OXIDES OF POTASSIUM. 387 caution is essential; for, if it be neglected, a black detonating compound is speedily formed by exposure to the atmosphere, and is even produced spontaneously, although the metal be kept under naphtha; this substance explodes violently upon the slightest friction. The purified metal amounts to about two-thirds of the quantity operated on. A third distillation may be necessary if the potassium be required in a state of perfect purity. A little impure potassium almost always remains in the tube attached to the retort; and in order to prevent the possibility of the formation of the detonating compound already mentioned, it is best to detach this tube as soon as it is cold, and to immerse it in water. (561) OXIDES OF POTASSIUM.-Potassium forms three wellestablished compounds with oxygen: a protoxide, which constitutes potash, the basis of the salts of the alkali; and two other oxides which do not form corresponding salts with acids. A blue suboxide appears also to be formed upon the surface of the metal during its gradual oxidation in dry air. Potash K. e. K,0 = 94'2, or 82.97 + 17:03 = 100 2 Deutoxide of potassium Ke, = 110'2 Peroxide of potassium Ke = 142°2 Tetroxide of Potassium (K ̧Ð ̧ = 1422; Harcourt, Q. J. Chem. Soc. xiv. 267). This substance is formed when potassium is heated gently in a current of dry air; the operation must be completed in a current of dry oxygen gas: if formed at a temperature of 536°, it slowly cakes together, but below that temperature it furnishes a powder of a chrome-yellow colour. It absorbs moisture rapidly when exposed to the air, and is decomposed by water with extrication of oxygen and formation of a solution of binoxide of potassium. (562) Potash (K,O=94'2, or KO=471).—This compound can be procured in an anhydrous form by oxidating potassium in thin slices, in air perfectly free from moisture and carbonic anhydride; or by heating potassium with an equivalent quantity of the hydrate, when hydrogen is expelled and pure potash is formed, 2 KHO + = 2 KKO + H2. It is white, very deliquescent, and caustic; when moistened with water it becomes incandescent after it has thus become hydrated, no degree of heat is sufficient to expel the water. Anhydrous potash fuses at a red heat, and is volatilized at a high temperature. For most purposes the presence of water is immaterial, potash is therefore generally procured in the state of hydrate, in which form it may be obtained without difficulty. Hydrate of Potash, or Caustic Potash (KHO, or KO,HO= 561; Sp. Gr. 2.2) is prepared by dissolving carbonate of potassium, of which the pearlash of commerce is an impure variety, in 10 or 388 HYDRATE OF POTASH. 3 12 times its weight of water, and adding to the boiling solution a quantity of caustic lime equal in weight to half the carbonate of potassium used; the lime should be slaked, made into a thin paste with water, and added in small portions at a time, so that the liquid may be maintained at the boiling-point: a crystalline carbonate of calcium is precipitated, and hydrate of potash remains in solution; K ̧¤¤ ̧ + ¤aH,, giving 2 KHO + ¤â¤¤ ̧. After decantation from the precipitate the liquid is evaporated rapidly in a clean iron or silver basin, till when the heat is raised nearly to redness, it flows without ebullition, like oil: it is then either cast into cylinders in a metallic mould, or is poured upon a cold stone slab, and solidifies on cooling. Hydrate of potash may be obtained crystallized in acute rhombohedrons, (KHO, 2 H2O), from a hot concentrated aqueous solution. Hydrate of potash is one of the most indispensable reagents to the chemist. It is therefore necessary that he should be able readily to ascertain its purity, and if needful prepare it for himself: when required pure, acid-carbonate of potassium, in crystals, may be decomposed in the manner above described by means of lime obtained from black marble. The impurities which occur most frequently in ordinary caustic potash are carbonates, sulphates, chlorides, and silicates of calcium, aluminum, iron, and lead, and peroxide of potassium. If pure, it is perfectly soluble in water without effervescence; a diluted solution gives no precipitate with baryta-water, showing the absence of carbonates and sulphates; it yields no precipitate with oxalate of ammonium, showing the absence of salts of calcium. neutralizing it with nitric acid, nitrate of