Protolytic and complexing properties of some isomeric aromatic amino acids in aqueous solution

The protolytic and complexation properties of some isomeric aromatic amino acids in aqueous solution were studied by a combination of pH-potentiometric and UV-spectrophotometric titration methods at I = 0.1 mol/dm3 (KCl/NaClO4) and t = (25 ± 1)°С. The acid dissociation constants of ammonium (pKa0) and carboxyl groups (pKa1) in the structure of isomeric benzene-carboxylic amino acids were determined: anthranilic acid (L1), meta-aminobenzoic acid (L2) and para-aminobenzoic acid (L3); ammonium groups (pKa1) in the structure of isomeric benzenesulfonic amino acids: orthanilic acid (L4), methanilic acid (L5), sulfanilic acid (L6). It is shown that the basicity of the amino group in the structure of the reagents decreases in the series of isomers: meta-, para-, ortho-isomer. Despite the fact that ortho-isomers are characterized by lower basicity of the amino group, it is shown that their metal complexes have the highest stability. It was found that anthranilic acid exhibits selective properties towards copper(II) ions, and orthanilic acid – towards silver(I) ions. The spectral characteristics of metal complexes of isomeric benzenesulfonic amino acids with transition metal ions: silver(I), copper(II), nickel(II) and cobalt(II) have been determined.