Octanol-Water Partition Coefficient Kow

The experimental approaches are similar to those for solubility, i.e., employing shake flask or generator-column techniques. Concentrations in both the water and octanol phases may be determined after equilibration. Both phases can then be analyzed by the instrumental methods discussed above and the partition coefficient is calculated from the concentration ratio Q/Cw. This is actually the ratio of solute concentration in octanol saturated with water to that in water saturated with octanol. 

As with solubility, Kow is a function of the presence of electrolytes and for dissociating chemicals it is a function of pH. Accurate values can generally be measured up to about 107, but accurate measurement beyond this requires meticulous technique. A common problem is the presence of small quantities of emulsified octanol in the water phase. The high concentration of chemical in that emulsion causes an erroneously high apparent water phase concentration. 

Considerable success has been achieved by calculating Kow from molecular structure thus, there has been a tendency to calculate ATow rather than measure it, especially for difficult hydrophobic chemicals. These calculations are, in some cases, extrapolations and can be in serious error. Any calculated log KqW value above 7 should be regarded as suspect, and any experimental or calculated value above 8 should be treated with extreme caution. 

For many hydrophilic compounds such as the alcohols, Kow is low and can be less than 1.0, resulting in negative values of log Kow. In such cases, care should be taken when using correlations developed for more hydrophobic chemicals since partitioning into biota or organic carbon phases may be primarily into aqueous rather than organic media. 

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The BUSES model provides an estimate of the organic carbon/water partition coefficient (Koc) based on the octanol/water partition coefficient (Kow)- From these data, it is evident that the methyltins are less likely to partition onto organic carbon (in sediments, soils, biota) than are the butyl- and octyltin compormds due to then-lower partition coefficients and higher water solubilities. The 7/oc value can then be used to derive sohds/water partition coefficients in suspended matter, in sediment, and in soil using values of 10%, 5%, and 2% for organie carbon, representing typical organic carbon contents of suspended matter, sediment, and soil, respeetively. 

Octanol-Water Partition Coefficient (Kow) — The equilibrium ratio of the concentrations of a chemical in -octanol and water, in dilute solution. 

The octanol-water partition coefficient Kow is widely used as a descriptor of hydrophobicity. Variation in /fow is primarily attributable to variation in activity coefficient in the aqueous phase (Miller et al. 1985) thus, the same correlations used for solubility in water are applicable to /fow. Most widely used is the Hansch-Leo compilation of data (Leo et al. 1971, Hansch and Leo 1979) and related predictive methods. Examples of Kow correlations are ... 

Rapaport, R.A., Eisenreich, S. (1984) Chromatographic determination of octanol-water partition coefficients (KoW s) for 58 PCB congeners. Environ. Sci. Technol. 18, 163-170. 

Then there are a number of pesticides, e.g. the phenolic herbicide dinoseb and the fungicide pentachlorophenol, whose speciation varies strongly in the environmental pH-range. For this reason, one has to consider the pwhen estimating their environmental fate. Structures of the compounds discussed in this section are depicted in Table 1, together with a listing of their pand octanol-water partition coefficients, Kow, of the neutral species (unless otherwise indicated). Typical basic pollutants include the industrial chemicals aniline and jV.jV-dimethylaniline. 

The octanol-water partition coefficient, Kow, is the most widely used descriptor of hydrophobicity in quantitative structure activity relationships (QSAR), which are used to describe sorption to organic matter, soil, and sediments [15], bioaccumulation [104], and toxicity [105 107J. Octanol is an amphiphilic bulk solvent with a molar volume of 0.12 dm3 mol when saturated with water. In the octanol-water system, octanol contains 2.3 mol dm 3 of water (one molecule of water per four molecules of octanol) and water is saturated with 4.5 x 10-3 mol dm 3 octanol. Octanol is more suitable than any other solvent system (for) mimicking biological membranes and organic matter properties, because it contains an aliphatic alkyl chain for pure van der Waals interactions plus the alcohol group, which can act as a hydrogen donor and acceptor. 

The most notable correlation found for the organic carbon partition coefficient (Koc) of hydrophobic chemicals is that obtained with the octanol/water partition coefficient (Kow). Recently, a single correlation has been proposed for all these compounds based on previous data (log(Koc/Kow) versus logAow) the specific approximation proposed was Koc = 0.35 Kow, but this could vary by a factor of 2.5 in either direction [39]. 

