Interaction of pyridine- and 4-hydroxypyridine-2,6-dicarboxylic acids with heavy metal ions in aqueous solutions

Interactions between pyridine-2,6-dicarboxylic acid and 4-hydroxypyridine-2,6-dicarboxylic acid with Cu (II), Pb (II), and Cd(II) ions were characterized in aqueous solutions (20degreesC; I = 0.4 (KNO3)) by means of dc-polarography. In solutions with excess of ligand, Cu(II) Pb(II), and Cd(II) form 1:2 complexes with the tridentate dianion of pyridine-2,6-dicarbox,lic acid (dipic(2-)) from weak acid to alkaline solutions. The values of logbeta(2) for Cu(II), Pb(II), and Cd(II) are 16.1, 11.8, and 11.0, respectively. The complexing ability of pyridine-2,6-dicarboxylic acid is higher in acid solutions and lower in alkaline solutions than that of 4-hydroxypyridine-2,6-dicarboxylic acid. This difference is attributed to the OH-group, which can deprotonate in basic pH. In acid solutions the OH-group acts as an electron acceptor and reduces the electron donation available to the nitrogen atom in 4-hydroxypyridine-2,6-dicarboxylic acid, whereas in alkaline solutions the OH-group is deprotonated, and the deprotonated O- group acts as an electron donor and increases the coordination ability of the ligand. The triple-deprotonated anion of 4-hydroxypyridine-2,6-dicarboxylic acid (chel(3-)) forms a stable diligand complex with Cu(II), the stability constant logarithm being 21.5 +/- 0.2. (C) 2003 Wiley Periodicals, Inc.