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I had the reaction:

$$\ce{Cr(H2O)6^3+ -> Cr(H2O)6^2+}$$ using $\ce{Zn}$ as a reducing agent.

the colour turning from green to blue and I know that with contact with air the solution becomes green again because the $\ce{Cr(II)}$ oxidizes to $\ce{Cr(III)}$ but what is the reaction for that?

I was thinking something along the lines of:

$$\ce{Cr^2+ + O2 -> Cr2O3}$$

and since $\ce{Cr2O3}$ is green it explains the colour we see after $\ce{Cr(H2O)6^2+}$ reacts with air.

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    $\begingroup$ You get the reactant back, your very first equation. Use the oxygen half cell oxygen in an acidic medium. $\endgroup$ – M. Farooq Jun 22 at 22:32

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