Determination of Formation Constants of Calcium Complexes of Difluoromethylenediphosphonic Acid and Related Diphosphonates

Formation constants for the 1:1 and 2:1 complexes formed between Ca²⁺ and difluoromethylenedlphosphonate (F₂MDP), dichloromethylenediphosphonate (Cl₂MDP), and 1-hydroxy-ethane-1,1-diphosphonate (EHDP) were determined by a Potentiometrie titration procedure where the Indicator electrode Is a Ca Ion selective electrode and the titrant is a CaCl₂ solution. Measurements were made in the absence of Ionic strength control and in 0.10 M NaCl and 0.10 M NH₄Cl solution. Logarithm formation constants, although similar for the 1:1 complex, change In the order EHDP > Cl₂MDP > F₂MDP which is opposite to the acid strength exhibited by the ligands. For the 2:1 Ca²⁺:llgand complex the EHDP complex formation constant is similar to the 1:1 complex and the complex is significantly more stable than either of the 2:1 Ca²⁺:Cl₂MDP or Ca²⁺:F₂MDP complexes. This Is probably the result of coordination Involving the hydroxyl group In EHDP. Weak 1:1 Na⁺:ligand complexes were also detected.