Indigo carmine, or 5,5′-indigodisulfonic acid sodium salt, is an organic salt derived from indigo by aromatic sulfonation, which renders the compound soluble in water. It is approved for use as a food colorant in the United States and European Union to produce a blue color. It has the E number E132, and is named Blue No. 2 by the Federal Food, Drug, and Cosmetic Act. It is also a pH indicator.