The complexes of holmium with methylenediphosphonate and 1-hydroxyethylidenephosphonate

The composition and stability of holmium methylenediphosphonate (MDP) and 1 -hydroxyethylidenephosphonate (HEDP) complexes were studied by potentiometr ic titration methods in 0.1M NaCl at 25 degreesC. It was found that besides L4- anions the protonated HnL(4-n)- species (n = 1-3 for MDP and n = 1-4 f or HEDP) are present in the pH region 3 to 10. The presence of the undissoc iated acids (H4L) has not been unambiguously proved for MDP. The complexes of the composition HoHnL (n varies from 1 to -2 for MDP and from 1 to -1 fo r HEDP) have been found if the concentration of the ligand is higher than t he concentration of holmium. The protonation constants of both acids and th e stability constants of the complexes discussed were determined and the co mparison with literature data of analogical complexes of other lanthanides was performed.