Electronic absorption spectra and acid—base equilibria of some arylaminoanthraquinones in aqueous medium

The visible absorption spectra of 1,4-di-(4′-methylanilino)anthraquinone (Alizarin Cyanin Green) and its disulphonated derivative (Quinizarin Green); 7,8-dihydroxy-1,4-di-(2′-sulpho-4′-methylanilino)anthraquinone (Alizarin Viridine); 1-hydroxy-4(2′-sulpho-4′-methylanilino)anthraquinone (Alizarin Irisol R) and 1,4-dihydroxy-2-(2′-sulpho-4′-methylanilino)anthraquinone (Alizarin Heliotrope) have been recorded in aqueous solutions at different pH values. The spectral changes are explained in terms of shifts in equilibria between different molecular and ionic species in the solutions. The pKa values have been determined for different acid-base equilibria and discussed in relation to molecular structure. Release of a proton from the NH-group in 1,4-bis(arylamino) derivatives is liable to occur through mesomeric interaction between the imino-nitrogen and CO group. The mesomeric shift is influenced by the pH of solution as well as the nature of compound.