2,4-Bis(4-nitrobenzazo)resorcinol-6-sulphonic acid as a new acid-base indicator

2,4-Bis(4-nitrobenzazo)resorcinol-6-sulphonic acid is suggested as a new visual acid-base indicator, which because of its colour sharpness and other qualities is useful in titrations of weak acids. There are two colour transitions, one from yellow to red at pH 6.8-8.2, the other from red to blue-violet at 10.5-13.0. The corresponding dissociation constants are pK(1) 7.64, and pK(2) 11.64.