4,7-Dihydroxynaphthalene-2-sulfonic acid [6357-94-4]. (2,5-dihydroxynaphthalene-7- sulfonic acid), dioxy J acid, C10H8O5S, Mr 240.2, is readily soluble in water; its sodium salt is much less soluble and is easily salted out. Amination with NH3– NaHSO3 yields J acid.
?Although not used as a precursor for J acid itself, 4,7-dihydroxynaphthalene-2-sulphonic acid can be used advantageously for the production of some N-substituted J acids.
2-Hydroxynaphthalene-5,7-disulfonic acidis heated with aqueous NaOH – KOH for 5 h at 175℃. The melt is poured into water and acidified with hydrochloric acid, and the precipitated solid is filtered off and washed with brine; the yield is 80 %.
