Abstract
The acid dissociation equilibria in aqueous solution of dopamine were determined by potentiometry and complementary tristimulus colorimetry (CTS method), at 25°and μ=0.1 (NaClO4). From these data, microscopic equilibrium constants were calculated by the use of the substitution method. At physiological pH region (pH 7.2-7.4), dopamine is present as an ammonium species (I), and at pH 9.75 as zwitter-ionic species, (III) and (IV) in about 30% and 20%, respectively. The concentration of mono-phenolate anionic forms, (V) and (VI), are about 40% and 30%, respectively, at pH 11.0.
