Research of aluminium (third analytical group)

Aluminum is sought on the third analytic group such as Al +3. Along with Cr +3 and Fe +3 is precipitated as hydroxide. The aluminum hydroxide is a white gelatinous precipitate.

aluminum hydroxide

The aluminum, with the chromium, is separated from the iron on the basis of its amphoteric characteristics. And 'at its maximum oxidation state so it is inert with respect to hydrogen peroxide.

Treatment with soda and hydrogen peroxide


Thus we have shown chromium and aluminum in solution thanks to their anfoterismo. We perform the recognition aluminum essays in the same solution containing chromate.

aluminum recognition

It starts from pressupposto that aluminum, if present, is in the form of aluminate (AlO 2 -), a complex soluble in an alkaline environment.

The first method that we see is very simple, based on the acid-base properties of aluminum. The second one is very specific and very sensitive.

1) Treatment with ammonium chloride (NH 4 Cl)

To the solution containing aluminate is added solid ammonium chloride. These acts as an acid (the ammonium ion by acid hydrolysis), neutralizing hydroxyl functions OH - and bringing back the necessary conditions to precipitate aluminum hydroxide. The chromium (III) hydroxide and relative interference because we did not damage the treatment with hydrogen peroxide, and the chromium and now present as chrome. In any case, we aim to make reprecipitated the same rooms that we saw earlier.

AlO 2 - + H 2 O + NH 4 + Al (OH) 3 + NH 3

The addition of H + moves this equilibrium towards the hydrate forming.


2) Treatment with Alizarin S

aluminum complex Alizarin

The use of alizarin is surely the best way to detect even minute traces of aluminum. It forms a colored complex in violet definitely noticeable. Indeed, the essay is too sensitive, so much so that the complexing Alizarin may also really slightest amount of aluminum present on the tube glass.

For this reason it is recommended that you begin a blank test (solution not containing aluminum) to be sure of the veracity of the essay.

You can also do an additional check. At the violet colored solution in acetic acid is added (CH 3 COOH). The solution turns lighter color, tends to pale pink, more and more diluted. You should form a red precipitate (Alizarin).

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