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I am in a physics lab studying dislocation, and we submerged a bismuth antimony sample in nitrogen acid (about 50% concentration, if I remembered it right) as the first step to etch it. There were bubbles, and the solution turned green. We googled and found bismuth nitrite should be colorless, so the greenish solution became a mystery.

I suspect there might be residues of copper in the beaker, the only possibility I can think of because I am terrible at chemistry. But we did clean the beaker with soap and paper towel. Can I get some chemist insight to this situation?

Edit: I forgot to mention that the green color persisted even after we took the sample out.

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    $\begingroup$ I think I've heard of this happening in some cases of metal dissolution with nitric acid due to transient formation of lower nitrogen oxides which can be blue-green in colour (e.g. dinitrogen trioxide). You could also be getting some nitrosyl metal complexes. $\endgroup$
    – Nicolau Saker Neto
    Commented Oct 23, 2023 at 23:08
  • $\begingroup$ @NicolauSakerNeto That was really helpful! I forwarded your insight to my lab members. Thank you so much. $\endgroup$
    – Yuting Z
    Commented Oct 23, 2023 at 23:35
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    $\begingroup$ What is "nitrogen acid"? Do you mean Nitric acid? $\endgroup$
    – Nilay Ghosh
    Commented Oct 24, 2023 at 3:30
  • $\begingroup$ @NilayGhosh Oh yes sorry I kinda improvised on that name $\endgroup$
    – Yuting Z
    Commented Oct 25, 2023 at 6:10

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Copper could have come from the sample. From Belmont Metals:

Bismuth as an alloying element can be used in the production of fusible alloys ( low melting point alloys ) and Bismuth based low temperature solders. Standard and special compositions are Bismuth alloyed with either Antimony, Cadmium, Copper, Indium, Lead or Tin.

Copper might be only a minor element, but a little bit can go a long way when it comes to color perception.

One way to check for copper is to add thiourea to a sample of your solution. Thiourea will decolorize solutions containing copper(II) by reducing it to copper(I).

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