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.
ASJC Scopus subject areas
- Analytical Chemistry