Alum (Alumen; Potassa-alum; Sulphate of Alumine and Potassa; K2 Al2SO; 24H3O; 948.) -This is the commercial alum, consisting of sulphate of alumine and sulphate of potassa. Large, colorless, octahedral crystals, sometimes modified by cubes, acquiring a whitish coating on exposure to air, odorless, having a sweetish, astringent taste, and an acid reaction. Soluble in 10.5 parts of water at 15° 0. (59°F.), and in 0.3 part of boiling water; insoluble in alcohol. Alum is used in the manufacture of mineral waters to introduce the insoluble component "alumina" in soluble form. It is employed where chloride of aluminium is excluded, but sulphuric acid or sulphate of potassium are admitted. It is generally dissolved for immediate use in half its weight of boiling water.

Solution

A ready-made solution may be prepared by dissolving one part by weight in 99 parts of distilled water (one per cent). Filter. This diluted solution will not separate crystals in cold weather. Proportion to be employed: 100 to 1.

Alum (Alumen Natricum; Soda-alum; Sulphate of Alumine and Soda; NaO,S03; Al2O33S03; 242.4.) - This consists of sulphate of alumine and sulphate of soda. Soda-alum is soluble in 2 parts of cold water, and easily decomposes by atmospheric influence. In these two parts it differs from potassa-alum. It is employed in the manufacture of artificial mineral waters to introduce the insoluble "alumina" in soluble form, where chloride of aluminium is excluded, but sulphuric acid or sulphate of sodium are admitted.

Solution

One part by weight in nine parts of distilled water (10 per cent). Filter. Proportion 10 to 1.