VAPOR-PRESSURE OF AMMONIUM SULFATES

Equilibrium partial pressures of NH3 over samples of ammonium sulfate are measured in the temperature range 45-180°C. Ring oven techniques are employed; data at 150 and 180°C are corroborated by separate determinations using (1) the NH3 mass lost by the sample and (2) titration of the effluent vapor. The standard entropy of formation of NH4HSO4, S° = 36 cal/formula unit K, is derived from these measurements. This value should make it possible to predict NH3 vapor pressures over solid (NH4)2SO4 within an order of magnitude at room temperature. Further, the data indicate that NH4HSO4 is unstable at warmer temperatures, effectively decomposing at 50°C; it has no boiling temperature, though a maximum boiling solid that is nearly pure (NH4)2SO4 boils at approximately 470°C. The vapor pressures themselves are complex functions of temperature and decomposition. The NH3 partial pressure over (NH4)2SO4 depends dramatically on its purity; it is lowered in proportion to its acidity; at 25°C a 99.99% pure (NH4)2SO4 sample (0.01% acid) exhibits an NH3 partial pressure of 4 × 10 -12 atmospheres, whereas a 99% pure (NH4)2SO4 sample (1% acid) has an NH3 vapor pressure of only 4 × 10-14 atmospheres. The extraordinary nature of the vapor pressure dependencies suggests that (NH4)2SO4 should be the most stable form of the solid in the atmosphere, except in the complete absence of ammonia. These data also suggest that concentration and/or temperature gradients should allow homogeneous, heteromolecular nucleation of new particles in the atmosphere. © 1979.