Inorganic Chemicals

Formula: Al2(SO4)3 18H2O; MW 648.41

Synonyms: alum, cake alum (the term alum also refers to aqueous solutions of this substance, as well as other hydrate salts containing varying number of waters of crystallization; also the term alum applies to a whole class of sulfate double salts, such as potassium aluminum sulfate or ammonium aluminum sulfate.)

Uses
The compound is used heavily in the paper industry. In its acidic solution form, which contains a slight excess of H2SO4, it is used for pH control of pulp slurries, treatment of process waters, setting of dyes and also for precipitating dissolved resin for setting the size on the fibers. In slight basic form (containing a slight excess of Al2O3), it is used for treatment of drinking and waste waters (e.g., for reducing phosphorus content). Other major applications include dyeing, tanning, catalysts, modification of concrete, and in the manufacture of various chemicals and pharmaceutical products.

Physical Properties
White crystal; sweet taste; density 1.62 g/cm3: decomposes at 86.5°C; soluble in water.

Preparation
Prepared from bauxite, kaolin or aluminum compounds on reaction with H2SO4. The insoluble silicic acid is filtered out; the hydrate salt forms on crystallization.