EFFUSION STUDIES OF THE VAPORIZATION-DECOMPOSITION OF POTASSIUM-SULFATE

The vaporization of K2SO4 was studied by the torsion-effusion method and by mass spectrometry in the range 1180°-1280°K, from which the total pressure, molecular weight, and composition of the effusing vapor were determined. Both the sublimation process K2SO4(s) = K2SO2(g) and the decomposition process K2SO4(s) = 2K(g) + SO2(g) + O2(g) contribute to the vaporization flux, with the sublimation pressure about 63% of the total pressure in the range of our measurements. The directly measured vapor molecular weights are in good agreement with values deduced from the magnitudes of the sublimation and decomposition pressures. A small but reproducible variation of pressure with effusion orifice area was observed, equivalent to an upper bound of 0.03 to the over-all sublimation coefficient. Second law slope heats derived from the temperature dependence of parent K+ and SO2+ ions were in close agreement with each other and with the heat of decomposition calculated from established thermochemical data. Similar measurements on parent K2SO4+ showed the temperature dependence of sublimation and decomposition pressures to be virtually identical. The heat of sublimation of K2SO4(g) at 298°K was determined as 86.5 ± 1.5 kcal/mole, leading to the standard heat of formation Hf298° (K2SO4, g) = -257.0 ± 1.5 kcal/mole. From the slope heat and the absolute pressure, one evaluates the entropy S°1200(K2SO4, g) = 135.8 ± 1.2 cal/deg mole. © 1979, The Electrochemical Society, Inc. All rights reserved.