Hello, I was looking formward to perform some quick metal analysis from a 0,5 g piece of aluminum alloy which is stated to contain 8 % silicon and copper and iron at 3 %. Since the alloy resists both hydrochloric and nitric acids, I tried to dissolve it using concentrated sodium hydroxide (sodium is not going to be analyzed on the sample) and then return it to a low pH (less than 2) with excess nitric acid to dissolve the remnants.
The sample did dissolve in NaOH quickly, but after adding acid a voluminous foam-like gray solid formed first and after some time, a white precipitate settled. This white precipitate looks a little like flocs, but is impervious to acids. Adding hydrogen peroxide only got some bubble formation from the solid, so it is probably catalizing the decomposition.
I doubt the flocs are aluminum related since those doesn't form at low pH. What compound could it be?