Journal of Solution Chemistry p. 523 - 532 (1981)
Update date:2022-08-29
Topics:
Farmer, Richard M.
Heubel, P.-H.C.
Popov, Alexander I.
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 Kf(ML) and monoprotonated Kf(MHL) forms of the PAA anion.The respective values for the complexes are: log Kf(CaL) = 4.68 +/- 0.03, log Kf(CaHL) = 2.61 +/- 0.08; log Kf(MgL) = 5.58 +/- 0.09, log Kf(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 Kf(ML), are: ΔHo(Ca) = 0.6 +/- 0.2 kcal-mol-1, ΔSo(Ca) = 21.4 +/- 0.6 cal-mol-1-K-1, ΔHo(Mg) = 3.0 +/- 0.7 kcal-mol-1, and ΔSo(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.
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