Question

Presence of $A{l}^{3+}$ cations in a solution can be confirmed using:

• A. potassium Ferro cyanide
• B. vandate ion
• C. Both A and B
• D. $NaOH$

Answer 1

The correct option is C Both A and B

The first indicative test for $A{l}^{3+}$ is the formation of a white ppt with $NaOH$ and Ammonia, with the dissolving on excess $NaOH$, but not with excess ammonia. Then, the blue ppt or deep blue solution (depending conc. of $A{l}^{3+}$) with acidified Potassium Ferro(II)Cyanide, or the Yellow-Orange precipitate obtained from the Vandate ion are valuable unique tests to confirm Al, since Tin, Zinc, Lead and Bismuth have different or no reactions with these two. For instance lead and zinc forms a white ppt with the ferrocyanide. Additionaly, the intense white ppt with Sodium Benzoate, and a white ppt with the Thio Sulphate are enough to further confirm the Aluminium cation.