Dissociation quotients of oxalic acid in aqueous sodium chloride media to 175°C

Abstract

The first and second molal dissociation quotients of oxalic acid were measured potentiometrically in a concentration cell fitted with hydrogen electrodes. The emf of oxalic acid-bioxalate solutions was measured relative to an HCl standard solution from 25 to 125°C over 25^o intervals at nine ionic strengths ranging from 0.1 to 5.0 molal (NaCl). The molal dissociation quotients and available literature data were treated in the all anionic form by a five-term equation that yielded the following thermodynamic quantities at infinite dilution and 25°C: logK_1a=−1.277±0.010, ΔH ^o_1a =−4.1±1.1 kJ-mol⁻¹, ΔS ^o_1a =38±4 J-K⁻¹-mol⁻¹, and ΔC ^o_p,1a =−168±41 J-K⁻¹-mol⁻¹. Similar measurements of the bioxalate-oxalate system were made at 25^o intervals from 0 to 175°C at seven ionic strengths from 0.1 to 5.0m. A similar regression of the experimentally-derived and published equilibrium quotients using a seven-term equation yielded the following values at infinite dilution and 25°C: logK_2a=−4.275±0.006, ΔH ^o_2a =−6.8±0.5 kJ-mol⁻¹, ΔS ^o_2a =−105±2 J-K⁻¹-mol⁻¹, and ΔC ^o_p,2a =−261±12 J-K⁻¹-mol⁻¹.