Microscopic Acid Dissociation Constants of 3,4-Dihydroxyphenethylamine (Dopamine)

Abstract

The acid dissociation equilibria in aqueous solution of dopamine were determined by potentiometry and complementary tristimulus colorimetry (CTS method), at 25° and |x= 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. © 1978, The Pharmaceutical Society of Japan. All rights reserved.