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/NaClO 4 ) and T = (25 ± 1)°C. Acid dissociation constants were determined for ammonium (p Ka 0 ) and carboxy groups (p Ka 1 ) in isomeric benzenecarboxylic amino acids: for anthranilic acid ( L1 ), meta -aminobenzoic acid ( L2 ), and para -aminobenzoic acid ( L3 ); and for ammonium groups (p Ka 1 ) 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.