide; nor could it by any means be detected until the constitution of the body was broken up, and its cyanogen (Cy=CN) destroyed by the action of some powerful oxidizing agent: fusion (e.g.) with nitrate or chlorate of potassium would restore the iron to the state in which it could be easily recognized. This, coupled with the fact that in ordinary decompositions the substance 2(KCy)FeCy divided itself into K, and FeCy,, led to the supposition that it was in reality a true salt containing a compound acid-radical, FeCy,, which was thenceforth called "ferrocyanogen ;" and to its salts the name of "ferrocyanides" was applied. It is a good instance of the way in which metamorphoses take place among complex molecules.

Chromate of po

THE CHROMATE does not appear to exist. tassium gives a yellowish-brown precipitate with ferrous salts, which appears to be a mixture of the chromic and ferric sesquioxides. This precipitate is partially soluble in nitric acid, yielding a green solution.

The Hydrate is produced by the action of the hydrates of potassium or ammonium on neutral solutions of ferrous salts, also by the carbonates of the second subdivision, but only on boiling. The hydrate of ammonium precipitates only half the metal present in the solution of a ferrous salt, the ammonium salt formed combining with the remaining half of the unchanged ferrous salt, and producing a double salt undecomposable by the excess of hydrate of ammonium present: this has been already observed in the case of the salts of magnesium (p. 93); it occurs thus:

2(FeC1)+2(NH HO)=FeHO+FeC1,NH C1+NH HỌ.

ppt.

When absolutely free from ferric salt, this precipitate is white; but as ordinarily prepared, it is of a greenish colour. It absorbs oxygen very rapidly from the air, and is converted into red ferric hydrate.

G

The formula is FeHO.

It is insoluble in excess of its precipitants; but the presence of ammonium salts prevents its precipitation by these reagents, especially by the hydrate of ammonium. Ammonium salts also dissolve it when once precipitated, although imperfectly. It is readily soluble in acids.

The Sulphide is produced by the action of the sulphides of potassium or ammonium; it is also produced by the passage of hydrosulphuric acid gas into a solution containing a ferrous salt dissolved in an ammonium compound, and containing hydrate of ammonium, while in neutral or acid solutions of ferrous salts hydrosulphuric acid yields no precipitate, except in rare cases, such as the neutral ferrous acetate (FeÃ), in which partial precipitation occurs. It is a black precipitate.

Its formula is Fe, S; but it probably contains some water.

It is insoluble in excess of its precipitants. It is slightly soluble in water, forming a green solution, from which it is reprecipitated by the addition of sulphide of ammonium. It dissolves in hydrochloric and nitric acids, but is less soluble in acetic acid.

THE SULPHATE is soluble, 1 part of the crystals, which have the formula Fe, SO,+7aq, dissolving in 2 parts of cold, and in three-fourths of boiling water.

THE DOUBLE SULPHATE OF IRON AND POTASSIUM (KFeSO1+3aq) is also soluble.

THE CARBONATE is produced by the action of the carbonates of potassium or ammonium (but not by carbonate of calcium); it is a white precipitate, which becomes green and then reddishbrown by exposure to the air, from a gradual conversion at first into the hydrate of the magnetic oxide (FeO) and afterwards into ferric hydrate.

Its formula is 2(Fe2 CO2)+aq.

It is insoluble in excess of its precipitants; but ammonium salts prevent its precipitation by these reagents: it is soluble in carbonic and other acids, in most cases suffering decomposition.

« PreviousContinue »