3 parts of water, producing considerable cold; it is more soluble in hot water, the solubility increasing uniformly with the temperature, and almost insoluble in alcohol. Chloride of potassium is capable of combining with anhydrous sulphuric acid, forming a compound of the formula KC1.2SO,. It also unites with terchloride of iodine. Chloride of potassium is sometimes used to convert into nitrate of potassa the nitrate of lime obtained in artificial nitrification (§ 145); it is also occasionally employed as the source of potassa in alum.

BROMIDE OF POTASSIUM, KBг.

This salt is formed under the same conditions as the chloride; it is usually prepared by dissolving a slight excess of bromine in potassa, separating the bromate of potassa by crystallization, evaporating the mother-liquid, and decomposing any bromate of potassa by igniting the residue, which may then be dissolved in water, and crystallized. The bromide crystallizes in white cubes, which are anhydrous, and behave like the chloride when heated. It is readily soluble in water, producing cold, and nearly insoluble in alcohol. A solution of bromide of potassium is capable of dissolving considerable quantities of bromine. Bromide of potassium is used in photography.

IODIDE OF POTASSIUM, HYDRIODATE OF POTASSA, KI.

Preparation. The iodide of potassium is produced under similar conditions. to those which yield the bromide, and may be prepared by the same process, substituting iodine for bromine. The usual method of preparing iodide of potassium, however, consists in digesting 2 parts of iodine and 1 part of iron filings in 10 parts of water, till they have combined to form a solution of a pale green color; the solution of iodide of iron thus obtained is decomposed with exactly the requisite quantity of solution of carbonate of potassa :

Fel+KO.CO, KI+FeO.CO,.

The solution is filtered from the precipitated carbonate of iron, and evaporated to the crystallizing point.

Properties.-Iodide of potassium crystallizes in anhydrous cubes, sometimes opaque, but more generally transparent, which decrepitate when heated, fuse easily, and volatilize at a moderate heat. After fusion, it has an alkaline reaction. The pure salt deliquesces slightly, and dissolves very readily in less than its own weight of water, with production of cold; the solution has a very slight alkaline reaction; but if, as is often the case, the salt be adulterated with carbonate of potassa, its solution will be strongly alkaline, and the solid iodide will deliquesce rapidly in air. Iodide of potassium is dissolved by alcohol. The aqueous solution of the salt is capable of dissolving a large quantity of iodine, which gives it a deep brown color. This property is turned to advantage in the compound solution of iodine, which is used in medicine. Iodide of potassium is an important medicinal agent, especially in scrofulous diseases. It is also used in photography.

The iodide of potassium of commerce often contains, in addition to carbonate of potassa, caustic potassa, iodate of potassa, bromide and chloride of potassium. In order to ascertain the degree of purity of any specimen of iodide of potas sium, we may employ a solution of chloride of mercury; 1 equivalent of this salt

This depression of temperature, being much greater than that caused by chloride of sodium, has been applied by Gay-Lussac to the determination of the amount of the former in mixtures of the two salts.

The iodate of potassa may be detected by adding solution of sulphurous acid, which will liberate a quantity of iodine, imparting a brown color to the solution. The other impurities in iodide of potassium may be detected by the ordinary methods of analysis.

completely precipitates 1 equivalent of iodide of potassium, in the form of the bright red iodide of mercury; thus:

HgCl+KI=HgI+KCl;

another equivalent of iodide of potassium redissolves this precipitate, so that if 1 equivalent of chloride of mercury be added to 2 equivalents of iodide of potassium, no precipitate is produced. In order to apply this principle, 1 equivalent (135.5 parts) of chloride of mercury (HgCl, corrosive sublimate), and 2 equivalents (166.1 parts) of the iodide of potassium to be tested, are dissolved in two equal quantities of water. The solution of chloride of mercury is poured from a burette into that of iodide of potassium, constantly stirring, till a permanent precipitate begins to appear; when this is the case, of course a little more than 1 equivalent of chloride of mercury must have been added for every 2 equivalents of iodide of potassium present, so that if the salt tested were absolutely pure, the whole of the mercury-solution should have been added, whereas an impure specimen will require only one-half, two-thirds, &c., of the solution, according to the amount of impurity present. 1

1

FLUORIDE OF POTASSIUM, KF.

This salt is formed when hydrofluoric acid is decomposed by potassium, hydrogen being evolved; it is best prepared by supersaturating potassa, or carbonate of potassa, with hydrofluoric acid, and evaporating in a platinum dish. It crystallizes in anhydrous cubes, which deliquesce in air, and dissolve readily in The solution has an alkaline reaction, and acts upon glass.

water.

Hydrated crystals of fluoride of potassium may be produced at low tempe

ratures.

POTASSIUM AND SULPHUR.

§ 160. Potassium combines with sulphur, when gently heated with it, with vivid combustion, producing several sulphides of potassium.

SULPHIDE OF POTASSIUM, KS.

This sulphide is formed when hydrogen is passed over sulphate of potassa at a red heat, and also when sulphur is fused with excess of hydrate of potassa; thus:

3(KO.HO)+S=2KS+KO.S,O,+3н0.

