Indigo Carmine, also known as 5,5′-indigodisulfonic acid sodium salt, is an organic salt derived from indigo pigment by sulfonation, which renders the compound soluble in water. It is approved for use as a food colorant in the United States and the European Union.
Usage as a pH indicator
Indigo Carmine in a 0.2% aqueous solution is pH indicator. Indigo Carmine turns blue at pH 11.4 and yellow at 13.0. Indigo Carmine is also a redox indicator, which makes it turn yellow upon reduction.