2,​7-​Naphthalenedisulfoni​c acid, 4-​amino-​6-​[2-​[4-​[[[4-​[2-​(2,​4-​diaminophenyl)​diazenyl]​phenyl]​amino]​sulfonyl]​phenyl]​diazenyl]​-​5-​hydroxy-​3-​[2-​(4-​nitrophenyl)​diazenyl]​-​, potassium salt (1:2)