silver gives no precipitate, showing the absence of chlorine. Freedom from iron or metallic impurities is shown by the absence of any precipitate on the addition of sulphide of ammonium. Caustic potash, when pure, is wholly soluble in alcohol, the impurities above mentioned remaining undissolved. Common potash is therefore often purified by forming a solution of it in alcohol, and boiling down to dryness in a silver vessel, till it flows tranquilly; the alcohol is thus expelled, the melted hydrate is poured off upon a silver plate from the black crust which forms over its surface, and when cold it is broken up and placed in a well-closed bottle. On A dilute solution of pure potash may be readily obtained by adding a hot solution of hydrate of baryta to a solution of sulphate of potassium, until the liquid gives no further precipitate either with baryta or with the sulphate of potassium; K2SO + BaH2O = BaᎦᎾ, + 2 ᏦᎻᎾ, 4 HYDRATE OF POTASH. 389 Hydrate of potash, after fusion, is a hard, greyish-white substance it rapidly absorbs both moisture and carbonic anhydride from the air; it is soluble in about half its weight of water, with the extrication of considerable heat; it is likewise soluble in alcohol to an almost equal extent. Hydrate of potash has a peculiar nauseous odour, and an acrid taste; it is a powerful cautery, and quickly destroys both animal and vegetable matters; for this reason its solution cannot be filtered except through pounded glass or sand, and is always best clarified by allowing the impurities to subside, and then decanting the clear liquid. The solution should be preserved in glass bottles into the composition of which no oxide of lead enters, as the solution gradually dissolves this oxide out of the glass. It also attacks vessels even of green glass and of porcelain when heated in them. The following table gives approximatively the proportion of anhydrous potash contained in 100 parts by weight of solutions of the alkali of various densities: Strength of Solutions of Potash (Dalton). The Liquor Potasse of the Pharmacopoeia contains nearly 6 per cent. of the solid hydrate, and has a sp. gr. of 1058. The concentrated solution used for organic analysis may be obtained by dissolving I part of the hydrate in 3 parts of water. At a high temperature hydrate of potash is wholly volatilized; consequently the water cannot be expelled from this hydrate by the mere application of heat. Its chemical attractions are so powerful that few vessels are found capable of resisting its action; those which contain silica are decomposed by it, and platinum itself is oxidized when heated in contact with it: gold and silver resist it better. Caustic potash decomposes the fixed oils, and converts them into soluble soaps: when fused with siliceous minerals it displaces the bases, and combines with the silica, forming silicate of potassium. Potash is extensively employed in the arts to the soap-boiler and the glass-maker it is indispensable; in combination in the form of nitre, it enters largely into the 390 NATURAL SOURCES OF POTASH. manufacture of gunpowder; and in greater or less quantity it furnishes important aids to a variety of processes employed in the manufactures of the country. In the laboratory, potash is in constant use for absorbing acid gases, such as carbonic acid, and for separating the metallic oxides from solutions of their salts, since owing to the powerful attraction of the alkali for acids, it readily decomposes the salts of all the metals which produce oxides insoluble in water. Potash is present in small proportion in all fertile soils, the grand reservoirs of this alkali being the different varieties of clay, which contain 2 or 3 per cent. of it, derived from the disintegration of felspar, in which it exists in the proportion of from 10 to 12 per cent., and certain kinds of mica, which yield 5 or 6 per cent. By exposure to the air and atmospheric vicissitudes, these rocks become gradually disintegrated; their soluble constituents, potash amongst the number, are taken up by the water which falls upon the earth's surface, and are assimilated by the plants which spring from its bosom; they accumulate it, especially in the leaves, young shoots, and succulent parts. Owing to this circumstance large quantities of potash may be obtained with facility dried brushwood is incinerated, and the remaining ash, which seldom constitutes more than 1 per cent. of the dry wood, contains the potash in the form of carbonate: the salt is extracted by water from the insoluble portions. M. Merle now extracts considerable quantities of chloride of potassium from the motherliquors of sea-water by a modification of the method of Balard (note, p. 415).