NOTE ON THE ENTHALPIES OF FORMATION OF SRWO4 AND BAWO4 DETERMINED BY HIGH-TEMPERATURE DIRECT SYNTHESIS CALORIMETRY FROM SRCO3+WO3 AND BACO3+WO3

A comparison has been made between the experimentally measured value o f enthalpy change of the reaction CaO(s, 298 K) + WO3(s, 298 K) = CaWO 4(s, 1473 K), and the value for the same reaction derived from the mea sured enthalpy change of the reaction CaCO3(s, 298 K) + WO3(s, 298.K) = CaWO4(s, 1473 K) + CO2(g, 1473 K). The two values of Delta H, namely 13.97 +/- 2.45 and 9.34 +/- 3.55 kJ mol(-1), are roughly comparable w hen experimental uncertainties are taken into consideration. This sugg ests that the error induced in the value of the enthalpy of formation, from the component oxides, of CaWO4 by using CaCO3 instead of CaO as one of the reactants in direct synthesis experiments would not be very great. The lower value of Delta H, 9.34 +/- 3.55 kJ mol(-1), indicate s that the CO2 formed in the reaction between CaCO3 and WO3 is release d at a temperature somewhat lower than the calorimeter temperature. Fo r all partial pressures of CO2, SrCO3 and BaCO3 decompose at much high er temperatures than CaCO3. Hence, it can be inferred that the agreeme nt between the two values of Delta H would be significantly better for SrO-SrCO3 and BaO-BaCO3, than for CaO-CaCO3. In a recent communicatio n (Q. Guo and O.J. Kleppa, Thermochim. Acta, 288 (1996) 53-61), we ass umed that the CO2 formed from SrCO3 + WO3 and BaCO3 + WO3 in the high- temperature calorimeter is released at or very near the calorimeter te mperature. Under this assumption, which we believe is valid, the entha lpies of formation for SrWO4 and BaWO4, from the component oxides, rep orted by these authors should be reliable. (C) 1997 Elsevier Science B .V.