POTASSIUM (Neo-Lat., from potassa, pot ash, from Eng. potash, from pot + ash ). A metallic chemical clement first isolated by Sir Hmnphry Davy in 1807. The carbonates of potassium and of sodium, undistinguished from each other, were called alkali by the alchemist Geber, and they were known as fixed alkali in order to distinguish them from ammonium car bonate, known as the volatile alkali. Duhamel, in 1730, discovered that the alkali contained in common salt is different from that contained in the ashes of land plants, and thenceforth the first-named was called 'mineral alkali' and the second 'vegetable alkali.' In 1758 Marggraf showed that the salts of the common alkali gave a violet tinge to the flame of a spirit lamp, while those derived from common salt showed a yellow color. Klaproth next pointed out that the 'vege. table alkali' was contained also in several miner als, such as leucite, subsequent to which the special name of potash was applied to this alkali, and that of natron or soda to the mineral alkali. Both of these alkalies remained undecomposed up to the time of Davy's experiments. (See CHEMISTRY.) Davy decomposed potash by pass ing the electric current from a voltaic pile of 200 plates through a piece of potash placed in a platinum dish, the result being the formation of "small globules having a high metallic lustre, some of which burned with explosion and bright flame as soon as they were formed, and others remained and were merely tarnished and finally covered by a white fllm.which formed on their surfaces," and these, he concluded, were the pe culiar principle, the basis of pot ash." Potassium is not found native, but is widely distributed, in combination, especially as the chloride and sulphate, in sea water and other natural waters: also as a constituent of many silicates, as the feldspars and micas, forming • from 1.7 to 3.1 per cent. of the granite composing .1 the earth's solid crust. As sylvite (potassium ' chloride) and as carnallite (potassium and mag nesium chloride) it occurs in the beds overlying the salt deposits of Stassfurt, Germany; and as nitre or the nitrate it is found as an efflorescence on the soil, usually with the sodium salt, in Chile, Peru, etc.: also as alunitc (hydrous sulphate of potassium and occurring in the older rocks, where its formation is attributed to the action of sulphurous gases. It is found as bi tartrate in wines, and as sulphate, carbonate, and chloride in molasses from beets. As chloride and carbonate, or as an organic salt. it occurs in soils • and in vegetable and animal substances; wood ashes and the ash of marine plants contain much potassium carbonate. The suint from the wool of sheep contains a large proportion, sometimes as much as one-third. of an organic potassium salt which is separated as carbonate, together with the wool fat.
The original electrolytic method used by Davy for the preparation of metallic potassium has already been referred to. It was soon superseded by methods which had for their purpose the re duction of the carbonate by means of carbon; thus an intimate mixture of potassium carbonate with charcoal, obtained by igniting crude acid potas sium tartrate in an iron crucible, yielded a porous mass which was heated to a white heat in an iron bottle connected with a receiver, into which the potassium distilled over and was con densed. The process now generally used is prac tically the one invented by Castner, and consists in reducing a potassium salt, such as the hydrate, by a mixture of carbon and a metallic carbide, or a mixture of very finely divided metal and car bon, which is heated in an iron crucible with an exit tube passing through the lid. The potassium distills over into the receiver and at the end of the operation is placed under petroleum so as to prevent oxidation.
Potassium (symbol. K : atomic weight, 39.11) has, when freshly cut, a bright silvery metallic lustre and a specific gravity of 0.875 at 13' C., being lighter than any other metal except lithium. It is brittle at 0° C., and possesses a crystalline fracture; at 15° C. it becomes soft like wax, and it melts at 62.5° C. (144.5° F.), forming a liquid that closely resembles mercury in appear ance. With the exception of cesium and rubid ium, it is the most electropositive element known, and it acts as a powerful reducing agent. On exposure to the air it rapidly becomes con verted into the hydrate, and finally into the car bonate. It decomposes water with sufficient energy to ignite the liberated hydrogen. When brought into contact with the halogens and with sulphur, selenium, tellurium, and phosphorus, it unites with them. It also alloys with most metals, usually by being heated with them, and with sodium it forms an alloy of a low melting point. For commercial purposes its use. especial ly as a reducing agent, has been almost entirely superseded by sodium, owing to the cheapness of the latter. With oxygen potassium combines to fern] two oxides. a monoxide and a per oxide The former is formed when potas siuth hydrate is heated with metallic potassium; it is a gray. brittle mass that has strong basic properties. When dissolved in water this oxide forms potassium hydroxide (KOH), or caustic potash, which, however, is generally prepared com mercially by decomposing a dilute solution of potassium carbonate with slaked lime. Potassium hydroxide is a hard, white, brittle, powerfully caustic substance, quickly destroying many aid mal and vegetable substances, and finds extensive use in the manufacture of soap. It is the liquor of the pharmacopteia, and is used as a caustic in surgical operations, and finds some application in analytical chemistry. Potassium peroxide is formed when metallic potassium is burned in the air. Potassium arsenate, called also `Macquer's salt,' is prepared by fusing to gether equal parts of potassium nitrate and arse nious anhydride, then dissolving the resulting mass in water and evaporating. On cooling, the arsenate crystallizes out. It is used in calico printing for the purpose of fixing the mordant on the fibre of the material. Potassium bromide may be made by dissolving bromine in a solution of potassium hydroxide, evaporating the solution, and igniting the mixture, with the addition of a little char coal, which reduces potassium bromate to bro mide, and the mass is dissolved in water, filtered, and allowed to evaporate and crystallize. A
