Indigo carmine, or 5,5'-indigodisulfonic acid sodium salt, also known as indigotine or FD&C Blue #2, is a pH indicator with the chemical formula C16H8N2Na2O8S2.
I 1275 (OTTO) Indigo carmine, GR Cas 860-22-0 - used as a pH indicator, often in a 0.2% aqueous solution. It is blue at pH 11.4 and yellow at 13.0. Indigo carmine is also a redox indicator, turning yellow upon reduction.