POTENTIOMETRIC INVESTIGATIONS OF EQUILIBRIA BETWEEN CAFFEIC ACID AND MANGANESE(II), COBALT(II), NICKEL(II) AND CADMIUM(II) IONS IN AQUEOUS-SOLUTION

Aqueous solution equilibria have been investigated for the systems caffeate (trans-3-(3, 4-dihydroxy- phenyl)-propenoate)-manganese(II),-cobalt(II),-nickel(II)and-cadmium(II) by means of glass electrode potentiometry. The identities of the complex species formed and the corresponding formation constants have been determined with the aid of computer analysis. Conditions were: temperature, 25ºC; ionic strength, 0.100 mol dm−3 with respect to chloride. Defining the ligand species, L2−, as the dianion, I, (one proton firmly bound to the catecholic moiety), the titration data can best be explained by postulating the following complexes: [Formula Omitted]. In each case the MLH−1complex constitutes the major species. By comparison with literature formation constants for related ligands, and considerations of the pH-ranges in which the MLH−1complexes form, it is concluded that the metal coordinates to the catecholate site of the caffeate ion. © 1992, Taylor & Francis Group, LLC. All rights reserved.