Air-water partition constant

Using the numbers given in Table 3.5 we can now inspect Table 3.4 in order to get some feeling of the temperature dependency of partition constants. Except for the hexadecane/water partitioning of hexane and benzene, there is a significant effect of temperature on the partition constants, particularly if one of the phases is the gas phase. For example, the air/water partition constant of diethylether is about 4 times larger at 25°C as compared to 5°C (An// = 46.8 kJ-mol 1). As we will see later in various other chapters, in cases in which equilibrium is not established, temperature may have an important effect on the direction of fluxes of compounds between environmental compartments. 

In the Appendix C you find the air-water partition constant (Kiaw) of naphthalene and its octanol-water partition coefficient (ATI0W) that you use as surrogate for the fat-water partition coefficient, Kifw). Note that these entities are given as ratios of molar concentrations. Use the fat (octanol) as phase 1 and calculate the fat-air (octanol-air) partition constant, Ki ... 

Figure 5.6 Illustration of the effect of a completely water-miscible solvent (CMOS, i.e., methanol) on the activity coefficient of organic compounds in water-organic solvent mixtures decadic logarithm of the activity coefficient as a function of the volume fraction of methanol. Note that the data for naphthalene (Dickhut et al., 1989 Fan and Jafvert, 1997) and for the two PCBs (Li and Andren, 1994) have been derived from solubility measurements whereas for the anilins (Jayasinghe etal., 1992), air-water partition constants determined under dilute conditions have been used to calculate y,f.

The Henry s Law Constant Effect of Temperature on Air-Water Partitioning Effect of Solution Composition on Air-Water Partitioning Illustrative Example 6.2 Evaluating the Direction of Air-Water Gas Exchange at Different Temperatures Illustrative Example 6.3 Assessing the Effect of Solution Composition on Air-Aqueous Phase Partitioning Availability of Experimental Data Estimation of Air-Water Partition Constants... 

Illustrative Example 6.4 Estimating Air-Water Partition Constants by the Bond Contribution Method... 

In the environmental chemistry literature, it is common to refer to the Henry s Law constant of a given compound when the solvent in question is water. In the following, we will adopt this nomenclature and denote the air-water partition constant as defined by Eq. 6-4 simply as K iU (i.e., we omit to indicate the solvent). 

Note that many of the activity coefficients of organic compounds in dilute aqueous solution, y , that we used in Chapter 5 were derived from experimental air-water partition constants (Ki lv/) using Eq. 6-7. Finally, we should point out that in the literature, similar to air-solid surface partitioning (Section 11.1), partition constants are quite often reported as the reciprocal quantity of the air-solvent partition constants as defined above, that is, as solvent-air partition constants. However, it does not really matter in what form such constants are given, as long as we pay... 

There are two general experimental approaches commonly used for determining air-water partition constants, the static and the dynamic equilibration approach. A detailed description of the different existing variations of the two methods can be found in the review by Staudinger and Roberts (1996) and in the literature cited therein. Here we will confine ourselves to a few remarks on the general concepts of these experimental approaches. 

Illustrative Example 6.4 Estimating Air-Water Partition Constants by the Bond Contribution Method Problem Estimate the Kiavi values at 25°C of (a) n-hexane, (b) benzene, (c) diethylether, and (d) ethanol using the bond contribution values given in Table 6.4. Compare these values with the experimental air-water partition constants given in Table 3.4. Note that for a linear or branched alkane (i.e., hexane) a correction factor of +0.75 log units has to be added (Meylan and Howard, 1991). 

Within a given class of apolar or weakly polar compounds (e.g., alkanes, chlorobenzenes, alkylbenzenes, PCBs), the variation in the air-octanol partition constants (Kiao) is much larger than the variation in the air-water partition constants (Kiaw). For example, the Kim values of the chlorinated benzenes vary between 10 3 5 (chlorobenzene) and 10-7 (hexachlorobenzene, see Hamer and Mackay, 1995), whereas their A, aw values are all within the same order of magnitude (Appendix C). Try to explain these findings. 

P 6.4 Getting the Right Air-Water Partition Constant for Benzyl Chloride... 

P 6.5 Experimental Determination of the Air-Water Partition Constant of CF3I (From Roberts, 1995)... 

If we may assume that the mutual saturation of the two liquid phases has little effect on Ym and Yu, we may relate Kuw to the respective air-solvent and air-water partition constants (see Eq. 6-11) ... 

Multiplication of Eq. 8-31 with [HA]W / [HA]W (=1) and rearrangement shows that Diavi is simply given by the product of the fraction in nondissociated form (aia) and the air-water partition constant of the neutral compound (Kiaw) ... 

Kiaoc can be approximated by the natural organic matter-water partition coefficient, Kioc, and by the air-water partition constant, Kiav/ (Eq. 6-11) ... 

Several studies have shown that the concentrations of many organic pollutants in fog water are much higher than would be expected from the compound s equilibrium air/ water partition constant (see Chapter 6), Kiaw (= gaseous concentration of compound i in the air/dissolved concentration of compound i in pure bulk water). In order to describe the observed enrichment of compounds in fog water, an enrichment factor EF can be defined (see Goss, 1994 and references cited therein) ... 

For this problem, assume an air-water partition constant, cH4aw = 30 and an organic carbon-water partition coefficient Kch oc = 10 L-kg-1oc. Also assume the air-plus-water-filled soil porosity is 0.4, the soil solid density is 2.5 g-mL"1, the soil water content is 12% by weight, and the organic carbon content is 5% by weight. Finally, assume the methane biodegradation obeys d[methane]totai/[Pg.774]

Traditionally, water is used as the test substance for determining v,a. Its air-water partition constant at 25°C is A)a/w = 2.3 x 10 5, which is much smaller than Kac cal of Eq. 20-4. Thus, the exchange of water vapor at the air-water interface is solely controlled by physical phenomena in the air above the water surface. The flux of water into air (evaporation) is given by (see Eqs. 20-6, 20-7, 20-9a) ... 

Let us now examine some specific examples using a set of compounds with air-water partition constants, Ku/Vl, between 30 and lxlO-5 (Table 20.5a) and by choosing two extreme wind velocities, uU) = 1 m s 1 and 20 m s 1. From Table 20.4 we get the following reference transfer velocities ... 

Usually evaporation of water is formulated as a function of relative humidity. Explain how this formalism is linked to the usual two-phase air-water model in which the nondimensional Henry s law constant (i.e., the air-water partition constant) of the exchanged chemical appears. 

In Illustrative Example 20.3 it is shown how water temperature influences the air-water exchange velocity, v,Ww. An additional temperature effect of the air-water flux results from the temperature dependence of the air-water partition constant, A,Ww. If water and air temperatures are different, the question arises whether the equilibrium between the air and water phase is determined by water temperature, by air temperature, or even by a mixture of both. Explain why Ki3lv/ should be evaluated for the temperature of the water, not the air. 

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