Lanthanide-fluoride ion association in aqueous sodium chloride solutions at 25°C

New results on lanthanide-fluoride ion association constants from potentiometric measurements in 1M NaCl at 25°C are presented for 13 lanthanide elements and compared with literature values. All data have been evaluated by taking two successive association steps into account and difluoro association constants are given where this is justified from the range of ligand numbers that could be reached before precipitation occurred. The lanthanide-monofluoride association constants may be arranged into several series usually referred to as the tetrad effect, and they show a maximum at Dy in the middle of the lanthanide series. Data on the ionic strength dependence of the lanthanide-monofluoride association constants in I=0.1, 0.5, 1.0, and 3.0M NaCl solutions for La, Nd, Tb, Er, and Lu are interpreted applying an extended Debye-Hückel formulation.