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

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.