Page:EB1911 - Volume 22.djvu/212

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198
POTASSIUM


bringing a platinum wire, connected to the positive of the battery, to the surface of the potassium a vivid action was observed: gas was evolved at the upper surface of the fused globule of potash, whilst at the lower surface, adjacent to the platinum plate, minute metallic globules were formed, some of which immediately infiamed, whilst others merely tarnished. In 1808 Gay-Lussac and Thénard (Ann. chim. 65, p. 325) obtained the metal by passing melted potash or wire heated' to

effected the same

This last process

and especially by

down a clay tube containing iron turnings

whiteness, and Caradau (ibid. 66, p. 97)

decomposition with charcoal at a white heat.

was much improved by Brunner, Wohler,

F. M. L. Donny and I. D. B. Mareska'(/lun. chim. phys., 1852, (3), 35, p. 147). Brunner's process consisted in forming an intimate mixture of potassium carbonate and carbon by igniting crude tartar in covered iron crucibles, cooling the mass, and then distilling it at a white heat from iron bottles, the vaporized metal being condensed beneath the surface of parafiin or naphtha contained in a copper vessel. It was found, however, that if the cooling be not sufficiently rapid explosions occurred owing to the combination of the metal with carbon monoxide (produced in the oxidation of the charcoal) to form the potassium salt of hexaoxybenzene. In Mareska and Donny's process the condensation is effected in a shallow iron box, which has a large exposed surface, capable of being cooled by damped cloths. When the distillation is finished the iron box, after cooling, is unclasped and the product turned out beneath the surface of parafiin. It is purified by re distilling and condensing directly under parafiin. Electrolytic processes have also been devised. Linnemann (Journ. Prak. Chem., 1858, 73, p. 413) obtained the metal on a small scale by electrolysing potassium cyanide between carbon electrodes; A. Matthiessen (Jourh. Chem. Soc., 1856, p. 30) electrolysed an equimolecular mixture of potassium and calcium chlorides (which melts at a lower temperature than potassium chloride) also between carbon electrodes; whilst Castner's process, in which caustic potash is electrolysed, is employed commercially. The metal, however, is not in great demand, for it is generally found that sodium (q.v.), which is cheaper, and, weight for weight, more reactive, will fulfil any purpose for which potassium may be desired.

Pure potassium is a silvery white metal tinged with blue; but on exposure to air it at once forms a hlm of oxide, and on prolonged exposure deliquesces into a solution of hydrate and carbonate. Perfectly dry oxygen, however, has no action upon it. At temperatures below 0° C. it is pretty hard and brittle; at the ordinary temperature it is so soft that it can be kneaded between the fingers and cut with a blunt knife. Its specific gravity is o-86 5; hence it is the lightest metal known except lithium. It fuses at 62-5°C. (Bunsen) and boils at 667°, emitting an intensely green vapour. It may be obtained crystallized in quadratic octahedral of a greenish-blue colour, by melting in a sealed tube containing an inert gas, and inverting the tube when the metal has partially solidified. When heated in air it fuses and then takes fire, burning into a mixture of oxides. Most remarkable, and characteristic for the group it represents, is its action on water. A pellet of potassium when thrown on water at once bursts out into a violet fiame and the burning metal fizzes about on the surface, its extremely high temperature precluding absolute contact with the liquid, except at the very end, when the last remnant, through loss of temperature, is wetted by the water and bursts with explosive violence. The reaction may be written 2K+2H2O= 2KOH-I-Hg, and the fiame is due to the combustion of the hydrogen, the violet colour being occasioned by the potassium vapour. The metal also reacts with alcohol to form potassium ethylate, while hydrogen escapes, this time without inflammation: K+ CQHS-OH=C2H5-OK-l-H. When the oxide-free metal is heated gently in dry ammonia it is gradually transformed into a blue liquid, which on cooling freezes into a yellowish-brown or flesh-coloured solid, pot ass amide, KNH2. When heated to redness the amide is decomposed into ammonia and potassium nitride, NK3, which is an almost black solid. Both it and the amide decompose water readily with formation of ammonia and caustic potash. Potassium at temperatures from 200°to 400°C. occludes hydrogen gas, the highest degree of saturation corresponding approximately to the formula KZH. In a vacuum or in sufficiently dilute hydrogen the compound from 200° upwards loses hydrogen, until the tension of the free gas has arrived at the maximum value characteristic of that temperature (Troost and Hautefeuille).

Compounds.

