The Formation of Proton and Alkali-Metal Complexes with Ligands of Biological Interest in Aqueous Solution. Thermodynamics of H+, Na+ and K+ -Oxalate Complexes

The protonation constants of oxalic acid were determined potentiometrically at 37°C in different media—NaNo 3 , KNO 3 and Et 4 NI, 0.03⩽ I ⩽ 0.3. From these data it was possible to determine the formation of the complexes [Na(ox)] − and [K(ox)] − and [K(ox)] − , and to calculate their stability constants. Simultaneous analysis of potentionmetric and calorimetric data (this work and literature) enabled the temperature and ionic strength dependendence of the equilibrium parameters to be obtained for the protonation of oxalate. Recalculation of some literature data gave the Δ H value for the formation of the [Na(ox)] − complex. The thermodynamic parameters obtained allowed us to confirm the hypothesis that dicarboxylate anions chelate with alkali-metal ions and that these complexes are mainly entropically stabilized.