Experiment 7 - Gravimetric Determination of Aluminum as Oxinate
Introduction
A number of metals form insoluble complexes with the bidentate
ligand oxine (8-hydroxyquinoline) and can therefore be
precipitated from aqueous solution on addition of this reagent.
The precipitation is dependent on the pH of the solution and this
aspect will be dealt with later on in this course.
Precipitation of Al(oxine)3
Advantages
(1) Aluminum forms an oxine complex that can be quantitatively
precipitated from aqueous solution between pH 4.2-9.8; it can
thus be precipitated from an acetic acid - acetate buffer
solution or from an ammoniacal solution.
(2) The precipitate is
crystalline, easily filtered and can be readily dried between 102
- 120°C.
(3) Precipitation from an acetic acid - acetate buffer solution
serves to separate aluminum from beryllium, barium, calcium,
strontium, and magnesium which are often associated with
aluminum.
Disadvantages
(1) Oxine is not a highly selective reagent and if aluminum
is to be precipitated from an acetic acid - acetate buffer
solution, all metals except the alkalis, alkaline earths and
magnesium must be absent.
(2) The precipitate tends to retain the reagent leading to high
results. However the method is sufficiently satisfactory for
almost all practical purposes.
(3) It is carcinogenic.
Method
Carry out the experiment in simultaneous duplicate as follows:
Pipette 10 cm3 of the aluminum solution into a 400
cm3 beaker. Add about 150 cm3 of distilled
water containing 1 cm3 of 0.1 M HCl and warm the
solution to 60 - 70 oC. Add 20 cm3 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
cm3 of 2 M ammonium acetate for each 100
cm3 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 (g/dm3) of aluminum in
the solution. Aluminum oxinate contains 5.874% aluminum.
Questions
1. Draw the structure of the aluminum complex of
oxine. Hint: see
1 form and
another form.
2. What is the purpose of the ammonium acetate in the
experiment?
3. How would you recover the oxine from the
complex?
4. Give some industrial uses of Al compounds.
5. Suggest another method to determine Al. Return to Chemistry, UWI-Mona,
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Created and maintained by Prof. Robert J.
Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created Oct 2002. Links checked and/or last
modified 19th October 2009. URL
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