Around my faucet was some mysterious dirt from a previous tenant. I poured acetic acid with a concentration of 25 percent over it. I came back after a few hours and noticed some blue stains which hadn't been there before. What is this blue substance? Is it poisonous? What must have been in the dirt to produce this?
In Wikipedia I came across a statement about acetic acids and blue color:
A more sensitive test uses lanthanum nitrate with iodine and ammonia to give a blue solution.
Could the mechanism described there be what I'm looking at? Is it really possible that this dirt contains lanthanum nitrate?