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, 25[ddot]C; 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: MnLH− −1, logβ11–1, = -4.88; MnLH2− −2, logpβ11–2 = -15.55; NiLH− −1, logβ11–1 = -3.62; NiLH2− −2, logβ11–2 = -13.45; Ni2LH+ −1, logβ21–1 = -1.56; CoLH− −1, logβ11–1, = -4.12; Co2LH+ −1, logβ21–1 = 1.75; CdLH− −1, logβ11–1, = -4.91; CdL2H4− −2, logβ12–2 = -12.29; CdL3H− −3, Iogβ13_3 = -19.50. In each case the MLH−1, complex constitutes the major species. By comparison with literature formation constants for related ligands, and considerations of the pH-ranges in which the MLH−1 complexes form, it is concluded that the metal coordinates to the catecholate site of the caffeate ion.