The procedure is applicable if
only alkali metals are present with the magnesium.
To a solution containing about 20mg of magnesium ion in 100mL of water, add 2 g of ammonium chloride and 0.5 mL of o-cresolphthalein indicator (0.2 per cent solution in ethanol). Neutralise with 6M aqueous ammonia solution until a violet colour is obtained (pH about 9.5) and then add 2mL of the ammonia solution in excess. Heat to 70-80 °C, add very slowly and with constant stirring a 1 per cent solution of 8-hydroxyquinoline (oxine) in 2M acetic acid until a small excess is present as shown by the yellow colour of the supernatant liquid (a large excess of the reagent should be avoided as there is a danger that the 'oxine' itself may precipitate). Digest the precipitate on the steam bath for 10 minutes with frequent stirring, and collect the precipitate on a weighed sintered-glass filtering crucible. Wash the precipitate with dilute ammonia solution (1:40) until the washings are colourless, dry to constant weight at 100-110 °C and weigh as Mg (C9H6ON)2'2H2O. Alternatively the precipitate may be dried at 155-160 °C and weighed as the anhydrous compound Mg(C9H6ON)2.