Aluminum confirmatory test
We start from the assumption that, if present, aluminum, is in solution as aluminate (AlO2-), soluble complex in an alkaline environment.
The first method that we are going to explain is really simple, based on the acid-base properties of aluminum. The second one, on the other hand, is very specific and very sensitive.
1) Treatment with ammonium chloride (NH4Cl)
To the solution that might contain aluminate is added with solid ammonium chloride. NH4Cl acts as an acid (the ammonium ion gives acidic hydrolysis), neutralizing OH- and restoring the conditions to precipitate aluminum hydroxide. Chromium (III) and its hydroxide would not interfere because we previously did the treatment with hydrogen peroxide, and chromium (if present) would now be present as chromate.
AlO2- + H2O + NH4+ Al(OH)3 + NH3
The addition of H+ moves this equilibrium to the hydroxide form. NH4+ is not acidic enough to solubilize the hydroxide.
2) Treatment with Alizarin
The use of alizarin is surely the best way to detect even traces of aluminum. It forms a colored complex in violet, definitely noticeable. The essay is probably too sensitive, Alizarin may detect indeed even really really slight amount of aluminum, even the traces present on the test tube glass.
For this reason it is recommended that you run a blank test (solution not containing aluminum).
You can also perform a double check. Add acetic acid (CH3COOH) to the violet colored solution. The solution turns lighter, tends to pale pink, more and more diluted. Subsequently a red precipitate should form (Alizarin).