Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/613

POTASSIUM 591 acid. When the oxide-free metal is heated gently within the dry gas it is gradually transformed into a blue liquid, which on cooling freezes into a yellowish-brown or flesh- coloured solid. This body is known as " potassamide," KNH 2 . When heated by itself to redness the amide is decomposed into ammonia and nitride of potassium, 3NH 2 K = NK 3 +2NH 3 . The nitride is an almost black solid. Both it and the amide decompose water readily with for mation of ammonia and caustic potash. Potassium at temperatures from 200 to 400 C. " occludes " hydrogen gas, as palladium does (see " Palladium," under PLATINUM, supra, p. 193). The highest degree of saturation corre sponds approximately to the formula K 2 H for the " alloy," or to about 126 volumes of gas (measured cold) for one volume of metal. In a vacuum or in sufficiently dilute hydrogen the compound from 200 upwards loses hydro gen, until the tension of the free gas has arrived at the maximum value characteristic of that temperature (Troost and Hautefeuille). Potassium Oxides, singularly, can be produced only from the metal, and another remarkable fact is that the one with which all chemical students imagine they are so familiar namely, "anhydrous potash," K 2 is little more than a fiction. According to Vernon Harcoiirt, when the metal is heated cautiously, first in dry air and then in dry oxygen, it is transformed into a white mass (K 2 2 ?), which, however, at once takes up more oxygen with formations ultimately of a yellow powdery tetroxide (KjOJ, fusible at a red heat without decomposition. At a white heat it loses oxygen and leaves a residue of lower oxides (K 2 ?). When heated in hydrogen it is reduced to ordinary potash, KHO. When dissolved in excess of dilute acid it yields a mixed solution of the respective potash salt and peroxide of hydrogen, with abundant evolution of oxygen gas. Potassium Salts. There is only one series of these known, namely, the salts produced by the union of potash (KHO) with acids. Chloride, KC1. This salt (commercial name, "muriate of potash ") is at present being produced in immense quantities at Stassfurt from the so-called " Abraumsalze. " For the purpose of the manu facturer of muriate these are assorted into a raw material contain ing approximately in 100 parts 55-65 of carnallite (representing 16 parts of chloride of potassium) ; 20-25 of common salt ; 15-20 of kieserite, a peculiar, very slowly soluble sulphate of magnesia, MgSo 4 .H 2 0; 2-4 of tachhydrite (CaCl.,. 2MgCl, + 12H a O) ; and minor components. This mixture is now wrought mainly in two ways. (1) The salt is dissolved in water with the help of steam, and the solution is cooled down to from 60 to 70, when a quantity of impure common salt crystallizes out, which is re moved. The decanted ley deposits on cooling and standing a 70 per cent, muriate of potash, which is purified, if desired, by washing it by displacement witn cold water. Common salt prin cipally goes into solution, and the percentage may thus be brought up to from 80 to 95. The mother -liquor from the 70 per cent, muriate is evaporated down further, the common salt which separates out in the heat removed as it appears, and the suffi ciently concentrated liquor allowed to crystallize, when almost pure carnallite separates out, which is easily decomposed into its components (see infra). (2) Ziervogel and Tuchen s method. The crude salt is ground up and then heated in concentrated solution of chloride of magnesium with mechanical agitation. The carnallite principally dissolves and crystallizes out relatively pure on cooling. The mother-liquor is used for a subsequent extraction of fresh raw salt. The carnallite produced is dissolved in hot water and the solution allowed to cool, when it deposits a coarse granular muriate of potash containing up to 99 per cent, of the pure substance. The undissolved residue produced in eithei process consists chiefly of kieserite and common salt. It is worked up either for Epsom salt and common salt, or for sulphate of soda and chloride of magnesium. The potassiferous bye -products are utilized for the manufacture of manures. Chemically pure chloride of potassium is most conveniently pre pared from pure perchlorate (see infra) by dioxygenating it in a platinum basin at the lowest temperature and then fusing the residue in a well-covered platinum crucible. The fused product solidifies on cooling into a colourless glass. Chloride of potassium dissolves in water and crystallizes from the solution in anhydrous cubes. 100 parts of water dissolve at 10 20 50 100 0. 