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In a Lowenthal-Permanganate titration, cider tannins are quantified by titrating the cider with indigo carmine indicator and sulfuric acid against potassium permanganate. Tannins are antioxidants, so that means they give up electrons to reduce the potassium permanganate. The endpoint of this titration is when the blue analyte turns a yellow colour. According to Wikipedia, indigo carmine turns yellow when reduced.

What I don't quite understand is why the indigo carmine didn't reduce when it was in the presence of the antioxidative tannins before the potassium permanganate was added and why it turned yellow only after potassium permanganate was added.

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  • $\begingroup$ Could there be an error of switched colours ? Titration blue to yellow makes little sense, analysts always try light to heavy color transitions. Check independent sources. $\endgroup$
    – Poutnik
    Oct 25, 2020 at 11:02
  • $\begingroup$ Thanks for the comment @Poutnik, I actually carried out this titration myself and the colours were correct. I based my procedure on procedures used in academic journals such as this one: onlinelibrary.wiley.com/doi/pdf/10.1111/1750-3841.14713. $\endgroup$
    – user100377
    Oct 25, 2020 at 11:06
  • $\begingroup$ Hm, I was just assuming that as an option. $\endgroup$
    – Poutnik
    Oct 25, 2020 at 11:14
  • $\begingroup$ Can it be gallates are not strong enough to reduce indicator, which is oxidized farther by permanganate ? Similarly as high pH discolour phenolphthalein ? $\endgroup$
    – Poutnik
    Oct 25, 2020 at 11:17
  • $\begingroup$ It's very possible I'm just not understanding this fully, but if the indigo carmine was oxidized by permanganate, wouldn't it be blue? $\endgroup$
    – user100377
    Oct 25, 2020 at 11:22

2 Answers 2

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After the endpoint of this titration you have an excess of permanganate, so your indicator is going to be oxidized, not reduced. It is confusing because most everything you find about indigo carmine as a redox indicator is about its reversible reduction to its leuco form, which is indeed yellow. However, like most dyes, indigo derivates can also be irreversibly degraded ("bleached") by strong oxidizers like acidic permanganate, and that is what's going to happen here.

The tannins do not reduce the indigo carmine before or during the titration, as that reaction would be either unfavorable or very slow, and are not supposed to. They do react rapidly with the much stronger oxidizer permanganate as it is added. Once all tannins are consumed, the excess permanganate will attack the indigo carmine, oxidizing it to, presumably, isatin sulphonic acid which in dilute solution is also yellowish.

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I did some brief looking but given that the carmine is also pH indicator that remains blue at below 11.4 pH it would appear as though your sulfuric acid is preventing the reduction from occurring in your indicator. Under acidic conditions KMnO4 is a strong oxidizer where carmine is a weak oxidizer under these conditions. However I am not sure how the carmine gets reduced to yellow upon equilibrium. Perhaps the pH changes. It appears as though permanganates acid conjugate is strong so its a weak base. Just throwing out ideas there though.

http://chem.lapeer.org/Chem1Docs/IndigoCarmine.html

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/permanganate

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