Bowman BT, Sans WW. 1983. Determination of octanol-water partitioning coefficients (Kow) of 61 organophosphorus and carbamate insecticides and their relationship to respective water solubility (S) values. J Environ Sci Health [B] 13 667-633. 

Octanol/water partition coefficient (Kow) other solvents)... 

Octanol/Water Partition Coefficient (Kow) — the equilibrium ratio of the concentrations of material partitioned between octanol and water. This coefficient is considered to be an index of the potential of a chemical to be bioaccumulated. Higher values of K, are associated with greater bioaccumulative potential. 

The driving force for the mass transfer of the solute in the three-phase system can be determined with the solvent/water partition coefficient, just as the partition coefficient for gas/liquid phases, the Henry s Law constant, is used to determine the driving force for the mass transfer of ozone. A solute tends to diffuse from phase to phase until equilibrium is reached between all three phases. This tendency of a solute to partition between water and solvent can be estimated by the hydrophobicity of the solute. The octanol/water partition coefficient Kow is a commonly measured parameter and can be used if the hydrophobicity of the solvent is comparable to that of octanol. How fast the diffusion or transfer will occur depends not only on the mass transfer coefficient in addition to the driving force but also on the rate of the chemical reaction as well. 

Bowman, B. T., and W. W. Sans, Determination of Octanol-Water Partitioning Coefficients (KOW) of 61 Organophosphorus and Carbamate Insecticides and Their Relationship to Respective Water Solubility (5) Values. J. Environ. Sci. Health, 1983 B18(6), 667-683. 

The exchange of chemical compounds from the gas phase to a surface, e.g. atmospheric particles, soil, water, vegetation or other surfaces, is controlled by the affinity of the compound to this surface. The ratio of vapour pressure to water solubility can be used as indicator between levels in the atmosphere and water surface (Henry s law H constant). In many model calculations, the ratio between POP levels in octanol and water, the octanol-water partitioning coefficient (Kow), is used as reference for the distribution of POP in organic material [14]. Consequently, the expression ///RT (Cair/Cwalcr) and Kow (Coctanoi/Cwater) provide the octanol-air partitioning coefficient (Koa) ... 

Fig. 14 Bioaccumulation factor (BAF) in Lake Superior lake trout versus octanol-water partition coefficient (.Kow) for 19 toxaphene congeners plus B9-715 and B10-1010 [74], Results for 6 congeners (P31, B8-1413, B9-1679, B9-1046, B9-715 and B10-1010) were estimated assuming water concentrations at the detection limit. The coefficient of variation r2 = 0.35 P< 0.01...

PAHs have been found all over the globe in all compartments of the environment. They are ubiquitous because the are persistent. Recalcitrance in PAHs may stem, in part, from the delocalized electrons in the planar pi orbitals of the aromatic structure. Their relatively high octanol-water partition coefficients, Kows, make them rather lipophilic. The lipophilicity of PAHs forces them from the dissolved phase to particles and also into lipid rich organisms, but they can be metabolized in higher organisms. However, these metabolites are often more toxic than their parent PAHs. When combined with other stressors, particularly ultraviolet radiation, PAHs can exert enhanced toxicity. 

M Company (2001a) Kow (Solubility in Water, Natural Seawater, An Aqueous Solution of 3.5% Sodium Chloride, and n-Octanol with Subsequent Calculation of the n-Octanol Water Partition Coefficient (Kow) of PFOS for each of the Aqueous Matrices). USEPA Docket No. AR226-1030a031... 

Three general classes of estimation method are in frequent use. The first exploits the considerable data on octanol water partition coefficient Kow and the well developed estimation methods for that quantity by seeking a relationship between solubility and KqW. The second attempts to relate molecular structure to solubility by a variety of techniques involving counting bonds or groups or calculating molecular volumes, areas, or other topological indices such as connectivity. 

Correlation with Physical and Chemical Properties — The Octanol/Water Partition Coefficient (Kow)and Aqueous Solubility (S)... 

Gill uptake rate relationships. General relationships between the gill uptake rate constant (kj), gill elimination rate constant (k2), fecal egestion rate constant (kE), and the bioconcentration factor, based on chemical equilibrium (BCFeq) and on the truly dissolved chemical concentration in the water (BCFd) and the total chemical concentration in the water (BCFt), the field based bioaccumulation factor based on the truly dissolved chemical concentration in the water (BAF) and the octanol-water partition coefficient Kow. 

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