All ye brilliant ones out there.....?

Question

Plz explain the blue lake test for Aluminum....What gives the blue colour???....Something to do with litmus adsorption...plz help..Clear answer will be judged BEST....

Answer 1

The presence of aluminium ions  in a salt is confirmed by performing a test called blue lake test The test is carried out by taking a solution of aluminium salt in a test tube add few drops of litmus solution then acidify with dil. HCl then add NH4OH(ammonium hydroxide) and allow to stand . The result is a blue ppt.  in colourless solution.

A Lake pigment is a pigment manufactured by precipitating a dye with an inert binder, usually a metallic salt. The metallic salt or binder used must be very inert and insoluble in the solution. Aluminium hydroxide fills this role. What hppens during the test is to form aluminium hydroxide with the NH4OH. This lakes with the litmus dye, which is blue in the alkaline solution, and the lake precipitates as it is insoluble in water. If the litmus was not added, the Al(OH)3 would be difficult to see - the litmus acts as a colour indicator that makes the Al(OH)3 easy to see.

When you do this test make sure that you definitel add sufficient NH4OH to make the solution alkaline, blue to litmus.