Alum

ALUM (Alumen, VII), double salts of sulfuric acid, one of the metals (cations) of which is trivalent and the other monovalent—M₂^III(SO₄)₃ + M^I₂SO₄ + 24H₂O, where M^I is replaced by K, Na, NH₄, Rb, Tl, or Ag; M^III is Al or a 3-valent element from the Fe or In group, or V, Co, Ir, or Rh. If M^III is Al and M^I is K, then potassium alum is obtained. If in the latter K is replaced by Na, NH₄, etc., then sodium, ammonium, etc., alums are obtained; if Al is replaced, for example, by Cr, then chrome-potassium alum is obtained; if in potassium alum Al is replaced by Fe and K by NH₄, then iron-ammonium alum is obtained. Alums usually crystallize well, as a result of which they are easily obtained free from foreign impurities, but they are isomorphous with each other and cannot be separated by crystallization. They are soluble in water, insoluble in fats, alcohol, etc. Due to the weakly expressed basic character of the trivalent cations, alums possess an acidic reaction and an acidic taste (kwas in Polish means acid). (In medicine, only potassium alum is used.) Alums are widely used in microscopic technique in view of their ability to form coloring lakes with a whole series of dyes, for example, with carminic acid and hematein. These lakes are insoluble in water and soluble in the corresponding alums; therefore, alums can serve both as a mordant and for differentiation. The most commonly used is potassium alum (usually called simply alum), Al₂(SO₄)₃ + K₂SO₄ + 24H₂O, large colorless regular octahedral crystals; it is highly soluble in acidified water (at 15°—12%, at 100°—35.7%). If alkali is carefully added to the resulting solution, a precipitate forms, which dissolves upon further addition of alkali. Upon careful heating to 100° with access to air, potassium alum loses its water of crystallization and turns into burnt alum (Alumen ustum), soluble in water up to 5%. In air, it effloresces slightly. Sodium alum, Al₂(SO₄)₃ + Na₂SO₄ + 24H₂O, is similar to the previous one, effloresces more easily, and is more easily soluble. Ammonium alum, Al₂(SO₄)₃ + (NH₄)₂SO₄ + 24H₂O, is similar to potassium alum. Chrome alum, Cr₂(SO₄)₃ + K₂SO₄ + 24H₂O—dark violet octahedral crystals; it effloresces slightly; it dissolves in water to 14% and forms a violet liquid, which turns green upon boiling and becomes violet again upon cooling. Iron-ammonium ("iron") alum (Ferrum sulfuricum oxydatum-ammoniatum), Fe₂(SO₄)₃ + (NH₄)₂SO₄ + 24H₂O—large octahedral crystals of a violet color. It effloresces very easily and becomes covered with a yellowish coating. At 15° it dissolves up to 33%, forming a light yellow liquid, in which a precipitate of basic ferric sulfate forms over time.