Indigo carmine

Indigo carmine, or 5,5′-indigodisulfonic acid sodium salt, is an organic salt derived from indigo by sulfonation, which renders the compound soluble in water. It is approved for use as a food colorant in the U.S and E.U. It has the E number E132. It is also a pH indicator. Indigo carmine, or 5,5′-indigodisulfonic acid sodium salt, is an organic salt derived from indigo by sulfonation, which renders the compound soluble in water. It is approved for use as a food colorant in the U.S and E.U. It has the E number E132. It is also a pH indicator. Indigo carmine in a 0.2% aqueous solution is blue at pH 11.4 and yellow at 13.0. Indigo carmine is also a redox indicator, turning yellow upon reduction. Another use is as a dissolved ozone indicator through the conversion to isatin-5-sulfonic acid. This reaction has been shown not to be specific to ozone, however: it also detects superoxide, an important distinction in cell physiology. It is also used as a dye in the manufacturing of capsules. In obstetric surgery, indigo carmine solutions are sometimes employed to detect amniotic fluid leaks. In urologic surgery, intravenous injection of indigo carmine is often used to highlight portions of the urinary tract. The dye is filtered rapidly by the kidneys from the blood, and colors the urine blue. This enables structures of the urinary tract to be seen in the surgical field, and demonstrate if there is a leak. However, the dye can cause a potentially dangerous increase in blood pressure in some cases.