This procedure separates aluminium
from beryllium, the alkaline earths, magnesium, and phosphate. For the
gravimetric determination a 2 per cent or 5 per cent solution of oxine in 2M
acetic acid may be used, 1 mL of the latter solution is sufficient to
precipitate 3 mg of aluminium. For this determination, use about 0.40 g,
accurately weighed, of aluminium ammonium sulphate. Dissolve it in 100 mL of
water, heat to 70-80 °C, add the appropriate volume of the oxine reagent, and
(if a precipitate has not already formed) slowly introduce 2M ammonium acetate
solution until a preciritate just appears, heat to boiling, and then add 25 mL
of 2M ammonium acetili solution dropwise and with constant stirring (to ensure
complete precipitation).
If the supernatant liquid is yellow, enough oxine reagent has been added. Allow to cool, and collect the precipitated aluminium 'oxinate' on a weighed sintered glass (porosity No. 4) or porous porcelain filtering crucible, and wash well with cold water. Dry to constant weight at 130-140 °C. Weigh as A1(C9H6ON)3. Precipitation may also be effected from homogeneous solution. The experimental conditions must be carefully controlled. The solution containing 25-50 mg of aluminium should also contain 1.25-2.0 mL of concentrated hydrochloric acid in a total volume of 150-200 mL. After addition of excess of the oxine reagent, 5 g of urea is added for each 25 mg of aluminium present, and the solution is heated to boiling. The beaker is covered with a clockglass and heated for 2-3 hours at 95 °C. Precipitation is complete when the supernatant liquid, originally greenish-yellow, acquires an orange-yellow colour. The cold solution is filtered through a sintered-glass filtering crucible (porosity No. 3 or 4), the precipitate washed well with cold water, and dried to constant weight at 130°C.
The solution should contain 25-50 mg of aluminium and 1.0-2.0 mL of concentrated hydrochloric acid in a volume of 150-200 mL. For practice in this determination, weigh out accurately about 0.45 g of aluminium ammonium sulphate, dissolve it in water containing about 1.0 mL of concentrated hydrochloric acid, and dilute to about 200 mL. Add 5-6 mL of oxine reagent (a 10 per cent solution in 20 per cent acetic acid) and 5 g of urea. Cover the beaker with a clockglass and heat on an electric hotplate at 95 °C for 2.5 hours. Precipitation is complete when the supernatant liquid, originally greenish-yellow, acquires a pale orange-yellow colour. The precipitate is compact and filters easily. Allow to cool and collect the precipitate in a sintered-glass filtering crucible (porosity No. 3 or 4), wash with a little hot water and finally with cold water. Dry at 130 °C. Weigh as A1(C9H6ON)3.