Potentiometric Investigations of Equilibria Between Caffeic Acid and Manganese(II), Cobalt(II), Nickel(II) and Cadmium(II) Ions in Aqueous Solution

Abstract

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, logβ_11–1, = -4.88; MnLH²⁻ ₋₂, logpβ_11–2 = -15.55; NiLH⁻ ₋₁, logβ_11–1 = -3.62; NiLH²⁻ ₋₂, logβ_11–2 = -13.45; Ni₂LH⁺ ₋₁, logβ_21–1 = -1.56; CoLH⁻ ₋₁, logβ_11–1, = -4.12; Co₂LH⁺ ₋₁, logβ_21–1 = 1.75; CdLH⁻ ₋₁, logβ_11–1, = -4.91; CdL₂H⁴⁻ ₋₂, logβ_12–2 = -12.29; CdL₃H⁻ ₋₃, Iogβ_{13_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.

Key Words:

• Caffeic acid
• manganese(II)
• cobalt(II)
• nickel(II)
• cadmium(II)
• potentiometry