Question

Determine the purity of an Aluminum can. In words how would you determine the purity? Also, make up random numbers to calculate how to find the purity of an Aluminum can.

Answer 1

Principle:

Al³⁺ ions form an insoluble complex with the bidentate ligand oxine (8-hydroxyquinoline) and can therefore be precipitated from aqueous solution on addition of this reagent

So Al can be estimated gravimetrically as its oxinate.

Method:

Dissolve a known mass of apiece of the Al can in HCl, add water and make upto the mark in a say 250 mL std measuring flask

Pipette 10 cm³ of the aluminum solution into a 400 mL beaker. Add about 150 mL of distilled water containing 1 mL  of 0.1 M HCl and warm the solution to 60 - 70 ^oC. Add 20 mL  of a 2% solution of oxine (8-hydroxyquinoline) in 2 M acetic acid, then slowly add a 2 M solution of ammonium acetate until a precipitate forms (if one is not already formed). Then add a further 25 mL of 2 M ammonium acetate for each 100 mL  of solution. It should be faintly yellow at this stage, indicating that oxine is present in slight excess.
Allow the liquid to stand for one hour with frequent stirring then filter through a weighed No. 4 sintered glass crucible. Wash the precipitate thoroughly with cold distilled water, dry at 102 - 120 ^oC, cool and weigh. Repeat drying and weighing cycles until constant weight is obtained. (i.e perform two additional cycles)
Calculate the concentration of aluminum in the solution using the fact that

Aluminum oxinate contains 5.874% aluminum.