Dissociation Quotients for Citric Acid in Aqueous Sodium Chloride Media to 150°C

Abstract

The three molal dissociation quotients for citric acid were measured potentiometrically with a hydrogen-electrode concentration cell from 5 to 150°C in NaCl solutions at ionic strengths of 0.1, 0.3, 0.6, and 1 molal. The molal dissociation quotients and available literature data at infinite dilution were fitted by empirical equations in the all-anionic form involving an extended Debye-Hückel term and up to five adjustable parameters involving functions of temperature and ionic strength. This treatment yielded the following thermodynamic quantities for the first dissociation equilibrium at $\begin{gathered} 25^ \circ {\text{C: log }}K_{1{\text{a}}} = - 3.127 \pm 0.002{\text{, }}\Delta H_{{\text{la}}}^{\text{o}} = \hfill \\ 4.1 \pm 0.{\text{2 kJ - mol}}^{{\text{ - 1}}} {\text{, }}\Delta S_{{\text{1a}}}^{\text{o}} = - 46.3 \pm 0.{\text{7 J - K}}^{{\text{ - 1}}} - {\text{mol}}^{{\text{ - 1}}} {\text{, and }}\Delta Cp_{{\text{1a}}}^{\text{o}} = - 162 \pm \hfill \\ {\text{7 J - K}}^{{\text{ - 1}}} - {\text{mol}}^{{\text{ - 1}}} ; \hfill \\ \end{gathered}$ for the second acid dissociation equilibrium at $\begin{gathered} 25^ \circ {\text{C: log }}K_{{\text{2a}}} = - 4.759 \pm 0.001{\text{, }}\Delta H_{{\text{2a}}}^{\text{o}} = \hfill \\ 2.2 \pm 0.1,{\text{ }}\Delta S_{{\text{2a}}}^{\text{o}} = - 83.8 \pm 0.4{\text{, and }}\Delta Cp_{{\text{2a}}}^{\text{o}} = - 192 \pm 15 \hfill \\ \end{gathered}$, and for the third dissociation equilibrium at $\begin{gathered} 25^ \circ {\text{C: log }}K_{{\text{3a}}} = - 6.397 \pm 0.002{\text{, }}\Delta H_{{\text{3a}}}^{\text{o}} = \hfill \\ - 3.6 \pm 0.2,{\text{ }}\Delta S_{{\text{3a}}}^{\text{o}} = - 134.5 \pm 0.7{\text{, and }}\Delta Cp_{{\text{3a}}}^{\text{o}} = - 231 \pm 7 \hfill \\ \end{gathered}$.