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

Abstract

A speciation study on the interaction between Ca 2+ and three phosphonic derivative ligands of nitrilotriacetic acid in aqueous solution is reported. The ligands under study are N-(phosphonomethyl)iminodiacetic acid (NTAP), N,Nbis(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, CaLH 2 , 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 1 H nuclear magnetic resonance spectra on metal−ligand solutions, registered as a function of pH, have underlined a characteristic complexing behavior of each ligand toward Ca 2+ . 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 Ca 2+ was evaluated under different conditions of the temperature and pH, typical of natural waters.

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“…For the dependence on the temperature, the van’t Hoff equation was used, as for other systems ( Cordaro et al, 2019 ; Giuffrè et al, 2020 ): where logβ T is the stability constant at a given ionic strength and temperature (in Kelvin), logβ θ is the value at the reference temperature ( T = 298 K), and Δ H 0 is the formation enthalpy change expressed in kJ mol −1 at T = 298.15 K and R = 8.314472 J K −1 mol −1 .…”

Study on Metronidazole Acid-Base Behavior and Speciation with Ca2+ for Potential Applications in Natural Waters

“…The evaluation of the sequestering ability of L against Ca 2+ , widely present in natural waters, therefore, becomes interesting in order to be able to describe the effectiveness of the system studied. To evaluate this capacity for quantitative purposes, as already widely reported in other previous papers [ 53 , 54 ], the empirical parameter pL 0.5 was calculated. It represents the total ligand concentration necessary for the sequestration of a mole fraction equal to 0.5 of a specific cation present in trace.…”