Fugacity capacity

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Fugacity capacity, Z, is a simple way to describe how much of a chemical a phase can hold for a given fugacity. In many systems, the concentration in a phase m (C_m) is linked to the chemical’s fugacity (f) by C_m = Z_m · f.

At equilibrium, the fugacity is the same in all phases—a condition called equifugacity. Because of that, Z can vary between media, and the relationship between C and f may not always be a straight line. If needed, Z can be allowed to depend on C or f to capture non-linear behavior. Conceptually, Z is like a heat capacity: it reflects the phase’s capacity to absorb the chemical.

What affects Z
- The identity of the solute (the chemical)
- The nature of the medium or compartment (water, air, soil, octanol, etc.)
- Temperature

Z depends on the specific media, so Z_m must be determined for each phase of interest. In practice, Z values for different media are related through the equilibrium concentrations: the ratio Z_a/Z_b matches the ratio of the target chemical’s equilibrium concentrations in those media.

Key factors often used to describe Z
- R: the ideal gas constant
- T: temperature
- H: Henry’s law constant for the chemical
- Kow: octanol–water partition coefficient
- Ps: vapor pressure
- v: molar volume of the chemical

Applying Z in multi-media calculations
- If you know the total mass of the chemical (M_T) and the volume of each compartment (V_m), you can estimate the system’s prevailing fugacity f.
- If the chemical is present as a pure phase at equilibrium, its vapor pressure equals the system’s fugacity f.

In short, Z tells you how much of a chemical a phase can hold at a given fugacity, and equifugacity means that shared fugacity links all phases at equilibrium.