Indigo Carmine Related Compound 2

1,1',3,3'-Tetrahydro-3,3'-dioxo-Δ2,2'-bi[2H-indole]-5,7'-disulfonic acid disodium salt

disodium 2-(1,3-dihydro-3-oxo-7-sulphonato-2H-indol-2-ylidene)-3-oxoindoline-5-sulphonate