Molecular formula. Al(OH)3
Relative molecular mass. 78.00
Chemical name. Aluminium hydroxide; CAS Reg. No. 21645-51-2.
Description. A white, fine, amorphous powder; odourless.
Solubility. Practically insoluble in water and ethanol (~750 g/l) TS; soluble in hydrochloric acid (~70 g/l) TS and sodium hydroxide (~80 g/l) TS.
Storage. Aluminium hydroxide should be kept in a tightly closed container.
Definition. Aluminium hydroxide contains not less than 71.9% and not more than 94.9% of Al(OH)3.
Dissolve 0.10 g by heating in 5 ml of sodium hydroxide (~80 g/l) TS. To the clear solution add 0.5 g of ammonium chloride R; a white, gelatinous precipitate is produced.
Heavy metals. For the preparation of the test solution dissolve 0.5 g, while heating, in 5 ml of acetic acid (~300 g/l) TS, dilute to 10 ml with water and filter. Adjust the pH of the filtrate to 3-4, dilute to 40 ml with water and mix. Determine the heavy metals content as described under 2.2.3 Limit test for heavy metals, according to Method A; not more than 60 μg/g.
Arsenic. Use a solution of 3.3 g in 20 ml of sulfuric acid (~100 g/l) TS and 35 ml of water and proceed as described under 2.2.5 Limit test for arsenic; the arsenic content is not more than 5 μg/g.
Ammonium salts. Transfer 5.0 g to an ammonia distillation apparatus, add 25 ml of sodium hydroxide (~200 g/l) TS and 200 ml of water, distil about 100 ml, collecting the distillate in 25.0 ml of hydrochloric acid (0.1 mol/l) VS.
Titrate the excess acid with sodium hydroxide (0.1 mol/l) VS using methyl red/ethanol TS as indicator; not less than 22.5 ml of sodium hydroxide (0.1 mol/l) VS is required.
Chlorides. Dissolve 0.10 g in 2 ml of nitric acid (~130 g/l) TS, boil, cool, dilute to 10 ml with water and filter. Proceed with 5 ml of the filtrate as described under 2.2.1 Limit test for chlorides; the chloride content is not more than 10 mg/g.
Sulfates. Dissolve 0.10 g in 5 ml of hydrochloric acid (~70 g/l) TS, boil, cool, dilute to 10 ml with water and filter. Proceed with the filtrate as described under 2.2.2 Limit test for sulfates; the sulfate content is not more than 5 mg/g.
Neutralizing capacity. Pass a sufficient quantity of the test substance, triturated if necessary, through a sieve of nominal mesh aperture 150 μm, add 0.50 g to 200 ml of hydrochloric acid (0.05 mol/l) VS previously heated to 37 °C, and stir continuously, maintaining the temperature at 37 °C; the pH of the solution at 37 °C, after 10, 15, and 20 minutes, is not less than 1.8, 2.3, and 3.0, respectively, and at no time more than 4.0. Add 10 ml of hydrochloric acid (0.5 mol/l) VS previously heated to 37 °C, stir continuously for 1 hour, maintaining the temperature at 37 °C. Titrate the solution with sodium hydroxide (0.1 mol/l) VS to pH 3.5. The neutralizing capacity is not less than 83.3% of the theoretical amount when calculated by the formula (1000)(150-y)/A × W × 38.46, in which y is the number of ml of sodium hydroxide (0.1 mol/l) VS required, A is the percent of Al(OH)3 obtained in the assay, W is the quantity, in g, of test substance taken, and 38.46 is the theoretical value of each g of Al(OH)3.
Alkaline impurities. The pH of a 0.04 g/ml suspension in carbon-dioxide-free water R is not more than 10.0.
Assay. Proceed with about 0.15 g, accurately weighed, as described under 2.5 Complexometric titrations for aluminium. Each ml of disodium edetate (0.05 mol/l) VS is equivalent to 3.900 mg of Al(OH)3.