Phosphonic Derivatives of Nitrilotriacetic Acid as Sequestering Agents for Ca2+ in Aqueous Solution: A Speciation Study for Application in Natural Waters

A speciation study on the interaction between Ca2+ and three phosphonic derivative ligands of nitrilotriacetic acid in aqueous solution is reported. The ligands under study are N-(phosphonomethyl)iminodiacetic acid (NTAP), N,N-bis(phosphonomethyl)glycine (NTA2P), and nitrilotris(methylenephosphonic acid) (NTA3P). The most likely speciation models obtained from the elaboration of the potentiometric data include only species with a 1:1 metal/ligand ratio. In detail, two species, CaLH and CaL, for the NTAP ligand and three species, CaLH2, CaLH, and CaL, for the NTA2P and NTA3P ligands were observed. The potentiometric measurements were carried out at different ionic strengths (0.15 ≤ I ≤ 1 mol L-1, in NaCl) and different temperatures (288.15 ≤ T ≤ 318.15 K, at I = 0.15 mol L-1). The dependence of formation constants of the complex species upon the temperature and ionic strength was also determined, and the enthalpy change values were calculated. The 1H nuclear magnetic resonance spectra on metal-ligand solutions, registered as a function of pH, have underlined a characteristic complexing behavior of each ligand toward Ca2+. The spectra of the ligand and metal-ligand solutions are fully consistent with potentiometric results for all of the systems. These data confirm the complex formation as well as speciation models and stability of the species. The sequestering ability of NTAP, NTA2P, and NTA3P toward Ca2+ was evaluated under different conditions of the temperature and pH, typical of natural waters.