2,​7-​Naphthalenedisulfoni​c acid, 4-​hydroxy-​5-​[2-​[4-​[2-​[1-​hydroxy-​6-​(phenylamino)​-​3-​sulfo-​2-​naphthalenyl]​diazenyl]​-​5-​methoxy-​2-​methylphenyl]​diazenyl]​-​, sodium salt (1:3)