Locating the more acidic hydroxyl group on dihydroxy compounds o,o'-dihydroxyazo-dye metal-ion indicators

In o,o'-dihydroxyazo-dye metal-ion indicators, one hydroxyl group is 10(4) times more acidic than the other. The location of the more acidic group has been a mystery. By methylation of the acidic group, reduction of the monomethylated compound, isolation and characterization of the split methoxyamine and hydroxyamine, the location of the acidic group has been established. Indicators examined include Calmagite, Eriochrome Black T, Eriochrome Blue Black R and 4-(2-pyridylazo)resorcinol (PAR).