[SomeDude]

so i was in lab the other day, trying to test a solution for presence of aluminum(III)ions.  the procedure called for a solution containing
Al³⁺(aq) + 4H2O(l) + Cr2O7 ^2-(aq)

to be placed in a test tube.  The procedure called for 4 drops of the solution being tested.  The solution being tested was of unknown composition, however it was tested alongside a standard solution known to be of the composition above.
     So, i followed the procedure, which was as follows

"Confirmation of Al+3 Ion: Place four drops of the supernatant liquid saved from the above procedure into a clean test tube. Add 4 drops of 6M HC1 (fizzing will occur). Add 4 drops of aluminon solution and then add 6M NH3 until the solution is basic to litmus. Centrifuge for one minute and decant. A red precipitate confirms the presence of Al+3. Remember a clear red solution indicates no Al3+."

However, no red precipitate formed even after 2 runs through the centrifuge.  My professor did not care to comment on what could have gone wrong.  i knew that the standard solution had to contain aluminum ions, so i went back and tried something.

I had noticed the red coloring disappear as the solution became more basic, so i went to the back of the lab, added 2 drops of 6M HCl.  The solution was a slightly red color now, however the test called for a solid.  i placed the samples in a centrifuge and got red precipitates at the bottom of each test tube.

Im guessing that "red dye aluminon" meant red dye #40 which forms an insoluble solid with Al(OH)₃ called a "red dye lake".

im confused about how the addition of HCl, which surely made the solution acidic again, aided the precipitation of the red dye lake.

Im assuming that the HCL was added to react with Aluminum and convert it to Al³⁺, so that once the NH₃ was dissociated, its conjugate base OH^- would react to form Al(OH)₃.

Did i have some type of equilibrium issue?  did i get a false positive? i am at a loss trying to explain this. my last guess is that the red dye was affected by the NH₃ somehow, but i cannot say anything definitively.

Thanks for any help, sorry i wrote so damn much.

[SomeDude]

heres a link to the lab and procedure in case it is helpful
http://faculty.ccri.edu/aahughes/GenChemII/Lab%20Experiments/Group_B_Cation_Analysis.pdf