Potassium

Potassium bicarbonate, is obtained when carbonic acid is passed through a cold solution of the ordinary carbonate as long as it is absorbed. Any silicate present is also converted into bi carbonate with elimination of silica, which must be filtered off. The filtrate is evaporated at a temperature not exceeding 65° ; after sufficient concentration it deposits on cooling anhydrous crystals of the salt, while the potassium chloride, which may be present as an impurity, remains mostly in the mother-liquor; the rest is easily removed by repeated recrystallization. The bicar bonate forms large monoclinic prisms, permanent in the air. When the dry salt is heated to 190° it decomposes into normal carbonate, carbon dioxide and water.

Sulphur Compounds.

Potassium sulphide, was obtained by Berzelius in pale red crystals by passing hydrogen over potas sium sulphate, and by Berthier as a flesh-coloured mass by heating the sulphate with carbon. When it is prepared by treating pot ash with sulphuretted hydrogen and adding a second equivalent of alkali, a solution is obtained which on evaporation in a vacuum deposits crystals of K2S-5H20. The solution is strongly caustic. It turns yellow on exposure to air, absorbing oxygen and carbon dioxide and forming thiosulphate and potassium carbonate and liberating sulphuretted hydrogen, which decomposes into water and sulphur, the latter combining with the monosulphide to form higher salts. The solution also decomposes on boiling. The hydro sulphide, KHS, was obtained by Gay-Lussac on heating the metal in sulphuretted hydrogen, and by Berzelius on acting with sul phuretted hydrogen on potassium carbonate at a dull red heat. It forms a yellowish-white deliquescent mass, which melts on heating, and at a sufficiently high temperature it yields a dark red liquid. It is readily soluble in water, and on evaporation in a vacuum over caustic lime it deposits colourless, rhombohedral crystals of The solution is more easily prepared by saturating potash solution with sulphuretted hydrogen. The solu tion has a bitter taste, and on exposure to the air turns yellow, but on long exposure it recovers its original colourless appear ance owing to the formation of thiosulphate. Liver of sulphur or hepar sulphuris, a medicine known to the alchemists, is a mixture of various polysulphides with the sulphate and thiosulphate, in variable proportions, obtained by gently heating the carbonate with sulphur in covered vessels. It forms a liver-coloured mass. In the pharmacopoeia it is designated potassa sulphurata.

Potassium sulphite, is prepared by saturating a potash solution with sulphur dioxide, adding a second equivalent of potash, and crystallizing in a vacuum, when the salt separates as small deliquescent, hexagonal crystals. The salt may be obtained by crystallizing the metabisulphite, (from sulphur dioxide and a hot saturated solution of the carbonate, or from sulphur dioxide and a mixture of milk of lime and potas sium sulphate) with an equivalent amount of potash. The salt is obtained as oblique rhombic octahedra by crystal lizing the solution over sulphuric acid.

Potassium sulphate, K2SO4, a salt known early in the 14th century, was styled in the i 7th century arcanum or sal duplicatum, being regarded as a combination of an acid salt with an alkaline salt. It was obtained as a by-product in many chemical reactions, and subsequently used to be extracted from kainite, one of the Stassfurt minerals, but the process is now given up because the salt can be produced cheaply enough from the chloride by de composing it with sulphuric acid and calcining the residue. To purify the crude product it is dissolved in hot water and the solution filtered and allowed to cool, when the bulk of the dis solved salt crystallizes out with characteristic promptitude. The very beautiful (anhydrous) crystals have the habit of a double six-sided pyramid, but really belong to the rhombic system. They are transparent, very hard and absolutely permanent in the air. They have a bitter, salty taste. The salt is soluble in water, but insoluble in caustic potash of sp.gr. 1.35, and in absolute alcohol. It fuses at 1,078°. The crude salt is used occasionally in the manufacture of glass. The acid sulphate or bisulphate, is readily produced by fusing thirteen parts of the powdered normal salt with eight parts of sulphuric acid. It forms rhombic pyramids, which melt at 197°. It dissolves in three parts of water of o° C. The solution behaves as if its two congeners, and were present side by side uncombined. An excess of alcohol, in fact, precipitates normal sulphate (with little bisulphate) and free acid remains in solution. Similar is the be haviour of the fused dry salt at a dull red heat ; it acts on silicates, titanates, etc., as if it were sulphuric acid raised beyond its natural boiling point. Hence its frequent application in analysis as a disintegrating agent. The sulphate is also used in alum manufacture; fertilizer and medicine. For the salts of other sulphur acids, see SULPHUR.