Potentiometric and spectrophotometric studies of the complexation of lanthanum(III) with adrenaline, noradrenaline, and dopamine

The interaction of lanthanum(III) with the catecholamines adrenaline (AD), noradrenaline (NAD), and dopamine (DP) has been investigated by potentiometric and spectrophotometric methods at 298.15 K in an I = 0.2 M KCl ionic medium. The protonation constants for the ligands and the stability constants of the La(III) complexes are calculated from the potentiometric data using "BEST" software. The LaHL and La(HL)(2) complexes are reported in 1:1 and 1:2 molar ratios from the potentiometric and spectrophotometric data. The species distribution diagrams were also calculated. In all of the catecholamine systems, the coordination of the ligands to La(III) occurs via the phenolate sites (O; OH), but one of the phenolate protons does not dissociate. With respect to the ligands, the stability constants of the La(III) complexes decrease in the following order: dopamine > adrenaline > noradrenaline. The stability constants of these lanthanum(III) complexes are lower than the analogous Y(III) complexes due to the lower ionic potential of La(III)