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I was working with Aqua Regia in the lab today and was impressed by how quickly the solution (HCl and HNO3) changed colors from a colorless liquid to orange to a deep red. I didn't time it, but I would estimate that the change from clear to orange took under a minute while the color change from orange to red took about an hour.

Why does this color change occur? Why does the first stage (clear to orange) occur so quickly without mixing? I'm sure this information's out somewhere, but I couldn't find an answer with searching.

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The initial yellow/orange colour is mainly dissolved chlorine ($\ce{Cl2}$) and notrosyl chloride, ($\ce{ClNO}$), the latter affording Aqua Regia much of its potency.

The reds and browns are almost certainly due to its subsequent decomposition into nitric oxide ($\ce{NO}$) and nitrogen dioxide ($\ce{NO_2}$) as $\ce{NO_2}$ is strongly coloured.

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  • $\begingroup$ Awesome, thanks! That explains why the change of color from orange to red takes longer as well. $\endgroup$
    – Checkmate
    Jul 7, 2016 at 18:21
  • $\begingroup$ Nitric oxide is colorless. $\endgroup$
    – vapid
    Jul 8, 2016 at 13:27
  • $\begingroup$ Good point, @vapid . I'll reword it to make that clear. $\endgroup$
    – Dan Sheppard
    Jul 8, 2016 at 13:32

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