Prepn from p-phenylenediamine and aniline hydrochloride: Nietzki, Ber. 16, 474 (1883); from p-aminoazobenzene: Barbier, Sisley, Bull. Soc. Chim. Fr. [3] 33, 1232 (1905); by reducing bis(p-nitrophenyl)nitroxide with tin chloride and hydrochloric acid: Wieland, Roth, Ber. 53, 213, 226 (1920).