Spectrophotometric Study of Complexation Equilibria between Lanthanoids(III) and 2-(5-Bromo-2-pyridylazo)-5-(N-propyl-3-sulfopropylamino)phenol

Abstract

Complex formation (1:1) between lanthanoids(III) and 2-(5-bromo-2-pyridylazo)-5-(N-propyl-3-sulfopropylamino)phenol (5-Br-PAPS, HL) has been studied by spectrophotometry at 25 °C and at I=0.1 (NaNO3). The protonated complex MHL is found to exist in equilibrium with the normal complex ML at pH 5–6. The formation constants KML and KMHL are obtained by a graphical analysis. The KML values uniformly increase with increasing atomic number except for those in the Gd–Ho region where a distinct decrease is observed. However, there is little variation in the KMHL values with atomic number. A linear relation holds well between log KML values of 5-Br-PAPS and 4-(2-pyridylazo)resorcinol (PAR) complexes for the elements up to Eu(III), the intercept of which reflects the difference in the basicity of the oxygen atom ortho to the azo group between the two ligands. This is not the case for log KMHL of the two systems.