McKay, Donald. "Front matter" Multimedia Environmental Models ...

If H is low, k OA approaches k A, and when P/H is small, the flux approaches
k AC WK AW or k AC WH/RT. In such cases, volatilization becomes proportional to H and
may be negligible if H is very small. In the limit, when H is zero (as with sodium
chloride), volatilization does not occur at all.
Figure 7.7 is a plot of log vapor pressure, P S , versus log solubility in water on
which the location of certain solutes is indicated. Recalling that H or K AWRT is the
ratio of these solubilities, compounds of equal H or K AW will lie on the same 45°
diagonal. Compounds of H > 250 Pa m 3 /mol lie to the upper left, are volatile, and
are water phase diffusion controlled. Those of H < 2.5 or K AW < 0.001 lie to the
lower right, are relatively involatile, and are air phase diffusion controlled. There is
an intermediate band in which both resistances are important.
It is interesting to note that a homologous series of chemicals, such as the
chlorobenzenes or PCBs, tends to lie along a 45° diagonal of constant K AW. Substituting
methyl groups or chlorines for hydrogen tends to reduce both vapor pressure
and solubility by a factor of 4 to 6, thus K AW tends to remain relatively constant,
Figure 7.7 Plot of log vapor pressure versus log solubility in water for selected chemicals.
The diagonals are lines of constant Henry’s law constant. The dashed line corresponds
to a Henry’s law constant of 25 Pa m 3 /mol at which there are approximately
equal resistances in the water and air phases.
©2001 CRC Press LLC