Abstract
Thermodynamic acid dissociation constants were determined for phosphonoacetic acid (PAA) in aqueous solution at 25°C by coulometric titrations at different ionic strengths and extrapolation of the results to I=0. The respective values are pK1∼2.0, pK2=5.11±0.04, and pK3=8.69±0.05. The enthalpy and entropy of dissociation for the second and the third dissociation steps, determined from the temperature dependence of pK's, are ΔH 2 o =0.2±0.3 kcal-mole−1, ΔS 2 o =22.6±0.9 e.u., ΔH 3 o =1.3±0.4 kcal-mole−1, and ΔS 3 o =11.7±0.4 e.u. Phosphorus-31 and carbon-13 NMR studies of PAA solutions as a function of pH gave the deprotonation sequence of the triacid. Acidity constants were also determined for phosphonoformic acid, 2-phosphonopropionic acid, and 3-phosphonopropionic acid at an ionic strenght of 0.05.
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