How is indigo carmine used in Lowenthal-Permanganate titration as a redox indicator? 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.
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