Acidity of several chromotropic acid azo derivatives

Both potentiometric and spectrophotometric methods have been used for the determination of the stability constants of hydrogen complexes of 4,5-dihydroxynaphthalene-2,7-disulphonic acid (Chromotropic Acid, CA), 3,6-bis(phenylazo)-4,5-dihydroxynaphthalene-2, 7-disulphonic acid (Azo III, A III), 3,6-bis(2'-sulphophenylazo)-4, 5-dihydroxynaphthalene-2,7-disulphonic acid (Sulphonazo III, SA III), 3,6-bis(4'-methyl-2'-sulphophenylazo)-4,5-dihydroxynaphthalene 2,7-disulphonic acid (Dimethylsulphonazo III, DMSA III), 3-(4'-chloro - 2' - phosphonophenylazo) -4,5- dihydroxynaphthalene -2,7-disulphonic acid (Chlorophosphonazo I, CPA I), 3,6-bis(4'-chloro-2'-phosphonophenylazo)-4,5-dihydroxynaphthalene-2,7-disulphonic acid (Chlorophosphonazo III, CPA III), 3-(2'-arsonophenylazo)-4, 5-dihydroxynaphthalene-2,7-disulphonic acid (Arsenazo I, AA I) and 3,6-bis(2'-arsonophenylazo)-4,5-dihydroxynaphthalene-2,7-disulphonic acid (Arsenazo III, AA III).