392

SULPHIDES OF POTASSIUM.

alcoholic solution of KHS to the air till it begins to become turbid, and evaporating to dryness in vacuo. It fuses easily, and is of an orange colour.

2

The tersulphide (K,S,, or KS,) is obtained pure by passing the vapour of bisulphide of carbon over ignited carbonate of potassium so long as any gas makes its escape: carbonic anhydride and carbonic oxide are produced, as follows: 2 K2€Ð ̧+3 €S2=2 K„S3+ 4 €0+C02. In the old process of making liver of sulphur, 69 €Ꮎ parts of dry carbonate of potassium are fused with 40 parts of sulphur; the resulting yellowish-brown mass consists of 3 atoms of tersulphide and I atom of sulphate of potassium: part of the carbonate in this case yields oxygen to one portion of the sulphur, and forms sulphuric acid, as shown in the annexed equation : 4 K‚¤Ð ̧+5§‚=K2SO4+3 K2S ̧+4 €Ð 2·

2

A tetrasulphide (KS1, or KS) may be formed by reducing sulphate of potassium by the vapour of bisulphide of carbon.

The pentasulphide (KS, or KS) is formed by boiling a solution of any of the preceding sulphides with excess of sulphur till saturated or by fusing any of the dry sulphides with an excess of sulphur; the excess of sulphur separates and floats above the sulphide, which has a dark liver-brown colour; it is deliquescent, and forms a deep yellow solution in water.

All these sulphides have an alkaline reaction to test-paper, and an odour of sulphuretted hydrogen more or less distinct. On the addition of a stronger acid they are decomposed with extrication of sulphuretted hydrogen, attended, in the case of all but the protosulphide, by the precipitation of white, finely divided sulphur. On adding the persulphide to an excess of hydrochloric acid of sp. gr. about 11, the persulphide of hydrogen (429) is separated as an oily liquid. By exposing solutions of the higher sulphides to air, they become colourless, hyposulphite of potassium is formed, and the excess of sulphur is separated. When a solution of caustic potash is boiled with sulphur, a decomposition ensues similar to that which occurs when hydrate of potash and sulphur are fused together; a deep reddish-yellow liquid is formed, which contains hyposulphite of potassium, and one of the higher sulphides of the metal; 6 atoms of hydrate of potash and 12 of sulphur would thus furnish I atom of hyposulphite and 2 of pentasulphide of potassium; 6KHO+6 §,=K‚§‚H ̧Ð1+2K2S5+2 H ̧Ð.

4

(564) CHLORIDE OF POTASSIUM (KCl=746); Sp. Gr. 1'994 (Filhol) Comp. in 100 parts; K, 52'35; Cl, 47'65.—This salt is extracted in considerable quantity from kelp, the ashes of burnt sca-weed, and is used largely as a source of the potash required

CHLORIDE AND BROMIDE OF POTASSIUM.

393

in the manufacture of alum. It may be prepared pure by directly neutralizing either the acid or the normal carbonate of potassium with hydrochloric acid, and evaporating. It crystallizes in cubes, and is very readily soluble in cold water, which takes up about a third of its weight, attended with great depression of temperature. It is remarkable that this salt possesses the property of absorbing the vapours of sulphuric anhydride, forming a hard translucent mass (KCl,SO,), which is instantly decomposed by water. With chromic acid it forms a corresponding compound (KCl, reg), which is also decomposed by water: it is obtained in needles when a solution of acid chromate of potassium in hydrochloric acid is allowed to crystallize.

According to Bunsen, a blue subchloride of potassium also exists. A native compound of chloride of potassium and magnesium has recently been discovered in a bed of clay in the neighbourhood of Stassfurt, near Magdeburg, which is immediately above a bed of rock-salt, 100 feet in thickness. This is precisely the position which it would occupy, supposing the deposit to have been formed by the gradual drying-up of an inland sea, where the common salt would crystallize out first, and the salts of magnesium and potassium afterwards. This bed of clay contains the sulphates and chlorides of potassium and sodium, and magnesium, and the upper part of the deposit consists chiefly of a hydrated double chloride of magnesium and potassium, (KCl, MgCl2, 6H,), resembling rock-salt in appearance, but with a more pearly lustre, and extremely deliquescent. It has been worked for the sake of the chloride of potassium, which amounts to nearly one-fourth of its weight.

