 The sulfonic acid derivatives of 1-naphthylamine are used for the preparation of azo dye.These compounds possess the important property of dyeing unmordanted cotton.An important derivative is naphthionic acid (1-aminonaphthalene-4-sulfonic acid), which produced by heating 1-naphthylamine and sulfuric acid to 170–180 °C in the presence of crystallized oxalic acid.