Heat capacities of aqueous solutions of Na 2 SO 4 and Na 2 CO 3 up to near saturation (1.9 and 2.5 mol‚kg -1 , respectively) and of NaOH (to 7 mol‚kg -1 ) have been measured at 25 °C with a Picker flow calorimeter. The calorimeter performance was checked using concentrated NaCl(aq) solutions. On the basis of these measurements, an experimental protocol suitable for the reliable determination of the heat capacities of concentrated electrolyte solutions by Picker calorimetry was established. The heat capacities for Na 2 -SO 4 (aq), Na 2 CO 3 (aq), and NaOH(aq) and literature data for the apparent molar volumes of NaOH(aq) at 25 °C were correlated using the Pitzer formalism. A number of inadequacies in previous models at high concentrations and for extrapolation to infinite dilution are discussed. In particular, it has been confirmed that the heat capacity data for Na 2 CO 3 (aq) at low concentrations must be corrected for the hydrolysis of the carbonate ion. Standard partial molar heat capacities for the three salts and the standard partial molar volume of NaOH(aq) have been derived at 25 °C.

A Pitzer model representing the thermodynamic properties of synthetic Bayer liquor solutions, consisting of the components NaOH -NaAl(OH) 4 -Na 2 CO 3 -Na 2 SO 4 -NaCl-NaF-Na 2 C 2 O 4 (sodium oxalate)-NaHCOO (sodium formate)-NaCH 3 COO (sodium acetate)-H 2 O, is presented. The model calculates, in a thermodynamically consistent manner, heat capacities, enthalpies, activity and osmotic coefficients, and densities of these solutions as well as the solubilities of gibbsite, Al(OH) 3 , boehmite, AlOOH, sodium oxalate, sodium sulfate and other relevant solid phases in synthetic Bayer liquors over concentration and temperature ranges of industrial interest.

Densities of sodium hydroxide solutions at concentrations of 0.1 j m/mol‚kg -1 j 8 have been measured by vibrating-tube densimetry over the temperature range 323 e T/K e 573 at a pressure of 10 MPa. The purposebuilt densimeter, which is capable of measuring densities of corrosive fluids at temperatures up to 573 K and pressures up to 30 MPa, has a platinum-rhodium vibrating tube with inlet/outlet tubes of platinum and an optical detection system. Calibrations were carried out with water and aqueous solutions of sodium chloride at temperatures up to 523 K. At higher temperatures, nitrogen was substituted as the auxiliary density reference because of uncertainties in the NaCl(aq) densities. The apparent molar volumes of NaOH(aq) calculated from the measured densities were fitted with an extended Redlich-Meyer equation. The present results are in good agreement with literature data but are more precise. The current global accuracy in the densities of NaOH(aq) appears to be about 0.1 %, being significantly better at lower concentrations and temperatures but slightly worse at higher concentrations and temperatures. More accurate knowledge of the densities of the reference materials (other than water) used for calibrating vibrating-tube densimeters under extreme conditions is probably required to improve the accuracy of such data.

Heat capacities of aqueous NaOH/NaAl(OH) 4 solutions at total stoichiometric ionic strengths of up to 6 mol kg -1 and with up to 60 mol % substitution of OHby Al(OH) 4have been measured at 25 °C using a Picker flow calorimeter. The mean apparent molar heat capacities (C pφ ) of the NaOH/NaAl(OH) 4 mixtures were linear with respect to Al(OH) 4substitution, consistent with Young's rule. This enabled C pφ values to be derived for the hypothetical pure NaAl(OH) 4 (aq) solutions. The results so obtained are well described by a Pitzer model adapted from the literature, even up to high concentrations.