Complexation properties of phosphonocarboxylic acids in aqueous solutions

Abstract

The concentration formation constants of phosphonoacetic acid (PAA) complexes with the Ca2+ and Mg2+ ions were determined in aqueous solution at 25°C by potentiometric and coulometric titrations at different ionic strengths and were extrapolated to I=0 in order to obtain thermodynamic values of the formation constants. Complexes were formed by the completely deprotonated K f (ML) and monoprotonated K f (MHL) forms of the PAA anion. The respective values for the complexes are: log K f (CaL)=4.68±0.03, log K f (CaHL)=2.61±0.08; log K f (MgL)=5.58±0.09, log K f (MgHL)=3.0±0.3. The enthalpy and entropy of complexation for the deprotonated Ca2+ and Mg2+ PAA species, determined from the temperature dependence of the log K f (ML), are: ΔH0(Ca) =0.6±0.2 kcal-mol−1, ΔS0(Ca)=21.4±0.6 cal-mol−1-K−1, ΔH0(Mg)=3.0±0.7 kcal-mol−1, and ΔS0(Mg)=35±2 cal-mol−1-K−1. It is seen there-fore, that the complexes are entropy stabilized but enthalpy destabilized. Formation constants were also determined for Ca2+ and Mg2+ complexes with PAA analogs, phosphonoformic and 3-phosphonopropionic acids and the complexation of PAA was also studied at a single ionic strength, with Na+, Ag+, Tl+, Sr2+, Ba2+, Cd2+, Cu2+, and Pb2+ ions.