Potassium

Pure Potassium.

Pure potassium is a silvery white metal tinged with blue; but on exposure to air it at once forms a film of oxide, and on prolonged exposure deliquesces into a solution of hydrate and carbonate. Perfectly dry oxygen, however, has no action upon it. (See DRYNESS, CHEMICAL.) At temperatures below o° C it is 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 0.865; hence it is the lightest metal known except lithium. It fuses at 62.5° C and boils at about 760°, emitting an intensely green vapour. It may be obtained crystallized in quadratic octahedra 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 flame 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 and the flame 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 ethoxide, while hydrogen escapes, this time without inflammation: = +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, potassamide, When heated to redness the amide is decomposed into ammonia and potassium nitride, which is an almost black solid. Both it and the amide decompose water readily with forma tion of ammonia and caustic potash. The metal dissolves in liquid ammonia to give a blue solution. Potassium at temperatures from 200° to 400° C occludes hydrogen gas, the highest degree of saturation corresponding approximately to the formula but it seems probable that KH is the only compound formed. In a vacuum or in sufficiently dilute hydrogen the compound from 20o° upwards loses hydrogen, until the tension of the free gas has arrived at the maximum value characteristic of that temperature. The hydride is used for the manufacture of potas sium formate which results from the action of moist carbon dioxide : KH+CO2----H.0O2K.

Oxides and Hydroxide.

Potassium forms two well-defined oxides, K20 and K204, whilst several others, of less certain exist ence, have been described. The monoxide, may be obtained by strongly heating the product or burning the metal in slightly moist air; by heating the hydroxide with the metal: -- - or by passing pure and almost dry air over the molten metal. It forms a grey brittle mass, having a conchoidal fracture; it is very deliquescent, combining very energetically with water to form caustic potash.

Potassium hydroxide or caustic potash, KOH, formerly con sidered to be an oxide, may be obtained by dissolving the metal or monoxide in water, but is manufactured by double decomposition from potassium carbonate and slaked lime: or by electrolytic methods similar to those used for sodium hydroxide. (See ALKALI MANUFACTURE.) In the former case, a solution of one part of the carbonate in 12 parts of water is heated to boiling in a cast-iron vessel 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 liquors after concentration in iron vessels are evaporated in a silver dish, and the residual oily liquid is then poured out into a polished iron tray, or into an iron mould to pro duce 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. Nickel basins are better adapted than iron basins for the pre liminary concentration of potash lye. The latter begin to oxidize before the lye has come up to the traditional strength of specific gravity 1.333 when cold, while nickel is not attacked so long as the percentage of real KOH is short of 6o. 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 KOH can be produced only in gold vessels. Glass and (to a less extent) porcelain are attacked by caustic potash lye, slowly in the cold, more readily on boiling.

Solid Caustic Potash.—Solid caustic potash forms an opaque, white, stone-like mass of dense granular fracture ; specific gravity 2.1. 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. It is extremely soluble in even cold water, and in any proportion of water on boiling. On crystallizing a solution, the hydrate is deposited; and 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 contami nated with excess of water (over and above that in the KOH) 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, decant ing and evaporating the solution to dryness and fusing in silver vessels.