ABSTRACT
So far, we have considered the mass exchange within or between the liquid or dissolved phase and the solid phase . However, gas solution, exsolution, and volatilisation can cause significant mass exchange of atmospheric gases (O2, N2), gases produced by the decomposition of organic matter (CO2, CH4, NH3, H2S), and organic chemicals between the liquid phase and the atmosphere. The concentration of dissolved gas in solution is commonly modelled using Henry’s law , which relates the concentration of dissolved gas in solution to the partial pressure of that gas in an atmosphere in contact with the solution and is an example of a chemical equilibrium constant (see Chapter 2):
P K (14.1)
where KH = the Henry’s law constant usually expressed in units like atm l mol-1 or atm m3 mol-1 (note that some textbooks define the Henry’s law constant as the reciprocal of the Henry’s law constant given here; compare Equation 2.9), P = partial pressure of the chemical in the gas phase (atm), and Caq = the equilibrium molar concentration of the chemical in solution (mol l-1 or mol m3). The Henry’s law constant can also be expressed in dimensionless form, by dividing the gas concentration in air by the aqueous concentration in the same units (mol l-1, mol m3, mg l-1, or mg m-3):
C K ' (14.2)
where K 'H = the dimensionless form of the Henry’s law constant (-) and Cair = the molar concentration of the gas in air (mol l-1 or mol m3). The relation between partial pressure P and the concentration in air is given by the ideal gas law :
TR
K K
TR
P
V
n C
' (14.3)
where n = the number of moles of the chemical in air (mol), V = the air volume (l or m3), P = the partial vapour pressure of the gas (atm), R = the gas constant (= 0.082058 (l atm mol-1 K-1)), and T = the absolute temperature (K). The Henry’s law constant is tabulated in many handbooks. It can also be estimated by dividing the vapour pressure of a chemical at a given temperature by its aqueous solubility at that temperature. Table 14.1 lists the dimensionless Henry’s law constant and vapour pressure for some selected chemicals.
