Protolytic and Complexation Properties of Some Isomeric Aromatic Amino Acids in Aqueous Solution

Abstract— The protolytic and complexation properties of selected isomeric aromatic amino acids in aqueous solution were studied by a combination of potentiometric and UV-spectrophotometric titration at I = 0.1 mol/L (KCl/NaClO₄) and T = (25 ± 1)°C. Acid dissociation constants were determined for ammonium (pKa₀) and carboxy groups (pKa₁) in isomeric benzenecarboxylic amino acids: for anthranilic acid (L1), meta-aminobenzoic acid (L2), and para-aminobenzoic acid (L3); and for ammonium groups (pKa₁) in isomeric benzenesulfonic amino acids: for orthanilic acid (L4), methanilic acid (L5), and sulfanilic acid (L6). The amino group basicity in the reagents decreased in the series of meta-, para-, and ortho-isomers. Despite the lower basicity of the amino group in ortho-isomers, their metal complexes have the highest stability. Anthranilic acid exhibits selective properties towards copper(II) ions, and orthanilic acid towards silver(I) ions. The spectroscopic characteristics of transition-metal complexes of isomeric benzenesulfonic amino acids with silver(I), copper(II), nickel(II), and cobalt(II) ions were determined.