Preparation. It may be prepared by heating to bright redness, in an earthen crucible, an intimate mixture of three parts of sulphate of potassa and 1 part of charcoal:

The sulphide is thus obtained as a light red mass, containing always an admixture of a higher sulphide.

1 A new method recently proposed by Penny (Chem. Gaz. October, 1852), consists in ascertaining the amount of a solution containing a known weight of the iodide, which is required to decompose a given quantity of bichromate of potassa dissolved in water acidulated with hydrochloric acid. The point at which the chromic acid is completely reduced is indicated by dipping a glass rod into the solution, and touching a drop of a solution of protosulphate of iron and sulphocyanide of potassium placed upon a white plate; when a red color is no longer produced, the decomposition is complete. 10 grs. of KO.2CrO, correspond to 33.3 grs. of KI.

2 A hydrofluate of fluoride of potassium, KF.HF, has been obtained.

If lampblack be substituted for charcoal in this process, the resulting sulphide of potassium is pyrophoric.1

It may be obtained in a pure state by saturating a solution of potassa with hydrosulphuric acid, and afterwards adding a volume of the same solution of potassa, equal to that originally employed; the first operation gives rise to hydrosulphate of sulphide of potassium, KS HS, which is converted, by addition of potassa, into sulphide of potassium. The solution may be evaporated to dryness in a retort, when the sulphide of potassium remains as a white crystalline mass. Properties. Sulphide of potassium, when heated in air, absorbs oxygen, and becomes covered with a coating of sulphate of potassa. It is volatile at a high temperature. This sulphide deliquesces in air, and dissolves rapidly in water. with rise of temperature, giving a colorless solution, which possesses an alkaline reaction and a bitter taste; it is also soluble in alcohol. When exposed to air, solution of sulphide of potassium absorbs oxygen, and becomes gradually converted into a mixture of potassa with a higher sulphide, which imparts a yellow color to the solution. When a pure solution of sulphide of potassium is mixed with dilute hydrochloric acid, hydrosulphuric acid is evolved, and the solution remains clear :

KS+HCI=KC1+HS;

but if a higher sulphide be present, a deposition of sulphur will take place, rendering the solution milky; thus:

KS,+HCI KCl+HS+S.

Sulphide of potassium is a powerful sulphur-base (see p. 155); it combines with those metallic sulphides which play the part of acids, such as those of arsenic and antimony, to form sulphur-salts; advantage is taken of this property in analysis, where sulphide of potassium is sometimes used to dissolve the sulphuracids, and thus to separate them from other metallic sulphides.

When a solution of potassa, or of sulphide of potassium, is saturated with hydrosulphuric acid, the compound KS.HS, hydrosulphate of sulphide of potas sium, is produced, and if the solution be evaporated to a syrup in an atmosphere of sulphuretted hydrogen, and allowed to cool, the new compound may be ob tained in colorless prisms, which are exceedingly deliquescent and soluble. The solution of this substance is strongly alkaline, and emits an odor of hydrosulphuric acid, due to the action of the atmospheric carbonic acid; if evaporated in an open vessel, it loses hydrosulphuric acid, leaving sulphide of potassium; when exposed to air, it gradually absorbs oxygen, and becomes first yellow, from the formation of a higher sulphide of potassium, and is ultimately converted into a colorless solution of hyposulphite of potassa.

I. KS.HS+0=KS,+HO.
II. KS,+0,=KO.S2O,.

A pure solution of hydrosulphate of sulphide of potassium, when mixed with acids, evolves hydrosulphuric acid, and remains clear.

When sulphur is heated in a solution of hydrosulphate of sulphide of potassium, hydrosulphuric acid is expelled, and a higher sulphide of potassium

formed.

When an alcoholic solution of sulphide of potassium is mixed with bisulphide of carbon, an orange crystalline substance is deposited, which is known as sulpho

It has been noticed in another place that the flash attendant upon the discharge of fire-arms is due to the combustion of the vapor of sulphide of potassium issuing from the muzzle.

2 The excess of sulphur may be removed by means of finely-divided copper or silver. 3 This compound is also formed when potassium is gently heated in a current of dry hydrosulphuric acid, hydrogen being evolved.

carbonate of sulphide of potassium (KS.CS,), and may be viewed as carbonate of potassa, in which all the oxygen is replaced by sulphur. The higher sulphides of potassium, namely:

KS,, KS, KS, and KS,,

may be prepared by fusing sulphide of potassium with the proper proportion of sulphur; they have all a yellow or brown color, and their solutions are alkaline; these yellow solutions become colorless when exposed to the air, from the formation of hyposulphite of potassa, and sulphur is deposited in all cases, except that of bisulphide of potassium; when hydrochloric acid is added to the solutions of the higher sulphides of potassium, hydrosulphuric acid is evolved, and the excess of sulphur deposited, but if a solution of one of these sulphides be very gradually added to an excess of hydrochloric acid, a portion of persulphide of hydrogen is formed at the same time, since the excess of acid gives it a certain degree of stability.

Bisulphide of potassium (KS) is produced when bisulphate of potassa is reduced by charcoal.

