Complexation Properties of Phosphonocarboxylic Acids in Aqueous Solutions

Article abstract of DOI:10.1007/BF00646933

The concentration formation constants of phosphonoacetic acid (PAA) complexes with the Ca(2+) and Mg(2+) ions were determined in aqueous solution at 25 deg 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 Ca(2+) and Mg(2+) PAA species, determined from the temperature dependence of the log K_f(ML), are: ΔH^o(Ca) = 0.6 +/- 0.2 kcal-mol^-1, ΔS^o(Ca) = 21.4 +/- 0.6 cal-mol^-1-K^-1, ΔH^o(Mg) = 3.0 +/- 0.7 kcal-mol^-1, and ΔS^o(Mg) = 35 +/- 2 cal-mol^-1-K^-1.It is seen therefore, that the complexes are entropy stabilized but enthalpy destabilized.Formation constants were also determined for Ca(2+) and Mg(2+) complexes with PAA analogs, phosphonoformic and 3-phosphonopropionic acids and the complexation of PAA was also studied at a single ionic strength, with Na(1+), Ag(1+), Tl(1+), Sr(2+), Ba(2+), Cd(2+), Cu(2+), and Pb(2+) ions.