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: H0(Ca) =0.6±0.2 kcal-mol–1, S0(Ca)=21.4±0.6 cal-mol–1-K–1, H0(Mg)=3.0±0.7 kcal-mol–1, and S0(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. |