H2SO4 vapor pressure over sulfuric acid

Figure 9.4 Equilibrium vapor pressures of H2O, H2SO4, and SO3 over sulfuric acid, 80 °C (Perry and Green, 1997). H2O vapor pressure decreases and H2SO4 and SO3 vapor pressures increase with increasing H2S04-in-acid. All vapor pressures increase with increasing temperature, not shown.

Fig. 5. Vapor pressure of water over sulfuric acid solutions. Percentage of H2SO4 noted on each curve.

Venus upper atmosphere is even drier than the lower atmosphere, and the average water-vapor mixing ratio above the clouds is only a few ppmv. The very low H2O mixing ratios were hard to explain until it was realized that Venus clouds are 75% sulfuric acid, which is a powerful drying agent. When dissolved in the acid, most of the water reacts with H2SO4 to form hydronium (HaO ) and bisulfate (HSO4) ions. As a result, the concentrations of free H2O in the acid solution and in the vapor over the acid are extremely low. The partial pressure of water at Venus cloud tops is lower than that over water ice at the same temperature. Thus, the clouds are responsible for the extreme dryness of Venus upper atmosphere, and play an important role in the photochemical stability of Venus atmosphere (see Section 1.19.3.3). 

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