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. It is approved for use as a food colorant in the United States and the EU and has the E number E132.
Read more about Indigo Carmine: Uses, Health Concerns