Why Did the Permanganate Ion and Glucose Reaction Change Colors?

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danago
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Hi.
In class recently, we performed a reaction where we added potassium permanganate solution to a solution of sodium hydroxide, followed by a few grains of glucose. The resulting solution was green for a short time, and then quickly went to an orange colour.

Im not very sure about what happened in this reaction. I think that the permanganate ion was reduced to an MnO42- ion, which is the reason for the green colour, but why did it then go orange? And what is the purpose of the glucose? Was it the reducing agent?

Thanks in advance,
Dan.
 
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danago said:
Im not very sure about what happened in this reaction. I think that the permanganate ion was reduced to an MnO42- ion, which is the reason for the green colour
This sounds right. Here's a Pourbaix diagram for manganese:
https://www.wou.edu/las/physci/ch462/pourbxmn.gif

danago said:
but why did it then go orange?
My guess is that the Mn(VI) in the green MnO42- was reduced to brown solid MnO2.

danago said:
And what is the purpose of the glucose? Was it the reducing agent?
Yes. Glucose is a reducing sugar.

EDIT: To be completely explicit: with a solution of KMnO4, you start way up in the purple region of the Pourbaix diagram. When you add NaOH, that pushes you toward the right of the diagram. Then adding glucose pushes you downward, presumably through the green region into the brown and red regions.