THERMODYNAMIC STABILITY OF COPPER(II), MANGANESE(II), ZINC(II) AND IRON(III) COMPLEXES WITH CHLOROGENIC ACID

Protometric studies were performed in aqueous solution at 20 degrees C and 1 mol dm(-3) ionic strength (KNO3) in order to determine the comp lexing ability of chlorogenic acid (H(3)L) towards copper(II), mangane se(II), zinc(II) and iron(III). A detailed qualitative examination of titration curves indicated formation of ML, complexes (n = 1, 2 or 3). With Cu(II) and Fe(III) a protonated complex MLH is formed in acidic medium and with Zn(II) it seems likely that two mixed hydroxo complexe s ZnL(2)OH(5-) and ZnL(2)(OH)(2)(6-) are present above pH = 10. Stabil ity constants beta(m/h) defined as [M(m)L(l)H(h)]/[M](m)[L](l)[H](h) w ere computed and refined. The stability order of the complex species i s Fe > Cu > Zn > Mn.