Preparation
3-Amino-2-hydroxybenzoic acid and 5-Amino-2-hydroxybenzoic acid?diazotization, and 5-Aminonaphthalene-2-sulfonic acid and 8-Aminonaphthalene-2-sulfonic acid coupling, product to diazotization, and 3-Hydroxynaphthalene-2,7-disulfonic acid coupling.