Synthesis and spectroanalytical studies of a new heterocyclic acid-base indicator

New purple dyes of the imidazopyridine series were synthesized by the reaction of 6-chloro-3-nitroimidazo[1,2-a]pyridine with arylacetonitriles in basic MeOH solution via nucleophilic substitution of hydrogen atom. The obtained 2-aryl-2-[6-chloro-3-(hydroxyimino) imidazo[1,2-a]pyridin-2(3H)-ylidene)]acetonitriles were further converted to new 8-chloropyrido[2',1':2,3]imidazo[4,5-b]-quinoline-12-carbonitriles. The solution of 2-[6-chloro-3-(hydroxyimino)imidazo[1,2-a]pyridin-2(3H)-ylidene)]-2-(4-methylphenyl)acetonitrile undergoes a reproducible color change due to the addition of acid or base. A UV-Vis spectroscopic characterization and acid-base equilibrium study of this dye was carried out with the view of its possible use as an indicator. The results show that this dye is an amphoteric substance with two ionization constants K (a1) and K (a2) indicative of weak basic and acidic properties. The pK (a1) and pK (a2) values in aqueous solution determined by spectrophotometric method are 2.43 and 9.73, respectively. A comparison was made with existing acid-base indicators effective over the same pH range.