Conductivity, Thermal Measurements, and Phase Diagram of the Na 2 S 2 O 7 -NaHSO 4 System G. Hatem, ² M. Gaune-Escard, ² S. B. Rasmussen, ‡ and R. Fehrmann* ,‡ Institut UniVersitaire des Syste ` mes Thermiques Industriels (IUSTI), Technopole de Chateau-Gombert, 13453 Marseille, France, and Department of Chemistry, Technical UniVersity of Denmark, DK-2800 Lyngby, Denmark ReceiVed: October 6, 1998; In Final Form: NoVember 25, 1998 The conductivity of the Na 2 S 2 O 7 -NaHSO 4 binary system has been measured for 15 different compositions in the full composition range, and in the temperature range 400-700 K. Phase transition temperatures were obtained, and the phase diagram was constructed. It is of the simple eutectic type, where the eutectic is found to have the composition X NaHSO 4 ) 0.97, as calculated from the measured thermodynamic properties, and to melt at 179 °C. The partial enthalpy and entropy of mixing have been obtained, and the negative entropy points to a structural order of the melt, presumably due to hydrogen bonding, as found earlier for the K 2 S 2 O 7 - KHSO 4 system. For each composition measured of the Na 2 S 2 O 7 -NaHSO 4 system in the molten state, the conductivity has been expressed by equations of the form κ ) A(X) + B(X)(T - T m ) + C(X)(T - T m ) 2 , where T m is the intermediate temperature of the measured temperature range. Introduction The molten salt-gas system M 2 S 2 O 7 -V 2 O 5 -SO 2 /O 2 /O 3 /N 2 (M ) alkali) at 400-500 °C is considered a realistic model system for the industrial catalyst used for SO 2 oxidation. 1 Catalysts containing mixtures of K and Na as promoting alkali ions are very common. They are also used for purification of wet industrial off-gases by oxidation of SO 2 to SO 3 , followed by condensation of H 2 SO 4 as a commercial produc. 2 In the wet flue gases, M 2 S 2 O 7 is partly transformed to MHSO 4 , making the catalyst solvent system a M 2 S 2 O 7 -MHSO 4 (M ) Na, K) mixture. Recently 3 we have investigated the K 2 S 2 O 7 -KHSO 4 system by conductivity, NMR spectroscopy, and thermal methods and constructed the phase diagram of this binary system. The present paper concerns our measurements on the Na 2 S 2 O 7 -NaHSO 4 system and the results are compared with the phase diagram published 4 more than 75 years ago and the previously 5 measured conductivity of the pure molten compo- nent NaHSO 4 . Other fundamental thermodynamic data as densities, con- ductivities, heats of mixing, and heat capacities of the Na 2 S 2 O 7 - NaHSO 4 system, as well as the heat of fusion of the pure components, are virtually missing in the literature. These properties are the object of our previous 6 and present paper as well as ongoing investigations. Experimental Section Chemicals. The hygroscopic Na 2 S 2 O 7 was prepared by thermal decomposition of Na 2 S 2 O 8 (Merck, p.a.) in N 2 as earlier described, 7 kept on sealed ampules, and handled only in the drybox, with a measured water content less than about 5 ppm. NaHSO 4 (Merck, suprapur (99%)) was dried at 110 °C and also kept in sealed ampules until used. Conductivity Measurements. The borosilicate cell with gold electrodes and other experimental details have been described previously. 3 The temperature of the furnace was regulated to within (0.1 °C. It was decreased from the highest measuring temperature in steps of 2.5 or 10 °C to the temperature of crystallization, often obtained at large subcooling of the melt. Then the temperature was raised stepwise until the measured conductivity coincided with the value obtained on cooling, i.e., at the liquidus temperature of the mixture. Finally, the temper- ature was increased to the initially measured temperature, where the reproducibility of the measured conductivity was checked. The temperature was measured by a thermocouple, precalibrated against a Pt100 resistance thermometer, placed at the capillary tube of the cell. A Radiometer CDM-230 conductivity meter was used for the conductivity measurements, and the cell constants in the range 100-200 cm -1 could be measured, as earlier described, 8 by use of a 0.1 D KCl standard solution. Thermal Measurements. The thermal measurements were carried out by use of a Setaram DSC 121 differential scanning calorimeter, where the temperature was changed in the range 2-5 °C/min. The samples where contained in Pyrex cells, sealed under vacuum. The partial enthalpy of mixing of Na 2 S 2 O 7 in the Na 2 S 2 O 7 -NaHSO 4 melts has been measured at the following experimental conditions: X NaHSO4 ) 0.540 (T ) 663 K) and X NaHSO4 ) 0.954 (T ) 544 K), by using a Calvet micro- calorimeter. The melts were contained in a Pyrex crucible and the sodium pyrosulfate in a fragile Pyrex ampule. The measure- ments were carried out by breaking the ampules in the crucible. Results and Discussion The conductivity of 15 different compositions of the solid and liquid Na 2 S 2 O 7 -NaHSO 4 binary system was measured in the composition range X NaHSO4 ) 0-1. These conductivities are plotted as - ln(κ) vs 1/T in Figures 1 and 2. As observed previously 3,9-11 for the systems K 2 S 2 O 7 -KHSO 4 ,K 2 S 2 O 7 - V 2 O 5 , Cs 2 S 2 O 7 -V 2 O 5 , and M 2 S 2 O 7 -V 2 O 5 (M ) 20% Na + 80% K), respectively, a marked change of the conductivity takes place at the phase transition temperatures of the measured mixtures, i.e., the liquidus and solidus temperatures. However, for mixtures rich in NaHSO 4 only one transition temperature could be identified; i.e., the temperature of fusion of the eutectic ² Institut Universitaire des Syste `mes Thermiques Industriels. ‡ Technical University of Denmark. 1027 J. Phys. Chem. B 1999, 103, 1027-1030 10.1021/jp983974a CCC: $18.00 © 1999 American Chemical Society Published on Web 01/23/1999