Octanol-water volume ratio

The asterisked quantity is defined in concentration units of moles of species dissolved in the organic phase per liter of aqueous phase solution. The octanol-water volume ratio, r = Voci/Vwater. takes into account differences in volumes of the two phases. In pharmaceutical applications, prachcal values of row range from 0.003 to 3 (and sometimes higher). 

Using a pPLC system, log P for one unknown compound was determined in less than 1 hr. It is important to note that the excess capacity provided by the system (24 columns are available for simultaneous analysis) allows simultaneous determination of log P for six additional compounds. The same study required 5 hr using conventional HPLC, and consumed 300 mL of solvent, equivalent to 15 times the volume of solvent used for the evaluations via jtiPLC. A similar approach can be used to evaluate log D, the octanol-water distribution coefficient—a measure of the distribution ratios of all combinations (ionized and unionized) of octanol and pH-buffered water. 

It should be noted that Eq. (6.9) gives the partition coefficient in terms of mole fractions, as is common in the chemical engineering literature. We therefore denoted it as if. In chemistry it is usual to report partition coefficients in mol/l units, and to denote them by K. Although the partition coefficient is dimensionless in both conventions, it differs by the ratio of the molar volumes of the two solvents. As we will see in more detail in the next chapter, logarithmic partition coefficient data have been used heavily for the parameterization of COSMO-RS. The octanol-water partition... 

This equation is simply a rearrangement of the distribution constant equation that relates the mass sorbed to the stationary phase divided by the mass in the solution phase. The authors note that the amount of analyte sorbed by the coating is proportional to the initial analyte concentration in both Eqs. (12.1) and (12.2). However, the additional term of A"V, is now present in the denominator of Eq. (12.2). This term decreases the amount of solute sorbed (nJ when this term is comparable in size to When it is much smaller than then only the volume of sample is important. As KV becomes much greater than V2, then the terms KV in numerator and denominator cancel, and one is left with the conclusion that the majority of the original analyte, C20, is sorbed. Thus, the extraction is quantitative at this point. In practice, the authors have found that for 90% of the sample to be sorbed into the coating, the distribution coefficient must be about an order of magnitude greater than the phase ratio, V2/V,. For this to occur, the K must be approximately 1000, which is equivalent to compounds with an octanol-water partition coefficient ( ow) of approximately the same value, or log of 3. 

References 7, 20, and 28 describe various measurement techniques. The chemical in question is added to a mixture of octanol and water whose volume ratio is adjusted according to the expected value of Kow. Very pure octanol and water must be used, and the concentration of the solute in the system should be less than 0.01 mol/L [7]. The system is shaken gently until equilibrium is achieved (15 min to 1 hr). Centrifuga-tion is generally required to separate the two phases, especially if an emulsion has formed. An appropriate analytical technique is then used to determine the solute concentration in each phase. 

For example, in an adaptation of the standard methods [9S, 99], the FTIR analytical technique just described was used to determine the octanol/water partition coefficient of naphthenic acids as a function of the aqueous phase pH. In this case, the octanol/water partition coefficient (Kqw) is the ratio of the equilibrium concentration of naphthenic acids in octanol to that in the aqueous phase. K w was determined by equilibrating a known total amount of naphthenic acids in a 1 1 volume mixture of octanol-saturated water (buffered to the selected pH) and water-saturated octanol, then determining the aqueous phase concentration. Figure 13 shows the variation in log(Kow) with solution pH. It can be seen that Kow decreases from quite high values, near 1000, to about 1 as the... 

The concept of the equilibrium sampler is analogous to that of the octanol-water equilibrium partition coefficient (fQ,w) used since the 1970s to predict the potential for persistent nonpolar contaminants to concentrate in aquatic organisms [71]. The use of equilibrium-t) e passive samplers in the aquatic environment depends on the development of a sampler-water partition coefficient (fCs ) defined as the ratio of sampler to water concentration of the compound of interest at thermod)mamic equilibrium. The other key parameter determining the utility of an equilibrium-type passive sampler is the time taken to reach an approximate equilibrium condition. A range of approaches applied in developing equilibrium-t)q)e passive samplers include polyethylene or silicon sheets of various volume to surface area ratio [72] and solid-phase microextraction techniques [73]. 

Values of Henry s law constant k =plc, where p is the partial pressure of the solute in the gas above the solution and c is the concentration of the solute) is a quantity frequently apphed in the thermodynamic description of dilute aqueous solutions, which is used in environmental chemistry and atmospheric physics as a major criterion for describing air-water partitioning of solutes at near ambient conditions. It plays amajor role in evaluating the transport of pollutants between atmosphere and aquatic systems, rainwater and aerosols. The octanol-water partition coefficient is a dimensionless number defined as the ratio of the compound s concentration in a known volume of octan-l-ol (Cq) to its concentration in a known volume of water (c ) after the octan-l-ol and water have reached equihbrium. It has been found to be related to water solubility, soil/sediment absorption coefficients and bioconcentration factors of pollutants for aquatic life. The adsorption coefficient normalised to the organic carbon content of the soil (sediment) is a useful indicator of the binding capacity of... 

TABLE 3 CAS numbers, molecular weight, melting point (T ), molecular volume (v), fugacity ratio (F), solubility in octanol (Cg), activity coefficient in octanol (log yo), and octanol/water partition coefficient (log... 

TABLE 1 gives the reported values or ranges of the physical-chemical properties of chlorobenzenes (CBs), polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDDs). Fugacity ratios were obtained from a single estimated entropy of fusion of 56 J mol °K (Yalkowsky 1979), molar volumes were calculated by the Le Bas method, an additive group contribution method (Reid et al. 1977). Total surface area (TSA) values were obtained from Yalkowsky et al. (1979 a,b). Solubilities, vapour pressures and octanol/water partition coefficients (Andren et al. 1986 Shiu and Mackay 1986 Bobra et al. 1985) are also tabulated. Henry s law constants were calculated as PSl/C and the octanol solubility Q as C Kq, . 

Figure 6 Partitioning of LiCl between water and 1-octanol at 25°C, as taken from ref. [2l6]. The lithium distribution ratios Du were determined at 1 1 initial phase ratio by use of ion chromatography (IC), inductively coupled plasma (ICP) atomic emission spectrometry, and Li NMR spectrometry. A correction was made for the slight volume changes due to the mutual solubility of 1-octanol and water. Error bars are indicated only for the ICP data, which were the least precise data obtained by the three techniques. The solid curved line represents the equilibrium model calculated by SXLSQl using the values of log/Cs= = —6.85 and logX, = — 2.74 (Table 12). The dashed curved line is an extrapolation of the model to indicate the approach to the calculated asymptotic value of the distribution ratio at infinite dilution (3.76 X 10...

Since the ratio of the molar volumes is constant ATqw simply reflects the deviation from ideal behavior of the compound in the two solvent systems. It is also apparent that, for organic compounds, y is the major factor contributing to A ow Organic compounds would approach ideal behaviorl (y 1) in the octanol phase while in water nonpolar organics, y can be as high as 10 (Table 2.5). 

Overall Water-to-Oil Ratio Optimum Octanol Concentration (g/lOO ml) Middle Phase Volume (%)... 

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