Article

Abstract

Azo-hydrazone, acid-base equilibria were examined for the water-soluble, food color additive FD&C Yellow No. 5 (Y5), with the observation of dynamic NMR effects near, and at, pH 10, where the food colorant converts from a predominantly hydrazone tautomer (acidic to moderately basic pH) to a prevailing azo-anion resonance hybrid (high pH). Potentiometric titrations further indicate that the pKa of Y5 is ca. 10. These data suggest that observed NMR line broadening is due to slow proton transfer between a hydrazone-NH and water.Key words: 1H, 13C, and 15N NMR, azo-hydrazone/acid-base equilibria, FD&C Yellow No. 5.