Potassium

The peroxide, K204, discovered by Gay-Lussac and Thenard, is obtained by heating the metal in an excess of slightly moist air or oxygen, or better by 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 after wards 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 mon oxide. Exposed to moist air it loses oxygen, possibly giving the dioxide, ; water reacts with it, evolving much heat and giving caustic potash, hydrogen peroxide and oxygen; whilst carbon mon oxide gives potassium carbonate and oxygen at temperatures below ioo°. A violent reaction ensues with phosphorus and sul phur, and many metals are oxidized by it, some with incandescence.

Halogen Compounds.—Potassium fluoride, KF, is a very del iquescent salt, crystallizing in cubes and having a sharp saline taste; it is formed by neutralizing potassium carbonate or hydrox ide with hydrofluoric acid and concentrating in platinum vessels. It forms the acid fluoride when dissolved in aqueous hydro fluoric acid, a salt which at a red heat gives the normal fluoride and hydrofluoric acid. Other salts of composition KF•2HF and KF.3HF, have been described by Moissan.

Potassium chloride, KC1, also known as muriate of potash, closely resembles ordinary salt. It is produced in immense quan tities at Stassfurt from the so-called "Abraumsalze" (waste salts). When hydrogen chloride is passed into a solution of potassium chloride the salt is completely precipitated as a fine powder. If the original solution contained the chlorides of magnesium or calcium or sulphate of potassium, all impurities remain in the mother-liquor (the sulphur as KHSO4), and can be removed by washing the precipitate with strong hydrochloric acid. The salt crystallizes in cubes of specific gravity 1.995 ; it melts at about 800° and volatilizes at a bright red heat. It is extensively em ployed for the preparation of other potassium salts, but the largest quantity (especially of the impure product) is used in the production of artificial manures.

Potassium bromide, KBr, may be obtained by dissolving bromine in potash, whereupon bromide and bromate are first formed, evaporating and igniting the product in order to decompose the bromate: = 2KBrO3= 2KBr+302 (cf. CHLORATES) ; but it is manufactured by acting with bromine water on iron filings and decomposing the iron bromide thus formed with potassium carbonate. In appearance it closely re sembles the chloride, forming colourless cubes which readily dis solve in water and melt at 722°. It combines with bromine to form an unstable tribromide, Potassium Iodide.—Potassium iodide, KI, is obtained by dis solving iodine in potash, the deoxidation of the iodate being facilitated by the addition of charcoal before ignition, proceeding as with the bromide. The commercial salt usually has an alkaline reaction; it may be purified by dissolving in the minimum amount of water, and neutralizing with dilute sulphuric acid; alcohol is now added to precipitate the potassium sulphate, the solution filtered and crystallized. It forms colourless cubes which are

readily soluble in water, melt at 685° and yield a vapour of normal density. It is sparingly soluble in absolute alcohol. Both the iodide and bromide are used in photography. Iodine dis solves in an aqueous solution of the salt to form a dark brown liquid, which on evaporation over sulphuric acid gives black acicular crystals of the tri-iodide, The salt is very deliques cent; it melts at and at ioo° decomposes into iodine and potassium iodide. For the oxyhalogen salts see CHLORATE, CHLORINE, BROMINE and IODINE.

Carbonate of Potash,

popularly known as "potashes," was originally obtained in countries where wood was cheap by lixiviating wood ashes in wooden tubs, evaporating the solution to dryness in iron pots and calcining the residue; in more recent practice the calcination is carried out in reverberatory furnaces. This product, known as "crude potashes," contains, in addition to carbonate, varying amounts of sulphate and chloride and also insoluble matter. Crude potash is used for the manufacture of glass, and, after being causticized, for the making of soft soap. For many other purposes it must be refined, which is done by treating the crude product with the minimum of coil water re quired to dissolve the carbonate, removing the undissolved part (which consists chiefly of sulphate), and evaporating the clear liquor to dryness in an iron pan. The purified carbonate (which still contains most of the chloride of the raw material and other impurities) is known as "pearl ashes." Most of the carbonate which occurs in commerce is made from the chloride of the Stass furt beds by an adaptation of the "Leblanc process" for conver sion of common salt into soda ash. (See ALKALI MANUFACTURE.) Chemically pure carbonate of potash is best prepared by igniting pure bicarbonate in iron or (better) in silver or platinum vessels, or else by calcining pure cream of tartar. The latter operation fur nishes an intimate mixture of the carbonate with charcoal, from which the carbonate is extracted by thdviation with water and filtration. The filtrate is evaporated to dryness (in iron or plati num vessels) and the residue fully dehydrated by gentle ignition. The salt is thus obtained as a white porous mass, fusible at a red heat (838° C) into a colourless liquid, which solidifies into a white opaque mass. The dry salt is very hygroscopic ; it deliquesces into an oily solution ("oleum tartani") in ordinary air. The most sat urated solution contains 205 parts of the salt to 100 of water and boils at 135°. On crystallizing a solution monoclinic crystals of are deposited, which at I00° lose water and give a white powder of this is completely dehydrated at 13o°. The carbonate, being insoluble in strong alcohol (and many other liquid organic compounds), is much used for dehydration of the corresponding aqueous preparations. The pure carbonate is constantly used in the laboratory as a basic substance generally, for the disintegration of silicates, and as a precipitant. The in dustrial preparation serves for the making of flint glass, of potash soap (soft soap) and of caustic potash.