Potassium alum, K₂SO₄.Al₂(SO₄)₃

Potassium alum, K₂SO₄.Al₂(SO₄)₃.24H₂O is an article of commerce, being used as a mordant, in the production of other aluminium mordants, in the manufacture of lake pigments, in dressing skins to produce white leather, in sizing paper, and in the production of fire-proofing materials. To a small extent it occurs naturally, and it is manufactured from aluminium sulphate (prepared from bauxite or china-clay) and potassium sulphate, from alum-rock and from alum-shale.

Natural potassium alum occurs as fibrous crystals or as an efflorescence on aluminous minerals at Whitby, Campsie, etc. At Solfatara, near Naples, and in the islands of Volcano and Milo it occurs in some quantity, and when twice recrystallised from water furnishes very good alum.

The manufacture of alum from alum-rock or alunite is an industry which dates from very early times. It was introduced into Europe in the thirteenth century, and several alum works were established during the fifteenth century, notably the celebrated works at La Tolfa near Civita Vecchia. In this district the manufacture of the so-called Roman alum is still an important industry, and the alum prepared there has always been highly valued on account of its purity. Alunite may be regarded as a double salt of potassium sulphate and a basic aluminium sulphate, K₂SO₄.Al₂(SO₄)₃.4Al(OH)₃. It is a mineral which occurs in large quantities at La Tolfa, in Hungary, at Puy-de-Sancy and Madria in Auvergne, and in numerous other localities, having been formed by the action of volcanic gases upon felspathic trachyte. In the modern process, alunite is calcined at 500° to 1000° and the product treated with sulphuric acid. A solution is thus obtained which deposits alum when crystallised, leaving an excess of aluminium sulphate in the mother liquors. It is usual, therefore, before crystallisation, to add sufficient potassium sulphate to enable all the aluminium sulphate present to be converted into alum.

The production of alum from alum-shale, an industry of great antiquity, is at the present time only of slight commercial importance. Alum-shale is a kind of shale or slate containing iron pyrites disseminated throughout its mass in a very finely divided state. On prolonged exposure to the weather the pyrites undergoes oxidation and the sulphuric acid produced attacks the clay. The weathered product, when leached with water, gives a solution containing aluminium sulphate and other substances. It was known to Agricola and Libavius that this solution would not crystallise well unless an alkali had been added to it, and each of these writers describes the early practice of adding decomposed urine to the solution to facilitate its crystallisation. Hence the alum prepared must have been mainly ammonium alum. Later, the urine was replaced by potash in the manufacture of alum. The fact that alum contains two bases, alumina and potash, was discovered by Marggraf and emphasised by Lavoisier, but was not generally accepted until 1797, when Chaptal and Yanquelin showed that potash was an essential constituent of ordinary alum, but could be replaced by ammonia, and that alum could be prepared from alunite without the addition of potash because potash was already present in the mineral. It should be mentioned that the presence of potash in alum was known to Bergman and Scheele, but they looked upon it only as an impurity.

The alum-shales at Whitby have been largely worked for alum since the time of Queen Elizabeth, though not worked at the present time. The shales found in the West Riding of Yorkshire, however, are still utilised to a small extent. Unless sufficiently bituminous, the shales are mixed with fuel and slowly roasted; but some shales contain sufficient carbonaceous matter to render the addition of fuel unnecessary. The roasting lasts for ten days, and the temperature never reaches a red heat. The product, which is light red, soft, and porous, is treated with sulphuric acid (density, 1.35) at 110° in lead-lined vessels. The solution of impure aluminium sulphate thus obtained is mixed with potassium sulphate and rapidly cooled with stirring to produce small crystals of "alum meal." The meal is drained, washed with mother liquor from "block alum," dissolved to form a hot, saturated solution, and treated in leaden vessels with a little size, which precipitates a quantity of insoluble matter. The clear solution is then run into tubs fitted with movable, lead-lined staves. After several days the staves are removed and a hole is bored in each mass of " block alum " to allow the mother liquor to drain away. The " block alum " when broken up is ready for the market. When shale is employed containing much iron, it is the custom to add potassium chloride, either alone or mixed with sulphate, instead of adding potassium sulphate to the solution of aluminium sulphate. By this means contamination of the alum with isomorphous iron alum is prevented, the iron remaining behind in solution as ferric chloride.

Potassium alum becomes white and opaque on the surface when exposed to the air, a change brought about by the absorption of ammonia and formation of a basic salt. When dried over sulphuric acid or heated to 61°, alum loses 18 molecules of water of crystallisation.

When an alkali is slowly added to a solution of alum, a precipitate is produced which redissolves on stirring until a certain amount of alkali has been added, after which the further addition of alkali leads to the production of a permanent precipitate. The solution which is on the point of yielding a permanent precipitate is known in commerce as neutral alum, and is used in dyeing, since it readily gives up alumina to the colouring matter. When heated to 40°, a precipitate is produced the composition of which may be represented by the formula K₂SO₄.Al₂(SO₄)₃.4Al(OH)₃, i.e. it is identical in composition with alunite. The precipitate may be obtained in the crystalline form by heating the solution in a sealed tube to 230°.

Crystals of alum having a cubic habit are obtained by the crystallisation of an aqueous solution of the salt to which a little alkali has been added. A study of the crystallisation of alum from hydrochloric acid solutions of different concentrations, and at various temperatures, has shown that alum crystals occasionally exhibit the faces of the pentagonal dodecahedron (210).

Potassium aluminium sulphate octahydrate, K₂SO₄. Al₂(SO₄)₃.8H₂O, separates in slender crystals when alum is melted in its water of crystallisation and the fused mass maintained at about 86°. Analogous salts are known containing indium, thallium, and the rare earth metals in the place of aluminium.