Alizarin red S is an organic sodium salt having 3,4-dihydroxy-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate as the counterion.It has a role as a histological dye. It is an organic sodium salt and an organosulfonate salt. It contains a 3,4-dihydroxy-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate.