Besides the obvious fact that the structure of a chemical determines how it reacts chemically with the receptor, structure affects its equilibrium distribution in the environment and within organisms. Here we describe the key physicochemical properties affecting
Environmental Biology for Engineers and Scientists, by David A. Vaccari, Peter F. Strom, and James E. Alleman Copyright © 2006 John Wiley & Sons, Inc.
this distribution, which are used in Section 18.7 in our discussion of pharmacokinetics and environmental modeling.
The properties we are interested in here all relate to the tendency for a chemical to distribute itself between two connected phases. The phases involved may include air, water, soil, or sediment in the environment, and tissues, membranes, lipids, and so on, in organisms. The molecules of a chemical will be distributed according to the physico-chemical attractions they have for the other molecules surrounding them in each phase. The physicochemical attractive forces are the same as those described in Section 3.3: van der Waals, dipole moment, hydrogen bonds, and so on. The process by which a chemical distributes itself between two phases is called partitioning.
The equilibrium between two phases is described by exactly the same thermodynamics as was developed for biochemical reactions in Chapter 5. In phase transfer the "reaction" is comparatively simple:
This leads to a definition of the phase equilibrium constant, which states that the compound simply distributes itself between the two phases at a constant ratio, called the partition coefficient, KP:
where CA and CB are the concentrations of the compound in phases A and B, respectively. (For use in partition constants, the concentrations can be expressed either in molar or mass concentration units.)
One of the most basic partition parameters describing a chemical is its vapor pressure, Pv. This is the partial pressure at a given temperature that a chemical will have in equilibrium with its pure liquid or solid phase. For example, if a vial is filled partway with benzene liquid and sealed, the benzene will vaporize into the airspace until its partical pressure reaches an equilibrium value, which is the vapor pressure. The vapor pressure can be considered to be a measure of the attractive forces among a compound's molecules. The stronger the attraction, the lower the vapor pressure. A high vapor pressure can facilitate exposure to toxic substances by inhalation or atmospheric transport.
Because it is such a basic property, the vapor pressure can serve as a sort of reference concentration for comparison wtih concentrations in other phases. To be more precise, the concentrations in different phases can be converted to units of fugacity, which are equivalent to gas-phase partial pressure under ideal gas conditions. Put another way, the fugacity can be thought of as the partial pressure of a substance in equilibrium with whatever concentration and phase is under consideration. The vapor pressure, then, corresponds to a reference fugacity, or the concentration in any phase in equilibrium with the pure liquid. Some investigators use fugacity in place of concentration units to express the amounts of a contaminant in the environment.
At ambient pressures, the ideal gas law is very accurate for both pure and mixed gases and rarely needs correction. Expressed in terms that relate the partial pressure of compound i, Pi, to concentration in mass/volume units, C,-, and to temperature, T, and the gas law constant, R, the ideal gas law is compound in phase A o compound in phase B
where M is the molar mass. Concentrations of chemicals in air may be expressed as mole/ volume, mass/volume, or commonly as volume/volume. The latter is usually given as the parts per million by volume (ppmv). In ideal gas mixtures, the following expression relates ppmv with the partial pressure in equation (18.2):
Pt where PT is the total pressure. At 20°C and 1.0 atm total pressure, we can combine equations (18.2) and (18.3):
Example 18.1 What would be the concentration of benzene in air, Cb, in a poorly ventilated room containing an unsealed drum of liquid benzene?
Answer In this case the partial pressure of the benzene in the air will be approximately equal to the vapor pressure. The vapor pressure of benzene is 95.2 mmHg at 25°C. From equation (18.2) (note that 1.0 atm = 760 mmHg):
b RT 760mmHg/atm V : moy \0.08205L • at^y\298.15K
Example 18.2 The ambient air quality standard for ozone (M=48g/mol) is 0.120 ppmv. What is the concentration in mg/m3?
Answer From equation (18.3), we have PO3 = Pt • ppmv/106 = 1.2 x 10~7 atm. From equation (18.2), CO3 = PO3M/RT =[(1.2 x 10~7 atm) (48 g/mol)]/(0.08205 L/atm • mol • K • 293K) = 2.40 x 10~7 g/L = 0.240 mg/m3.
Another important partition coefficient is Henry's constant, Hc, which describes how a chemical distributes itself between vapor phase and a liquid mixture (usually water solution in the cases of interest here). The amounts in the vapor and water phases can be measured in numerous ways, including partical pressure, fugacity, mole fraction, or molar or mass concentration. When expressed as molar or mass concentration units, Henry's constant is
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