(565) BROMIDE OF POTASSIUM (KBr=119'1; Sp. Gr. 2672: Comp. in 100 parts; K, 32-78; Br, 67 22.-This is a very soluble salt, which crystallizes in cubes. It may be obtained by adding bromine to a solution of caustic potash until the liquid acquires a slight permanent yellow colour: bromine and bromate of potassium are formed. Löwig dissolves the mixed salts in water, decomposes the bromate by a current of sulphuretted hydrogen, warms gently to expel the excess of the gas, filters from the deposited sulphur, and evaporates till the solution crystallizes: 2 KBrÐ ̧+6 H2S=2 KBr+6 H2O+3 §2.

(566) IODIDE OF POTASSIUM, or Hydriodate of Potash (KI= 166); Sp. Gr. 3056: Comp. in 100 parts; K, 76'5; I, 23′5.— This valuable medicine may be procured in several ways. A simple method consists in adding iodine to a solution of caustic potash gently warmed, until the solution begins to assume a brown tint.

Iodide and iodate of potassium are formed; 3 I,+6 KHO

394 IODIDE, FLUORIDE, AND SILICOFLUORIDE OF POTASSIUM.

=5 KI+KIÐ,+3 H2O. By gentle ignition of the residue obtained on evaporation, the iodate is decomposed, and the remaining iodide fuses. The salt must not be strongly heated, as iodide of potassium is volatilized by a red heat. A better plan is to digest 2 parts of iodine and 1 part of iron, in a stoppered vessel, with 10 parts of water, the iron being purposely added in excess; under these circumstances iodide of iron is formed by the direct union of the metal with the iodine: the solution is decanted, and a quantity of iodine equal to one-third of that which it already contains is added. The liquid is then boiled, and a solution of carbonate of potassium is added in small quantities so long as effervescence is produced and a precipitate occurs; the solution is next filtered from the dense magnetic oxide of iron, and on evaporation it yields crystals of iodide of potassium ; Fe, +4 I1⁄2+4 K¿€Ð ̧ =8KI+Fe2+4 €Ð2•

2

Iodide of potassium crystallizes in anhydrous cubes, which in a dry air are not deliquescent. It is very soluble in water, and to a smaller extent in alcohol; it has a cooling, bitterish taste. Its solution has the property of dissolving an additional equivalent of iodine, with which it forms a deep brown liquid.

Iodide of potassium, if pure, should be completely soluble in six times its weight of alcohol (sp. gr. 0·83); it should not effervesce when moistened with hydrochloric acid (carbonate of potassium would be indicated by effervescence), and it should not turn brown by the action of the acid; if iodate of potassium were mixed with it, free iodine would be shown by the brown colour developed on adding the acid.

(567) FLUORIDE OF POTASSIUM (KF=581; Sp. Gr. 2'454) is a very deliquescent salt obtained by neutralizing hydrofluoric acid with a solution of caustic potash. Its solution has an alkaline reaction and corrodes glass.

(568) SILICOFLUORIDE OF POTASSIUM (2KF,SiF=220, or KF,SiF=110).-This salt is one of the most insoluble compounds of potassium; it falls as a transparent gelatinous precipitate whenever hydrosilicofluoric acid is added to a salt of potassium; it dries to a white earthy-looking powder. Advantage is occasionally taken of its insolubility to separate potassium from some of its salts: in this way chloric acid is sometimes prepared from chlorate of potassium.

(569) SULPHATE OF POTASSIUM (K2SO1=174,or KO,SO。=87); Sp. Gr. 264: Composition in 100 parts, K,, 54'02; SO3, 45'98.— This salt crystallizes either in anhydrous six-sided prisms, terminating in six-sided pyramids, or in four-sided oblique rhombic prisms; it requires about 16 parts of cold water for solution. The

SULPHATE, ACID SULPHATE, AND NITRATE OF POTASSIUM.

395

crystals decrepitate strongly when heated. Sulphate of potassium forms a series of double salts with sulphates of the metals isomorphous with magnesium, and another class of salts (the varieties of alum) with the sulphates of the metals isomorphous with aluminum. Jacquelain finds that if normal sulphate of potassium be dissolved in nitric acid, a little nitre and acid sulphate of potassium are formed, whilst a salt consisting of (HNO,,K,SO4) crystallizes in oblique prisms. A nanalogous compound may be formed with phosphoric acid (H ̧P, K2SO).

