Dielectric relaxation spectra have been measured for aqueous solutions of sodium malonate and sodium succinate over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 89) and solute concentrations (0.025 ≤ c/M ≤ 1.0). In addition to the usual processes associated with bulk water, two further relaxation processes were detected. The faster of these, with a relaxation time of ∼16 ps, is unusual and was attributed to the presence of “slow” water in the hydration shells of the anions. The extent of this hydration shell is rather “fragile” (concentration-dependent) for malonate but not for succinate. The second, slower, process was attributed to the presence of ion pairs NaX-(aq). Unambiguous determination of the structure of the ion pair was not possible because of the internal flexibility of the anions, but the data were most consistent with a chelated solvent-shared ion pair. The kinetics of formation and dissociation of the ion pairs were evaluated from the effects of solute concentration on their relaxation times.

The solubility of solid sodium oxalate (Na2Ox) has been measured in a variety of concentrated aqueous electrolyte solutions at T = 298.15, 323.15, and 343.15 K by titration of dissolved oxalate with permanganate. The electrolyte solutions studied (not necessarily at all temperatures) were NaCl, NaClO4, NaOH, LiCl, KCl, Me4NCl, and KOH at concentrations ranging from approximately 0.5 mol·kg–1 to at least 5 mol·kg–1. Where comparisons were possible, the present results were in excellent agreement with literature data. The solubility of Na2Ox(s) decreased markedly with increasing concentrations of Na+(aq), due to the common ion effect. This decrease was almost independent of the electrolyte anion. A number of ternary mixtures of these electrolytes were also investigated at constant ionic strength. Consistent with the binary mixtures, the solubility of Na2Ox(s) showed almost no dependence on solution composition at constant Na+(aq) concentrations. Solubilities in non-Na+ media, with the exception of Me4NCl, showed small but regular increases with increasing concentration of added electrolyte, probably reflecting activity coefficient variations. The solubility data in certain Na+-containing media could be correlated accurately at all temperatures and concentrations using a relatively simple Pitzer model with interaction parameters for Na2Ox­(aq) assumed to be identical to those available in the literature for Na2SO4(aq).

Apparent molar volumes (V φ ) and heat capacities at constant pressure (C pφ ) of aqueous solutions of sodium oxalate (Na 2 Ox), sodium succinate (Na 2 Suc), sodium malonate (Na 2 Mal), and potassium oxalate (K 2 Ox) were determined at 25°C up to their saturation limits using vibrating tube densitometry and flow calorimetry. These data were fitted using the Redlich-Rosenfeld-Meyer and Pitzer models. It was established using literature data that V φ and C pφ of Na + and K + salts with a common anion are additive within well-defined limits even up to high concentrations. This was used to estimate hypothetical values of C pφ and V φ at high ionic strengths for the sparingly soluble Na 2 Ox, which are required for the thermodynamic modeling of concentrated electrolyte mixtures such as Bayer process solutions. These values were in reasonable agreement with those estimated using a Pitzer model with parameters for Na 2 SO 4 (aq).

The formation constant β(NaOx−) of the extremely weak ion pair formed between sodium (Na+) and oxalate (Ox2−) ions in aqueous solutions has been determined at 25°C as a function of ionic strength in tetramethylammonium chloride by Na+ ion-selective electrode potentiometry. The effects of trace Na+ impurities from all reagents were accounted for. An extrapolated value for β o of 6.6 ± 0.5 M−1 was obtained at infinite dilution, which is in good agreement with literature values. Attempts to measure this constant in 1 M CsCl media gave a β(NaOx−) value of 0.00 ± 0.06 M−1, probably because of competition between Cs+ and Na+ for Ox2−.

The solubility of solid calcium oxalate monohydrate (CaC2O4·H2O, COM) has been measured in aqueous solutions of four 1:1 electrolytes (NaCl, KCl, CsCl, and Me4NCl) at temperatures T = 298.15, 323.15, and 343.15 K. Solubilities at industrially relevant electrolyte concentrations of up to 5.0 mol·kg–1 were determined by atomic absorption spectrophotometry of the dissolved calcium ions. The solubility of COM in 1.0 mol·kg–1 electrolyte solutions was 2–4 times higher than in pure water but varied little at higher ionic strengths for all of the systems studied. Most of the observed solubilities were well correlated at all temperatures and electrolyte concentrations using a simple Specific Ion-interaction Theory (SIT) model with only one adjustable, temperature-independent, interaction parameter. For Me4NCl solutions, an additional empirical ionic strength dependence of the SIT parameter significantly improved the fit.