29-2 32-0 347 42-8 56 6 parts of the salt. When a sufficiency of hydrochloric-acid gas i; passed into the solution the salt is completely precipitated as a fine powder. If the original solution contained chloride of mag nesium or calcium or sulphate of potash, all impurities remain in the mother-liquor (the S0 3 as KHS0 4 ), and can be removed by washing the precipitate with strong hydrochloric acid. Chloride of potassium fuses at 738 C. (Carnelley), and at a red heat vola tilizes rather abundantly. Chlorate, KC10 3 . This industrially important salt was dis covered in 1736 by Berthollet, who correctly designated it as " peroxidized muriate. " Chlorine gas is largely absorbed by cold caustic -potash ley with formation of chloride and hypochlorite, 2KHO + C1 2 = KC1 + KC10-I-H 2 0. When the mixed solution is boiled it suffers, strictly speaking, a complicated decomposition, which, however, in the main comes to the same as if the hypo chlorite broke up into chloride and chlorate, 3KC10 = 2KC1 + KC10 3 . Hence chlorate of potash is easily produced by passing chlorine into hot caustic - potash ley so as at once to realize the change, 6KHO + 3C1 2 = 3H 2 + 5KC1 + KC10 3 ; and this method used to be followed industrially until Liebig pointed out that five-sixths of the potash can be saved by first substituting milk of lime, Ca(OH) 2 = 2caOH, for the potash ley and from the mixed solution of lime -salts precipitating, so to say, the chloric acid as potash salt by adding 1KCI for every lcaC10 :! present, concentrating by evaporation, and allowing the KC10 3 ito crystallize out. This is the present industrial process. For the technical details we must refer to the handbooks of chemistry. Suffice it to say that in practice about 1 03 times KC1 are used for every lcaC10 3 , and that the salt produced is almost chemically pure after one recrys- tallization. By repeated recrystallization every trace of impurities is easily removed. The crystals are colourless transparent mono- clinic plates, which, unless formed very slowly, are very thin, so as often to exhibit the Newton s colours. 100 parts of water dissolve at 3-3 15 6 50 19 104 -8 (on boiling) 60 parts of the salt (Gay-Lnssac). The salt is almost insoluble in strong alcohol. It is permanent in the air. It fuses at 359 C. (Carnelley), and at about 18 above the temperature of its formation the liquid gives off oxygen with evolution of heat, and formation ultimately of chloride (and oxygen). The salt accordingly, in opposition to any combustible matter with which it may be mixed, behaves at the same time as a store of highly-condensed loosely-combined oxygen and of potential lieat. Hence its manifold applications in artillery and pyrotechnics are easily understood. To give one example of the readiness with which it acts as a burning agent : a mixture of it and sulphur when struck with a hammer explodes loudly, the mechanical blow sufficing to produce locally the temperature neces sary for starting the reaction. When the salt was still a novelty it was tried as a substitute for the nitre in gunpowder. Such powder, however, proved too good to be safe. More recently a mixture of 49 parts of the chlorate, 23 of sugar, and 28 of prussiate of potash was recommended by Pohl as a preferable substitute for gunpowder, but this powder has never come into actual use any where. We must not forget to point out that mixtures of chlorate of potash and combustible substances must on no account be made in a mortar ; this would be sure to lead to dangerous explosions. The several ingredients must be powdered separately and only then be mixed together on a sheet of paper or on a table, all unnecessary pressure or friction being carefully avoided. The decomposition of chlorate of potash by heat is greatly facili tated by admixture of even small proportions of certain solid oxides, e.g., oxide of copper, of iron, or of manganese. The oxygen, in the case of binoxide of manganese, for instance, comes off below the fusing point of the salt. Hence a salt contaminated with even a small proportion of heavy metallic chlorate cannot (in general) be fused without decomposition. The writer observed this anomaly with a commercial chlorate which happened to contain about one half per cent, of chlorate of zinc. The aqueous solution of the salt is neutral and bears prolonged boiling without decomposition. On acidification with dilute sulphuric acid it assumes the reactions of a solution of chloric acid, i.e., of a powerful but readily controllable oxydant. In this capacity it is used in calico-printing as a "discharge." In the same industry it serves for making the chlorate of soda needed for the production of aniline black. In the chemical laboratory it is in constant requisition as a source of oxygen and as an oxidizing agent. In the hands of Marignac it served for the determination of the important ratio KC1 : 30. Perchlorate, KC10 4 . The decomposition of chlorate of potash by heat, if catalytic agents like Mn0 2 , &c., are absent, proceeds by two stages. In the first the salt breaks up thus, 2KC10 3 =KCl-f O., + KC10 4 ; in the second the perchlorate at a higher temperature is decomposed into chloride and oxygen. The termination of the first stage is marked by a slackening in the evolution of the oxygen and by the residual salt (which, at the beginning, is a thin fluid) becoming pasty. From the mixture KC1 + KC10 4 the chloride is extracted by lixiviation with successive instalments of cold water. The residual perchlorate is very easily purified by recrystallization (compare pure chloride of potassium, supra). Perchlorate of potash dissolves in 88 parts of water of 1 0C., and in far less of boiling water. It is absolutely insoluble in absolute alcohol. It begins to give

off its oxygen at about 400 C. , which is below its fusing point.