more satisfactory method, however, consists in decomposing iron bromide with potassium car bonate, and allowing the resulting liquid to crystallize. Potassium bromide crystallizes in the form of white cubes that have a strong saline taste, and find extensive application in photog raphy and medicine. Potassium. carbonate, which was known to the ancients and is deseribed by Aristotle as being prepared by the burning of rushes, was long obtained by burning plants in dry pits and dissolving the ashes in water, then evaporating till the sulphates, chlorides, etc., separated out by crystallization, and then boil ing the mother liquor to dryness in iron pots, which was probably the origin of the name 'pot ashes.' The process subsequently introduced was similar to that used by Le Blanc for the manufac ture of soda ash, and consisted in fusing potassium sulphate with calcium carbonate and coal. Another source is from the saint of sheep's wool. The raw wool on washing yields a mixture which is evaporated to dryness, and the solid residue is calcined in retorts. The resulting mass is dis solved in water and allowed to cool, when the potassituti carbonate is deposited as a mass of crystals. This salt, which is a white solid with an alkaline and caustic taste, is used largely in the manufacture of soft soap, glass, potassium chromate, and potassium ferrocyanide. Potassium chloride, known commercially as mnriate of pot ash, is now largely obtained from the Stassfurt deposits, where it occurs native as sylvite, and in combination with magnesium chloride as ear nallite. The last-named is the principal source. The mineral is dissolved in warm water, heated by steam to about 120° C. (248° F.), and then allowed to cool. Between 60° and 70° C. (140° to 158° F.) the magnesium sulphate, calcium sul phate, and sodium chloride separate, and on fur ther cooling about 70 per cent. of the potassium chloride is obtained, the crystals of which are washed with a little cold water to remove any sodium or magnesium chloride, and a product is obtained containing about 95 per cent. of potas sium chloride. Small quantities are also manufac tured from the ashes of seaweed, which, previous to the working of the Stassfurt deposits, formed one of the principal sources of potassium salts. Potassium chloride is a white crystalline com pound with a strong saline taste. It is used chiefly in the preparation of other potassium salts, such as the carbonate and the chlorate, and in an impure state it is employed as a fertilizer. Potassium bichromate is prepared by heating to gether finely ground chromic iron ore with potas sium carbonate and lime. The resulting mass is extracted with hot water and the calcium chro mate precipitated out by means of potassium sul phate, leaving in solution potassium chromate, which is then converted into the bichromate by treatment with sulphuric acid. The solution is evaporated to crystallization, yield ing splendid garnet-red crystals. Potassium bi chromate finds extensive use in the preparation of chromium compounds. in the manufacture of various colors, as an oxidizing agent, and in certain photo-engraving processes, owing to the fact that when mixed with gelatin it becomes insoluble when exposed to the light. This par ticular property has also led to its employment in the manufacture of insoluble glue. Potassium cyanide may be prepared by melting potassium ferrocyanide with potassium carbonate in an iron crucible. It is a white crystalline compound, ex ceedingly poisonous. It finds some use in photog raphy, and as a reducing agent in chemical operations. especially in metallurgy. (See HY DROCYA N IC ACID.) Potassium iodide may he prepared by incline in potassium hy droxide, in the same way as the bromide is made by the action of the hydroxide on bromine. It is also manufactured by decomposing ferrous iodide with potassium carbonate, evaporating. dissolv ing the resulting mass, and crystallizing. This salt crystallizes in white cubes that have a sharp taste, and is used chiefly in photography and in medicine. Potassium nitrate, which is found na tive as nitre, or saltpetre, is described under SALTPETRE. Potassium. silicate is prepared by heating potassium carbonate with white sand in a reve•be•atoy furnace, usually with a small amount of charcoal, by means of which a compound is obtained that is put on the market as a thick solution, and is used as a substitute for sodium silicate in the manufacture of soaps and in fresco painting. (See WATER-GLASS.) Potassium su/phate is found in large quantities at Stassfurt, principally as kainite, which is a potassium and magnesium sulphate with mag nesium chloride. This mineral, when allowed to remain for some time exposed to the air, de liquesces, and as soon as the soluble magnesium chloride has run off the remaining salt is decom posed by boiling water, so that on cooling the sulphate crystallizes out. It is also obtained by the action of acid on a potassium chlo ride. It. is a crystalline salt with a bitter saline taste, and finds use in medicine as a purgative, while large quantities are used for the manufac ture of potash-alum and potassium carbonate. It was formerly called sal polychrest. The acid sulphate, o• bisulphate, the .cal enisrum of the older chemists, may be formed by heating potas sium sulphate with sulphuric acid, or by heating potassium nitrate with sulphuric acid. It is a white crystalline compound, occasionally used as a flux. Other potassium salts may be found described under the names of the acids contained in them. Consult: Pfeiffer, "Die Stassfurter Eali-Industrie," in vol. ii. of BoIley's Handbuch der chemischen Technologic (Brunswick, 1887) ; Lunge, A Theoretical caul Practical Treatise on the Manufacture of Sulphuric Acid and A lha/i (London, 1891) ; id., The Alkali-Maker's Hand book (ib., 1891). Consult also the dictionaries and industrial works reeommended in the article CIIEMISTRY.