Furthermore, azo dyes with hydroxyl group at ortho to azo band dis-
play tautomerism depending on the proton transfer. Tautomers are
structural isomers of the same chemical substance that usually have dif-
ferent technical properties. Small changes in molecular structure or sol-
vent environment can considerably change the ratio of tautomers.
Determination and characterization of tautomeric equilibrium are inter-
esting and extensively studied during the recent decades using both
theoretical calculations and experimental approaches [11–15].