Re-evaluation of the First and Second Stoichiometric Dissociation Constants of Oxalic Acid at Temperatures from 0 to 60 °C in Aqueous Oxalate Buffer Solutions with or without Sodium or Potassium Chloride

Abstract

Equations were developed for the calculation of the first stoichiometric (molality scale) dissociation constant (Km1) of oxalic acid in buffer solutions containing oxalic acid, potassium hydrogen oxalate, and potassium chloride from the determined thermodynamic values of this dissociation constant (Ka1) and the molalities of the components in the solutions. Similar equations were also developed for the second stoichiometric dissociation constant (Km2) of this acid in buffer solutions containing sodium or potassium hydrogen oxalate, oxalate and chloride. These equations apply at temperatures from 0 to 60 °C up to ionic strengths of 1.0 mol⋅kg−1 and they have been based on single-ion activity coefficient equations of the Huckel type. For the equations for Km1, the activity parameters of oxalate species and the Ka1 values were determined at various temperatures from the Harned cell data of a recent tetroxalate buffer paper (Juusola et al., J. Chem. Eng. Data 52:973–976, 2007). By using the resulting equations for Km1, the activity parameters of oxalate species for Km2 and the Ka2 values were then determined from the new Harned cell data and from those of Pinching and Bates (J. Res. Natl. Bur. Stand. (U.S.) 40:405–416, 1948) for solutions of sodium or potassium oxalates with NaCl or KCl. The resulting simple equations for calculation of Km1 and Km2 for oxalic acid were tested with all important thermodynamic data available in the literature for this purpose. The equations for ln (Ka1) and ln (Ka2) are of the form ln (Ka)=a+b(t/°C)+c(t/°C)2. The coefficients for ln (Ka1) are the following: a=−2.8737, b=0.000159, and c=−0.00009. The corresponding coefficients for ln (Ka2) are −9.6563, −0.003059, and −0.000125, respectively. The new activity coefficient equations were used to evaluate the pH values of the tetroxalate buffer solution (i.e., of the 0.05 mol⋅kg−1 KH3C4O8 solution) for comparison with the pH values recommended by IUPAC at temperatures from 0 to 60 °C and to develop a new two-component oxalate pH buffer of 0.01 mol⋅kg−1 KHC2O4+0.05 mol⋅kg−1 Na2C2O4 for which pH values are given from 0 to 60 °C. Values of p(mH) calculated from these equations are tabulated for these buffers as well as for buffer solutions with KCl and KH3C4O8 as the major component and minor component, respectively. Tables of p(mH) are also presented for 0.001 mol⋅kg−1 KHC2O4+0.005 mol⋅kg−1 Na2C2O4 solutions in which KCl is the supporting electrolyte.