Indigotindisulfonic acid

2-(1,3-dihydro-3-oxo-5-sulpho-2H-indol-2-ylidene)-3-oxoindoline-5-sulphonic acid

2-(1,3-Dihydro-3-oxo-5-sulfo-2H-indol-2-ylidene)-2,3-dihydro-3-oxo-1H-indole-5-sulfonic acid