title: Spectrophotometric determination of formation constants of 1:1 complexes of lanthanides with 4-(2-pyridylazo)resorcinol (par).
abstract: Complex formation (1:1) between lanthanides(III) and 4-(2-pyridylazo)resorcinol (PAR or H(2)R) has been studied by spectrophotometry. The method is based on indirect estimation of the protonated (MHR) and the normal (MR) complexes by measuring the absorbance at the peak for the ligand, which decreases with increasing metal concentration at a constant pH. Similar experiments were made at various pH values. Both MHR and MR complexes were found to be formed in the pH range 5-6. Their formation constants, logK(MHR) and logK(MR), determined by graphical analysis, ranged from 3.78 +/- 0.02 (Ce) to 4.39 +/- 0.02 (Lu), and from 9.61 +/- 0.06 (Ce) to 10.70 +/- 0.05 (Lu), respectively. The acidity of the MHR complexes parallels the order of stability of the MR complexes.
authors.affiliation: Yatsushiro College of Technology, Hirayamashinmachi 2627, Yatsushiro, Kumamoto, 866 Japan.
authors.forename: E
authors.initials: E
authors.lastname: Ohyoshi