Determination of aluminium back-titration

Pipette 25 mL of an aluminium ion solution (approximately 0.01 M) into a conical flask and from a burette add a slight excess of 0.01 M EDTA solution adjust the pH to between 7 and 8 by the addition of ammonia solution (test drops on phenol red paper or use a pH meter). Boil the solution for a few minutes to ensure complete complexation of the aluminium cool to room temperature and adjust the pH to 7-8. Add 50 mg of solochrome black/potassium nitrate mixture [see Section 10.50(C)] and titrate rapidly with standard 0.01 M zinc sulphate solution until the colour changes from blue to wine red. 

After standing for a few minutes the fully titrated solution acquires a reddish-violet colour due to the transformation of the zinc dye complex into the aluminium-solochrome black complex this change is irreversible, so that over-titrated solutions are lost. 

Every millilitre difference between the volume of 0.01 M EDTA added and the 0.01 M zinc sulphate solution used in the back-titration corresponds to 0.2698 mg of aluminium. 

The standard zinc sulphate solution required is best prepared by dissolving about 1.63 g (accurately weighed) of granulated zinc in dilute sulphuric acid, nearly neutralising with sodium hydroxide solution, and then making up to 250 mL in a graduated flask alternatively, the requisite quantity of zinc sulphate may be used. In either case, de-ionised water must be used. 

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