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?


closed as unclear what you're asking by Zhe, Jon Custer, Jan, NotEvans., Todd Minehardt Jun 8 '17 at 18:31

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    $\begingroup$ The intersection of plumbing and blue is copper. Copper oxide is black. It will react with acetic acid to make copper acetate which is blue when hydrated. $\endgroup$ – Ben Norris Jun 8 '17 at 19:24
  • $\begingroup$ @BenNorris It's definitely more likely then lanthanium nitrate, still unless OP finds out copper out there, it still may be like anything one could dump into sewage. $\endgroup$ – Mithoron Jun 8 '17 at 20:12
  • $\begingroup$ I understand that the information given might not be enough to provide a definite answer but I'm afraid I simply haven't got more information about my problem myself... $\endgroup$ – MrFrety Jun 9 '17 at 9:26