ACID SULPHATE, or BISULPHATE OF POTASSIUM: KHSO1=136; Sp. Gr. 2:475.-This salt is formed on a large scale as a residuary product in the preparation of nitric acid from nitrate of potassium. It crystallizes from a strongly acid solution in rhomboidal tables, which fuse at a heat below redness, and by prolonged ignition lose half their acid; they are very soluble in water and have a sour bitterish taste. If redissolved in water, the normal sulphate crystallizes first, and afterwards, when the liquid has become strongly acid, the bisulphate is deposited. This salt is sometimes used as a flux in cases where the action of an acid is required at a high temperature upon salts or metallic oxides with which it may be fused. The bisulphate occasionally crystallizes in anhy

drous needles, Ꮶ ᎦᎾ ᎦᎾ3.

2 5'

It ap

(570) NITRATE OF POTASSIUM (KNO,, or KO,NO,=101); Sp. Gr. 2.070; Comp. in 100 parts; K‚Ð, 46°54; N2Љ› 53°46.— Saltpetre, or Nitre as this salt is frequently termed, is one of the most important and valuable salts of potassium. The principal supply of nitre is derived from various districts in the East Indies, where it occurs sometimes as an efflorescence upon the soil, at other times disseminated through the superficial stratum itself. pears to be formed in the moist portions of the soil, at some little distance below the surface, towards which it is raised by capillary action. The nitre is obtained by lixiviating the soil, and allowing the solution to crystallize. The earth which furnishes it consists principally of loose, porous carbonate of calcium, mixed with decomposing felspar, and it always contains more or less of organic matters. In temperate climates, both nitrites and nitrates are almost always found in the shallow well-waters of towns, owing to the oxidation of nitrogen contained in the animal débris during their infiltration through the soil. Notwithstanding the investigations of J. Davy, of Kuhlmann, of Schönbein, and of others, the process of nitrification is still very imperfectly understood. The artificial formation of nitre has, however, been practised with considerable success in various countries of Europe, which furnish

annually a large amount of the salt. In Sweden this supply of nitre is considered of such importance that each landed proprietor is obliged to pay a certain tax in raw nitre, the quantity required being proportioned to the value of the estate (Berzelius).

Where animal matters are present in abundance, the formation of nitric acid is chiefly due to the gradual oxidation of ammonia developed in the process of putrefaction. The presence of a certain amount of moisture is necessary, and the oxidation is materially favoured by an excess of carbonate of potassium, of lime, or of some basic substance which can combine with the acid at the moment of its generation. Ozone appears to have the power of combining directly with nitrogen; it may possibly, as Schönbein conjectures, be concerned in the natural production of nitric acid, and indeed it is not improbable that nitrification is, in favourable cases, due to the slow combination of atmospheric nitrogen and oxygen. The process of nitrification becomes arrested if the temperature be allowed to fall much below 60o.

Nitre Plantations.-The method adopted in the artificial production of nitre consists in placing animal matters, mingled with ashes and lime rubbish, in loosely aggregated heaps, exposed to the air, but sheltered from rain. These heaps are watered from time to time with urine or stable runnings; at suitable intervals the earth is lixiviated, and the salt crystallized. Three years usually elapse before the nitre bed is washed: after this interval a cubic foot of the débris should yield between 4 and 5 ounces of nitre. As there is always a considerable quantity of the nitrates of calcium and magnesium present, which will not crystallize, carbonate of potassium, in the shape of wood ashes, is added so long as any precipitate occurs. By this means the nitrate of calcium is decomposed, and the insoluble carbonate of calcium separated. The clear liquor is then evaporated and crystallized. It is found by the saltpetre-boiler that the earth in which nitre has once been formed furnishes fresh nitre more readily than on the first occasion. Care is taken that the nitre plantations, as they are termed, shall rest upon an impervious flooring of clay, so that the liquid which drains away from them may be collected and preserved.

In Prussia, by a more methodical treatment, a cubic foot of the earth yields about 20 ounces of nitre. The heaps are so constructed as to form a terrace of steps, exposing the back in the form of an upright wall to the prevailing wind; the watering is thus facilitated, while the evaporation proceeds with rapidity upon the exposed side, and here, from capillary action, the nitre chiefly accumulates from time to time a layer of earth is removed from