Equilibrium Partitioning among All Phases Fugacity

Fugacity has units of pressure, and can be related to the concentration of a chemical in a system through a fugacity capacity constant, commonly with units of (mol/atm m3). Thus the chemical concentration in a given 

In the air phase, under pressures normally found in the environment, fugacity equals the pressure exerted by the chemical s vapor. (At higher pressures, vapors do not exactly obey the ideal gas law, and a correction must be applied this is small enough to ignore for practical purposes of fate and transport modeling in the environment.) By combining the ideal gas law (Eq. [1-29]) and Eq. [1-34], it is evident that the fugacity capacity for air is 1/RT, for all chemicals  

The fugacity capacity for other phases is a function of both the chemical s partition coefficient between that phase and water and the chemical s Henry s law constant. For water, the fugacity capacity is 

Once the fugacity capacity for each phase has been calculated, the moles of chemical in each phase are given by 

Fugacity modeling does not allow any new calculations to be made that cannot already be made with the partition coefficients described in the previous three sections. However, a comparison of the fugacity capacity of a chemical in different phases permits a direct assessment of which phase will have the highest chemical concentration at equilibrium. For further details, the reader is referred to Mackay and Paterson (1981) and Schwarzenbach et al. (1993). 

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