Acid-base and UV behaviour of 3-(3,4-dihydroxyphenyl)-propenoic acid (caffeic acid) and complexing ability towards different divalent metal cations in aqueous solution

The acid–base and the UV behavior of 3-(3,4-dihydroxyphenyl)-propenoic acid (caffeic acid) was studied by potentiometry and UV spectrophotometry at T = 298.15 K and 0.1 ≤ I/mol L− 1 ≤ 1 in NaCl aqueous solution. The complex formation with Zn2 +, Cu2 +, Cd2 +, (CH3)2Sn2 + at I = 0.1 mol L− 1 in NaCl has been also determined by using potentiometric and spectrophotometric titration techniques. The results obtained by the spectrophotometric measurements fully confirm those obtained by potentiometry, with a particularly good agreement for the system containing Cu2 +. The results showed that, for the ML species, at I = 0.1 mol L− 1 in NaCl and T = 298.15 K, the stability of the caffeate-(CH3)2Sn2 +, and -Cu2 + species is high, with logβML = 16.577 and 13.05, respectively, while that of the caffeate-Zn2 + and -Cd2 + species is considerably lower, with logβML = 8.526 and 7.272, respectively. Speciation profiles for all the systems under study were also calculated and sequestering ability of the caffeate towards the different metal cations considered in this investigation was also evaluated. For example, at physiological pH, I = 0.1 mol L− 1 in NaCl and T = 298.15 K, the order of sequestering ability is Cu2 + > (CH3)2Sn2 + > Zn2 + > Cd2 +.