Radiochimica ActazyxutsrqponmlihfedcbaSRPLHGDCA 61, 201 - 2 0 6 (1993) © R. Oldenbourg Verlag, München 1993 - 0033-8230/93 $ 3.00 + 0.00 Complexation Thermodynamics of Lanthanoids with 2,4-, 2,5- and 3,5-Dihydroxybenzoic Acids By Raimo Anttila, Anne-Marja Kiviniemi, Lauri H. J. Lajunen, Risto S. Laitinen, and Jukka Jokisaari* Department of Chemistry, University of Oulu, Linnanmaa, SF-90570 Oulu, Finland Dedicated to Professor G. R. Choppin on the occassion of his 65th birthday (Received March 31, 1992; revised October 5, 1992) Lanthanoids / Dihydroxybenzoic acids / Complex equilibria Abstract The thermodynamic parameters, log/?, AH and AS for the for- mation of [Ln(H 2 L)] 2 + complexes between lanthanoid cations and 2,4-, 2,5- and 3,5-dihydroxybenzoate anions (H 2 L~) were determined by Potentiometrie and calorimetrie titrations in aqueous solutions of 0.10 M (NaC10 4 ) ionic strength and at 25 °C. The A H and A S values of complexation were positive and largely compensatory reflecting that the dehydration plays a major role in the overall complexation reaction. The 13 C-NMR spectroscopic data indicate that, in all species, the metal ion coordinates mainly to the carboxyl oxygens. The increased sta- bility of 2,4- and 2,5-dihydroxybenzoato complexes compared to that of the 3,5-dihydroxybenzoato and benzoato complexes can be attributed to the electronic effects of the o-hydroxyl groups. Introduction The thermodynamic parameters for the formation of lanthanoid complexes of the benzoate, 5-sulfo- salicylate, 3- and 4-hydroxybenzoate, 3- and 4-fluoro- benzoate, and 4-nitrobenzoate have been recently studied [1 — 4]. The logarithmic values of the stability constants varied linearly with the carboxylatezyxwvutsrqponmlkjihgfedcbaVUTSRPONMLKIHGFEDCBA ρK a . However, those of the substituted o-hydroxycarboxy- lates constituted a separate group with an independent linear relationship and stabilities of at least one order of magnitude larger than the simple benzoates. This enhancement was taken as an evidence for chelate formation [2], Recent NMR studies of the lanthanoid complexes with 5-sulfosalicylate [5] as well as the crys- tal structure of the aquatris(salicylato)samarium(III) [6] supported this conclusion. In this paper we report the results of the Potent- iometrie and calorimetric titrations of trivalent lanthanoids with three aromatic dihydroxycarboxy- late ligands in 0.10 M (NaC10 4 ) and at 25 °C. The mode of coordination in these complexes was investi- gated by 13 C-NMR spectroscopy. Experimental Reagents Stock solutions of lanthanoid Perchlorates were pre- pared and analyzed as described elsewhere [1], Com- * Department of Physics, University of Oulu. mercially available 2,4-, 2,5- and 3,5-dihydroxy- benzoic acids (Fluka) were recrystallized from water before use. The buffer solutions of the ligands were prepared by dissolving the acids (H 3 L) in deionized water and adjusting the [H 3 L]/[H 2 L~] ratios with standard NaOH followed by dilution to the desired concentration. The exact molarity of the ligands were checked by Potentiometrie titration. NaOH solution was standardized by potassium hydrogenphthalate. Known amount of potassium hydrogenphthalate was dissolved in 50 ml of hot, deionized water and titrated with NaOH solution using Phenolphthalein as an indi- cator. The buffer solution (E° solution) for the cali- bration of the electrode system was 0.01 and 0.09 M with respect to HC10 4 and NaC10 4 , respectively. Procedures Potentiometry and calorimetry The investigation was carried out as a series of Potenti- ometrie and calorimetric titrations at 25 °C in 0.10 M (NaC10 4 ) solution. The Potentiometrie titrations were performed in a thermostated room (25 °C) and the cup temperature was maintained at 25±0.1°C. An automatic titration system consisting of an ABU 91 Autoburette and a VIT 90 Video Titrator was used for the Potentiometrie measurements. The indicator electrode was an Orion 90-01 glass electrode and the reference electrode was a Ag-AgCl electrode filled with 0.10 M NaC10 4 solution. The electrode was calibrated by titration of 50 ml of theE E° solution with standard 0.10 M NaOH solution and the E° and E } values were obtained by the Gran method. The protonation constants were determined by ti- tration of the ligand acid solutions with 0.10 M NaOH solution and the stability constants were obtained by titration of the metal solutions with buffered ligand solutions. The concentration of lanthanoid, C M , varied from 0.01 to 0.02 M, while that of the ligands varied from 0.026 to 0.055 M. Calorimetric titrations were performed on a Peltier-cooled calorimeter constructed locally. The ti- tration techniques were described elsewhere [7, 8]. For each individual titration, 50 ml of a lanthanoid Per- chlorate solution (0.012 < C M <0.020 M) was titrated Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 8/3/15 4:20 AM