* : (563) SULPHIDES OF POTASSIUM.-Potassium takes fire readily Much potash accumulates as an organic salt in the fleece of the sheep and is wasted. Maumené and Rogelet collected this potash salt by simply washing and evaporating the wash water. A fleece weighing 9 lb. contains about 7 ounces of pure potash, of which they consider nearly 6 is recoverable. A good deal of the potash which is carried off the land cultivated for beet-root is now recovered by suitable treatment of the waste after the sugar has been extracted. Messrs. Ward & Wynants have recently contrived a method of extracting potash from felspar in the form of carbonated or of caustic alkali, which, if commercially successful, will materially alter the source of supply of this alkali. In this process the felspar is ground to a fine powder, and mixed with a suitable quantity of chalk, slaked lime, and fluorspar. The materials are fritted in a furnace, at a heat about sufficient to fuse silver, the mixture not being allowed really to melt. The chalk during ignition evolves carbonic anhydride; it thus mechanically preserves the porosity of the mass, and facilitates the extraction of the soluble products. The mass obtained is then lixiviated, and rendered caustic by the addition of slaked lime. The proportion of lime required is indicated by the following formulæ, supposing the materials to be employed in a pure state:-(K,0, 3 SiO2, Al,0, 3 Si☺,) + 7§¤a✪ + €aF ̧. — (Hofmann's Jury Report, 1862.) - SULPHIDES OF POTASSIUM. 2 391 and burns with brilliancy when heated in the vapour of sulphur. It combines with this element in not less than 4 and possibly in 5 different proportions, K,S?, K,S,, K,S,, K,S4, and K,S. Owing to this circumstance, the reactions which occur when sulphur is heated with caustic or carbonated potash are somewhat complicated; but they are now well understood, and may be traced without difficulty. Protosulphide of Potassium (K,S, or KS).—Some doubt exists as to the possibility of forming this compound. The usual directions are to heat sulphate of potassium in a current of dry hydrogen, or to mix the sulphate intimately with finely powdered charcoal, and ignite in covered vessels. Bauer, however (Chem. Gaz. 1858, 468), finds that the result is not that usually represented by the equation, K2SO4+4 H2=K2S+4 H ̧Ð, but that a mixture of free alkali and a variable amount of one of the higher sulphides of potassium is the result. The residue obtained has a reddish-yellow colour; it is deliquescent, and acts powerfully upon the skin as a caustic. When a current of sulphuretted hydrogen is transmitted through a solution of caustic potash it is rapidly absorbed; and if the gas be allowed to pass till the liquid is completely saturated, the compound KHS will be obtained in solution. This solution is colourless when first prepared, but if exposed to the air it quickly absorbs oxygen, and acquires a yellow colour, owing to the formation of bisulphide of potassium; 4 KHS+0,2 K,S,+2 H2O. It is usually stated, and probably with truth, that if a solution of potash be divided into two equal portions and one be converted into the sulphide of potassium and hydrogen (KHS), and be then mixed with the other half of the solution of potash, a solution of pure protosulphide will be obtained, KHS+KHO becoming K,S+H2O. It is possible, however, that this is not so; but that both the caustic and potash and the double sulphide remain unaltered in the liquid. On the addition of a stronger acid sulphuretted hydrogen is given off abundantly, and this would occur whichever view were correct, no sulphur being deposited; for K2S+H2S→ ̧=H,S+ K2SO4, and KHS+ KHO+ H2SO1=H2S+H2O+K2S→ ̧. 2 22 If sulphate of potassium be mixed in fine powder with half its weight of lampblack, and heated in a covered crucible, the sulphate is reduced to sulphide of potassium, which remains in a finely divided state mixed with the excess of charcoal, and yields a pyrophorus, or compound which takes fire spontaneously in the air, owing to the heat emitted by its rapid absorption of oxygen. The bisulphide (KS2, or KS) may be formed by exposing an |