Is chromium (III) green or violet in solution? In terms of oxidation reactions between chromate and $\ce{Cr}$ (III) it forms a green color in solution, but when I searched online in different sources it seems to say that it is actually purple. Which is the right answer?

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    $\begingroup$ It can be anything, depending on the ligand environment. $\endgroup$ – Ivan Neretin Apr 17 '18 at 6:06
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    $\begingroup$ [Cr(H2O)6] is a violet solution. Cr(OH)3 is a grey/green precipitate. [Cr(OH)6] is dark green solution. As Ivan said, it depends on the ligands etc. $\endgroup$ – user60221 Apr 17 '18 at 6:18
  • $\begingroup$ Related: How hydrate isomerism results difference in colours? $\endgroup$ – Faded Giant Apr 17 '18 at 11:06
  • $\begingroup$ What if it was water alone though? I'm still curious on how many textbooks say for sure that the chromium (iii) in solution or in minerals such as ruby cause the green color but when I search on the internet most sources say its violet. $\endgroup$ – phi2k Apr 17 '18 at 15:50

Even in water this depends on the hydration process, $\ce{[Cr(H2O)6]Cl3}$ is purple. Also the anhydrous $\ce{CrCl3}$ when you buy it is a purple compound. The chloride-ligands exchange for water ligands like $\ce{[Cr(H2O)5Cl]Cl2 * H2O}$ which is blue-green. And in the end the $\ce{[Cr(H2O)3Cl3]*(H2O)3}$ is green. Some ions like fluoride are said to influence this process but I have not been able to confirm that yet. For Cr(III) there's this interesting phenomenon, and I think I already asked for that, that this exchange seems to be quite slow. I once prepared a Cr(III) citrate solution and it was blue-green, but after about a week it was deep purple. So this exchange may sometimes take some days.

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  • $\begingroup$ What if it was water alone though? $\endgroup$ – phi2k Apr 17 '18 at 15:49

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