Prepn from 5-chloro-1-naphthalenesulfonic acid + ammonia: Oehler, DE 72336 (1893); Frdl. 3, 435; from α-naphthylamine + concd sulfuric acid: Erdmann, Ann. 275, 193, 200 (1893); Sixma, Rec. Trav. Chim. 73, 235 (1954); from 2-naphthylamine-8-sulfonic acid: Blangey, Helv. Chim. Acta 39, 977 (1956).