Effusion studies of the decomposition of zinc sulfate and zinc oxysulfate

The torsion-effusion method has been used to measure the SO/sub 3/ decomposition pressure over the crystalline solids ZnSO/sub 4/ and ZnO x 2ZnSO/sub 4/ in the range of about 800 to 900 K. SO/sub 3/ is the only detectable vapor species under these conditions. Vapor composition was verified by mass spectrometry and by molecular weights obtained from simultaneous torsion and mass-loss effusion measurements. The vaporization process is kinetically limited in two respects, since SO/sub 2/ and O/sub 2/ are the dominant gaseous products at equilibrium and since the SO/sub 3/ effusion pressures show a pronounced dependence on orifice size. Equilibrium SO/sub 3/ pressures over both ZnSO/sub 4/ and ZnO x ZnSO/sub 4/, derived by extrapolation of steady-state pressures to zero orifice area, are more than a factor of five lower than values calculated from established thermodynamic data, indicating the likelihood that the product solid phases are formed in a finely divided metastable condition. Addition of a few mole percent of Pt powder to ZnO x 2ZnO/sub 4/ led to a dramatic pressure increase and to effusing gas compositions consistent with decomposition to SO/sub 2/ and O/sub 2/, while for ZnSO/sub 4/ the effusing gas was converted to SO/sub 2/ andmore » O/sub 2/ but the pressure increase was smaller and not entirely reproducible. 23 references, 2 figures, 6 tables.« less