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