text: 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.