§ 161. Hepar sulphuris, or liver of sulphur, is a brown-red mass, which is sometimes used in medicine, and is prepared by fusing sulphur with carbonate of potassa in closed earthen crucibles; it varies in composition according to the proportions in which these ingredients are employed; the common proportions are two parts of carbonate of potassa to one of sulphur; the resulting mass is a mixture of tersulphide of potassium, hyposulphite of potassa, and sulphate of potassa, the proportions of the two latter varying according to the temperature employed.

Thus, at a low temperature, the decomposition will be represented by the equation:

3(KO.CO1)+S ̧=2KS,+KO.S ̧0,+3C0,;

whilst, at a full red heat, a portion of the hyposulphite will be decomposed with production of pentasulphide of potassium, according to the equation:

4(KO.S,0,)=KS,+3(KO.SO2).

It is from hepar sulphuris, or some similar compound, that milk of sulphur is always obtained; for this purpose, the mass is prepared by fusing carbonate of potassa at rather a low temperature, with enough sulphur to produce the pentasulphide, which is, of course, mixed with hyposulphite; when an acid is added to this solution, it acts upon both these salts, precipitating sulphur, liberating hydrosulphuric acid from the pentasulphide, and evolving hyposulphurous acid from the hyposulphite; but the hyposulphurous acid is almost immediately resolved into sulphurous acid and sulphur, the latter being precipitated; the sulphurous acid, meeting with the hydrosulphuric, is decomposed according to the equation:

SO,+2HS=S,+2HO,

so that all the sulphur is precipitated.

The most economical method of preparing milk of sulphur, therefore, consists in fusing together equal weights of carbonate of potassa and sulphur, at a temperature of about 500° F. (260° C.), when the following decomposition takes place:

3(KO.CO,)+S,,=2KS,+KO.S,O,+3C0,;

12

if an acid be now added to the aqueous solution of this mass :

2KS,+KO.8,0,+3HCI=3KC1+S,,+3 HO.

The silico fluoride of potassium, 3KF 2SiF,, is sparingly soluble in water, whence hydrofluosilicic acid is sometimes employed for precipitating potassa. The description of the compounds of potassium with cyanogen and the gen-radicals, falls strictly within the province of organic chemistry.

SODIUM.

Sym. Na. Eq. 23. Sp. Gr. 0.97.

§ 162. Sodium was discovered by Sir H. Davy in soda, in the year 1807. This metal occurs in great quantity, and very widely diffused in nature, chiefly in combination with chlorine, in the form of sea-salt, which is found not only in the waters of the ocean, but also in those of most springs. Sodium also occurs in nature in combination with oxygen and certain acids; the silicate of soda is a constituent of many minerals, such as albite (silicate of soda and alumina, NaO.3SiO„AlО.SiO), analcime (silicate of soda and alumina, NaO. SiO,,A10.3SiO,+2Aq.), labradorite (silicate of soda, alumina, and lime, NaO. SiO,3(CaO.SiO),4(Al,O,.2SiO), kryolite (3NaF, Al,F), &c. The nitrate, carbonate, biborate, and sulphate of soda are found in nature. Soda is likewise found in all the animal fluids, and in plants, especially such as grow near the sea. Sodium and its compounds are prepared from the ashes of sea-plants, and from sea or rock salt.

Preparation.-Sodium may be obtained from the hydrate of soda, by the same processes as potassium from hydrate of potassa. The process which is employed in practice, consists in converting the acetate of soda into a mixture of carbonate of soda and charcoal by ignition, and distilling this with an additional quantity of carbon, exactly in the same way as in the preparation of potassium; the preparation of sodium, however, is far easier, since it is more readily reduced, and does not form any combination with the carbonic oxide, and there is hence no fear of explosion from the choking of the tube. By a careful operator, almost the theoretical quantity of sodium may be obtained in this manner. Sodium may be purified in the same manner as potassium.

Properties.-Sodium is a yellowish-white lustrous metal, more nearly resembling silver than potassium, to which, in its other physical properties, it is very similar. Its surface tarnishes rapidly in air, from oxidation; it is therefore preserved under petroleum. Sodium fuses at 194° F. (90° C.), and distils at higher temperatures; its vapor is said to be colorless. When heated in air or

oxygen, sodium burns with a bright yellow flame, and is (entirely, if the metallic surface be continually renewed by scraping) converted into soda. The specific gravity of sodium is about 0.97, so that, like potassium, it floats upon water, which it decomposes with great energy, producing soda, and liberating hydrogen; the heat is not sufficient to kindle this gas, unless the sodium be confined to one spot; for example, upon the surface of water thickened with gum, or upon moistened filter-paper; the hydrogen then burns with a fine yellow flame. Sodium being little inferior to potassium in its affinity for oxygen, may often be substituted for this metal in chemical experiments, and, since sodium is much less costly than potassium, it should always replace it if possible.

§ 163. By the imperfect oxidation of sodium, a grayish brittle substance is produced, which is believed to be a suboxide of sodium, corresponding to the suboxide of potassium :

1 Considered by some chemists to be a sesquioxide, Na,O,.