# How to know dichromate is being reduced from Cr⁶⁺ to Cr³⁺?

When potassium dichromate is being used as an oxidising agent, it gets reduced from chromium(VI) to chromium(III). Is there any way to know that or calculate that other than being familiar with general oxidation states?

• Long story short, no. – Ivan Neretin Aug 30 '19 at 5:45

Similar to other redox reactions (e.g., permanganate [purple] to either manganate [greenish, at a basic pH] or $$\ce{Mn^{2+}}$$ [colorless, at an acidic pH]), the reduction of chromate to $$\ce{Cr^{3+}}$$ in aqueous solutions occurrs with a significant change of color of this solution. Even more in acidic solutions if you have dichromate being reduced to $$\ce{Cr^{3+}}$$. And you could infer «being there» since there are both stable compounds with chromate / dichromate, as well as $$\ce{Cr^{3+}}$$ you can dissolve in water as a visual reference to check with.
That is the colour change. There are several reactions of colour change as you are probably more familiar with the orange dichromate(VI) ion ($$\ce{Cr2O7^{2-}}$$) than the yellow chromate(VI) ion ($$\ce{CrO4^{2-}}$$) . Changing between them is easy. If dilute sulfuric acid is added to the yellow solution, it turns orange. If you add sodium hydroxide solution to the orange solution it turns yellow. There are more reaction of this you can find on internet and this reaction itself is elaborated in the following manner: \begin{align} \ce{H2SO4 + 2Na2CrO4 &-> Na2Cr2O7 + Na2SO4 + H2O}\\ \ce{Na2Cr2O7 + 2NaOH &-> 2Na2CrO4 + H2O} \end{align}