NUCLEAR-MAGNETIC-RESONANCE INVESTIGATIONS OF AZO-HYDRAZONE, ACID-BASEEQUILIBRIA OF FD-AND-C YELLOW NO-6

Azo-hydrazone, acid-base equilibria were examined for the water-solubl e dye FD&C Yellow No. 6 (Y6), with the observation of dynamic NMR effe cts near, and at, pH 12, where the dye converts from a predominantly h ydrazone tautomer (acidic to moderately basic pH) to a prevailing azo- anion resonance hybrid (high pH). Semilogarithmatic pH-C-13 chemical s hift plots and a 2-bond, N-15(beta)-C-13(8a) coupling constants vs. pH relationship both indicate that the midpoint of the azo-hydrazone equ ilibrium occurs at ca. pH 12. Potentiometric titrations further indica te that the pK(a) of Y6 is ca. 12. These data suggest that the observe d NMR line broadening is due to slow proton transfer between the hydra zone-NH and water. (C) 1996 Elsevier Science Ltd.