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Narration : 2. Aluminon Lake

• Aluminum ion can be detected by the red lake formed by the absorption of organic pigment aluminon on aluminum hydroxide.
• However, because the aluminon reagent itself is red, the change is very subtle and you should observe the change very carefully.
• As aluminon lake is generated in the ammonium acetate buffer solution, you need to add it to the sample.
• Next, add the aluminon reagent.
• If the reaction solution is highly acidic, an orange precipitate will be produced. This is not aluminon lake but the protonated aluminon.
• Next, add ammonia water drop by drop and observe carefully the change of the reaction solution.
• At this time, do not continuously add ammonia water.
• When a proper amount of aqueous ammonia water has been added, the red color of the reaction solution turns deep and a gel-like precipitate is formed.
• This is aluminon lake. You should pay attention to not only the color change but also the formation of the precipitate.
• The pH value of solution at this time is between 5 and 7.
• Allow the reaction solution to stand a little while. When the red precipitate of aluminon lake separates from the colorless supernatant, the presence of aluminum is confirmed.
• If the gel-like precipitate remains in the solution, centrifuge and observe the reaction solution.
• When an excess amount of aluminon reagent is added, the supernatant will exhibit red color.
• Some of you may miss aluminon lake and add an excess amount of aqueous ammonia.
• In this case, the reaction solution becomes alkaline whose pH is outside the appropriate range.
• As a result, the color of lake fades away and aluminum ion cannot be detected.
• When an aluminum sample contains ferric ion, the color of the aluminon lake formed is not red but purple.
• Because both aluminum ion and ferric ion belong to group III, you should carefully separate these ions and observe the lake color when analyzing an unknown sample.