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Encyclopedia Britannica - Main :: ALM-ANC |
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ALUM , in chemistry, a term given to the crystallized double
formula
Potash alum is the common alum of commerce, although both soda alum and ammonium alum are manufactured. The presence of sulphuric acid in potash alum was known to the alchemists. J. H. Pott and A. S. Marggraf demonstrated that alumina was another constituent. Pott in his Lithogeognosia showed that the precipitate obtained when an alkali is poured into a solution of alum is quite different from lime and chalk, with which it had been confounded by G. E. Stahl. Marggraf showed that alumina is one of the constituents of alum, but that this earth possesses peculiar properties, and is one of the ingredients in common clay (Experiences faites sur la terre de l'alun, Marggraf's Opusc. ii.111). He also showed that crystals of alum cannot be obtained by dissolving alumina in sulphuric acid and evaporating the solutions, but when a solution of potash or ammonia is dropped into this liquid, it immediately deposits perfect crystals of alum (Sur la regeneration de l'alun, Marggraf's Opusc. ii. 86). T. 0. Bergman also observed that the addition of potash or ammonia made the solution of alumina in sulphuric acid crystallize, but that the same effect was not produced by the addition of soda or of lime (De confectione aluminus, Bergman's Opusc. i. 225), and that potassium sulphate is frequently found in alum. After M. H. Klaproth had discovered the presence of potassium in leucite and lepidolite
Vauquelin
double
Vauquelin
The word alumen, which we translate alum, occurs in Pliny's Natural History. In the 15th chapter of his 35th book he gives a detailed description of it. By comparing this with the account of vrrvnrrrtpia given by Dioscorides in the 123rd chapter of his 5th book, it is obvious that the two are identical. Pliny informs us that alumen was found naturally in the earth. He calls it salsugoterrae. Different substances were distinguished by the name of alumen"; but they were all characterized by a certain degree of astringency, and were all employed in dyeing and medicine, the light-coloured alumen being useful in brilliant dyes, the dark-coloured only in dyeing black or very dark colours. One species was a liquid, which was apt to be adulterated; but when pure it had the property of blackening when added to pomegranate juice. This property seems to characterize a solution of iron sulphate in water; a solution of ordinary (potash) alum would possess no such property. Pliny says that there is another kind of alum which the Greeks call schistos. It forms in white threads upon the surface of certain stones. Fromthe name schistos, and the mode of formation, there can be little doubt that this species was the salt which forms spontaneously on certain slaty minerals, as alum slate and bituminous shale, and which consists chiefly of the sulphates of iron and aluminium. Possibly in certain places the iron sulphate may have been nearly wanting, and then the salt would be white; and would answer, as Pliny says it did, for dyeing bright colours. Several other species of alumen are described by Pliny, but we are unable to make out to what minerals he alludes.The alumen of the ancients, then, was not the same with the alum of the moderns. It was most commonly an iron sulphate, sometimes probably an aluminium sulphate, and usually a mixture of the two. But the ancients were unacquainted with our alum. They were acquainted with a crystallized iron sulphate, and distinguished it by the names of misy, sory, chalcanthum (Pliny xxxiv. 12). As alum and green vitriol were applied to a variety of substances in common, and as both are distinguished by a sweetish and astringent taste, writers, even after the discovery of alum, do not seem to have discriminated the two salts accurately from each other. In the ;writings of the alchemists we find the words misy, sory, chalcanthum applied to alum as well as to iron sulphate; and the name at; amentum sutorium, which ought to belong, one would suppose, exclusively to green vitriol, applied indifferently to both. Various minerals are employed in the manufacture of alum, the most important being alunite (q.v.) or alum-stone, alum schist, bauxite and cryolite. In order to obtain alum from alunite, it is calcined and then exposed to the action of air for a considerable time. During this exposure it is kept continually moistened with water, so that it ultimately falls to a very fine powder. This powder is then lixiviated with hot water, the liquor decanted, and the alum allowed to crystallize. The alum schists employed in the manufacture of alum are mixtures of iron pyrites, aluminium silicate and various bituminous substances, and are found in upper Bavaria, Bohemia, Belgium and Scotland. These are either roasted or exposed to the weathering action of the air. In the roasting process, sulphuric acid is formed and acts on the clay to form aluminium sulphate, a similar condition of affairs being produced during weathering. The mass is now systematically extracted with water, and a solution of aluminium sulphate of specific gravity 1.16 is prepared. This solution is allowed to stand for some time (in order that any calcium sulphate and basic ferric sulphate may separate), and is then evaporated until ferrous sulphate crystallizes on cooling; it is then drawn
meal
In the preparation of alum from clays or from bauxite, the material is gently calcined, then mixed with sulphuric acid and heated gradually to boiling; it is allowed to stand for some time, the clear solution drawn
Potash alum, K2SO4Al2(SO4)s.24H20, crystallizes in regularoctahedra and is very soluble in water. The solution reddens litmus and is an astringent. When heated to nearly a red heat it gives a porous friable mass which is known as " burnt alum." It fuses at 92 C. in its own water of crystallization. " Neutral alum " is obtained by the addition of as much sodium carbonate to a solution of alum as will begin to cause the separation of alumina; it is much used in mordanting. Alum finds application as a mordant, in the preparation of lakes for sizing hand-made paper and in the clarifying of turbid liquids.Sodium alum, Na2SO4Al2(SO4),.24H2O, occurs in nature as the mineral
Chrome alum, K2SO4Cr2(SO4)3.24H20, appears chiefly as a by-product in the manufacture of alizarin, and as a product of the reaction in bichromate batteries. The solubility of the various alums in water varies greatly, sodium alum being readily soluble in water, whilst caesium and rubidium alums are only sparingly soluble. The various solubilities are shown in the following table: End of Article: ALUM If you wish, you can link directly to this article.
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