Stability constant for the lanthanide fluoride complexes in aqueous solution at 25°C

Stability constants for the mono- and difluoride complexes of fourteen lanthanides in aqueous solution 25°C and an ionic strenght of 0.5 M have been measured using the newly developed Orion expandable and digital ion analyzer and their fluoride specific ion electrode. A simplified potentiometric procedure was employed to measure the stability constant for the monofluoride complex and this value was used to compute analytically the stability constant for the difluoride complex in separate experiments. In the case of heavy lanthanides a linear regression method was also used to compute simultaneously the stability constant for both complexes. The stability constant for the monofluoride complexes may be arranged into four groups that shows the tetrad effect with maxima at Pr, Sm, Dy and Er. There is a gradual but unsystematic increase in the stability constant for the monofluoride complex with atomic number of the lanthanides.