DEFINITION
Content: 94.0 per cent to 102.0 per cent of AlPO4 ignited substance).
CHARACTERS
Appearance: White or almost white powder.
Solubility: Very slightly soluble in water, practically insoluble in alcohol. It dissolves in dilute solutions of mineral acids and alkali hydroxides.
IDENTIFICATION
A. Solution S (see Tests) gives reaction (b) of phosphates.
B. Solution S gives the reaction of aluminium.
TESTS
Solution S: Dissolve 2.00 g in dilute hydrochloric acid R and dilute to 100 ml with the same acid.
Appearance of solution: Solution S is clear and colourless.
pH: 5.5 to 7.2
Shake 4.0 g with carbon dioxide-free water and dilute to 100 ml with the same solvent.
Chlorides: Maximum 1.3 per cent.
Soluble phosphates: Maximum 1.0 per cent, calculated as PO4.
Sulphates: Maximum 0.6 per cent.
Arsenic: Maximum 1 ppm.
Heavy metals: Maximum 20 ppm.
Loss on ignition: 10.0 per cent to 20.0 per cent, determined on 1.000 g at 800 ± 50C.
Neutralising capacity: Add 0.50 g to 30 ml of 0.1 M hydrochloric acid previously heated to 37C and maintain at this temperature for 15 min while stirring. The pH of the mixture after 15 min at 37C is 2.0 to 2.5.
ASSAY
Dissolve 0.400 g in 10 ml of dilute hydrochloric acid and dilute to 100.0 ml with water. To 10.0 ml of the solution, add 10.0 ml of 0.1 M sodium edta and 30 ml of a mixture of equal volumes of ammonium acetate solution and dilute acetic acid. Boil for 3 min, then cool. Add 25 ml of alcohol and 1 ml of a freshly prepared 0.25 g/l solution of dithizone in alcohol. Titrate the excess of sodium edta with 0.1 M zinc sulphate until the colour changes to pink. 1 ml of 0.1 M sodium edta is equivalent to 12.20 mg of AlPO4.