Oxides and Hydroxide.-Potassium forms two well-defined oxides, KQO' and K2O4, whilst several others, of less certain existence, have been described. The monoxide, KQO, may be obtained by strongly heating the product or burning the metal in slightly moist air; by heating the hydroxide with .the metal: 2KHO+2K= 2K2O-l-Hz; or by passing pure and almost dry air over the molten metal (Kiihnemann, Chem. Centr., 1863, p. 491). It forms a grey brittle mass, having a conchoidal fracture; it is Very deliquescent, combining very energetically with water to form caustic potash. According to Holt and Sims (Iourn. Chem. Soc., 1894, p. 438), the substance as obtained above always contains free potassium. Potassium hydroxide or caustic potash, KOH, formerly considered to be an oxide but shown subsequently to be a hydroxide of potassium, may be obtained by dissolving the metal or monoxide in water, but is manufactured by double decomposition from potassium carbonate and slaked lime: K2CO3+Ca(OH)2=2KOH-I-CaCOf, . A solution of one part of the carbonate in 12 parts of water is heated to boiling in a cast-iron vessel (industrially by means of steam pipes) and the milk of lime added in instalments until a sample of the filtered mixture no longer effervesces with an excess of acid. The mixture is then allowed to settle in the iron vessel, access of air being prevented as much as practicable, and the clear liquor is syphoned off. The remaining mud of calcium carbonate and hydrate is washed, by recantation, with small instalments of hot water to recover at least part of the alkali diffused throughout it, but this process must not be continued too long or else some of the lime passes into solution. The liquors after a concentration in iron vessels are now evaporated in a silver dish, until the heavy vapour of the hydrate is seen to go off. The residual oily liquid is then poured out into a. polished iron tray, or into an iron mould to produce the customary form of “ sticks, ” and allowed to cool. The solid must be at once bottled, because it attracts the moisture and carbonic acid of the air with great avidity and deliquesces. According to Dittmar (Journ. Soc. Chem. Ind., May 1884), nickel basins are far better adapted than iron basins for the preliminary concentration of potash ley. The latter begin to oxidize before the ley has come up to the traditional strength of specific gravity 1-333 when cold, while nickel is not attacked so long as the percent< age of real KHO is short of 60. For the fusion of the dry hydrate nickel vessels cannot be used; in fact, even silver is perceptibly attacked as soon as all the excess of water is away; absolutely pure KHO can be produced only in gold vessels. Glass and (to a less extent) porcelain are attacked by caustic potash ley, slowly in the cold, more readily on boiling.

Solid caustic potash forms an opaque, white, stone-like mass of dense granular fracture; specific gravity '2'I. It fuses considerably below and is perceptibly volatile at a red heat. At a white heat the vapour breaks down into potassium, hydrogen and oxygen. lt- is extremely soluble in even cold water, and in any proportion of water on boiling. On crystallizing a solution, the hydrate KOH»2H2O is deposited; 2KOH»9H2O and 2KOH»5H2O have also been obtained. The solution is intensely “alkaline ” to test papers. It readily dissolves the epidermis of the skin and many other kinds of animal tissue-hence the former application of the " sticks ” in surgery. A dilute potash readily emulsionizes fats, and on boiling saponifies them with formation of a soap and glycerin. All commercial caustic potash is contaminated with excess of water (over and above that in the KHO) and with potassium carbonate and chloride; sulphate, as a rule, is absent. A preparation sufficing for most purposes is obtained by digesting the commercial article in absolute alcohol, decanting and evaporating the solution to dryness and fusing in silver vessels.

The peroxide, K2O¢, discovered by Gay-Lussac and Thénard, is obtained by heating the metal in an excess of slightly moist air or oxygen. Vernon Harcourt (Iourn. Chem. Soc., 1862, p. 267) recommends melting the metal in a flask filled with nitrogen and gradually displacing this gas by oxygen; the first formed grey film on the metal changes to a deep blue, and then the gas is rapidly absorbed, the film becoming white and afterwards yellow. It is a dark yellow powder, which fuses at a high -temperature, the liquid on cooling depositing shining tabular crystals; at a white heat it loses oxygen and yields the monoxide. Exposed to moist air it loses oxygen, possibly giving the dioxide, KZOQ; water reacts with it, evolving much heat and giving caustic potash, hydrogen peroxide and oxygen; whilst carbon monoxide gives potassium carbonate and oxygen at temperatures below 100°. A violent reaction ensues with phosphorus and sulphur, and many metals are oxidized by it, some with incandescence.