Determining the partition coefficient

When both the neutral and the n different charged forms of a compound partition into the organic phase, a minimum of n + 1) titrations with different phase volume ratios is necessary to determine the partition coefficient of each species. For a monoprotic substance, for instance, can be related to pK by an empirical equation [119] ... 

The easiest way to determine the partition coefficient is to extract V cm3 of saturated aqueous solution with Vz cm3 of solvent, and determine the concentration Cj in the latter. The amount left in the aqueous phase is (Ci V — C2V2) = M, so that the partition coefficient is given by... 

These new methods for determining the partition coefficients of ionized species are still e3q>erimenta1, but they are presented in a spirit that they may stimulate thinking and further refinement. Single-phase titrations with HCl in octanol have only recently been run. A possible concentration dependency of pKa in the single-phase titrations has been suggested by a referee and will be looked for. Further refinement of the two-phase titrations may incorporate ion-pair association constants. 

To test this, we determined the partition coefficients of two different solute extremes - vitamin B-12 and norethindrone. Vitamin B-12 is a relatively large, hydrophilic solute, while norethindrone is a relatively small, hydrophobic solute. Thus we would expect size exclusion to dominate partitioning with vitamin B-12, while hydrophobic interactions should dominate with norethindrone. This behavior is in fact observed in Figs. 19 and 20. The values of K... 

In the case of HOCs, equations are used to determine the partitioning coefficient between organic carbon and water (K0c), based on aqueous solubility (5) or an octanol/water partition coefficient (Kov/). 

To recap, first one needs to determine the partition coefficients for each of the analytes on each of the two liquid phases—each chosen because it is effective in separating most of the sample analytes when used alone. Second, one plots these K values versus < > by using the two K values for each analyte at 4>a- = 0 and < >a- = 1.0 and joining these two points with a straight line. Then, the window diagram is drawn using <(> values calculated from Eq. (10) at various a values. 

Because the chemical potential of component a in the liquid phase and that in the contacting gas phase are equal in equilibrium, it is possible to determine the partition coefficient for component a between the liquid and gas phases with the help of thermodynamic quantities. 

However, trinitrotoluene reacts with alkalis, yielding organo-metallic products. The readiness of trinitrotoluene to react with alkalis has suggested the idea that it is an acid. This problem was studied by Fanner [75] in 1901. He applied a method, based on determining the partition coefficient K for trinitrotoluene between two liquid phases water and benzene or water and ligroin phase, with addition of sodium hydroxide to the water phase. 

Several methods are used to determine the partition coefficient of solutes between water and liposomes. 

The ratio of organic to aqueous phase is expressed as r, PB is the partition coefficient of the unionised, PBH+ is the partition coefficient of the protonated analyte. Using this approach depending on the nature of the analyte, several titrations (for monoprotic compounds at least two experiments) using different ratios of lipid to water phase have to be carried out in order to determine the partition coefficients of the solute. The result is a partition coefficient-pH profile for the compound and its distinct ionisation states. 

Effect of Tween 80 on TFMS tt Values. Table VI lists octanol/water partitioning data for the 15 substituted TFMS compounds used in this study. Logarithms of the partition coefficients obtained both in the absence (log Px) and presence (log Px) of 0.1% (w/v) Tween 80 are given. (Procedures for determining the partition coefficients Px and Px have been outlined in the Experimental section.) tt, tt, and w" values in Table VI were calculated according to the following relationships ... 

Most of the static headspace methods determine the partition coefficient by quantifying volatile concentration above a sample by gas-chromatography. The vapour phase calibration method (VPC) uses an external vapour standard for calibration. One must assure that the pure component is completely vaporized before injection. A widely employed alternative is the Liquid calibration static headspace (LC-SH) method (YoiWey et al. 1991 Nedjma 1997). A third approach uses HS-SPME. SPME may be used to determine partition coefficients if short sampling times are applied the process must only sample the headspace and not disrupt the equilibrium (Jung and Ebeler 2003). This method has become very popular to study the effect of wine macromolecules on the liquid-vapor equilibrium, (Whiton and Zoecklein 2000 Escalona et al. 2002 Hartmann et al. 2002 Aronson and Ebeler 2004). 

The diffusion coefficient in the aqueous phases (donor and acceptor phase) would not play a role if practically no concentration gradient exists within these compartments, in other words, if the diffusion in the aqueous compartments is not rate determining. The partition coefficient between the lipid phases and aqueous phases, K, determines whether CmD and are higher or lower than Cd and Ca, respectively. 

The basic principle of the method is to measure the dissolution of the substance in two different phases, water and -octanol. In order to determine the partition coefficient, equilibrium between all interacting components of the system must be achieved after which the concentration of the substances dissolved in the two phases is determined. The shake-flask method is applicable when the log Kow value falls within the range from -2 to 4 (OECD 107, 1995). The shake-flask method applies only to essential pure substances soluble in water and -octanol and should be performed at a constant temperature ( 1°C) in the range 20-25 °C. 

Second-derivative electronic absorption spectrophotometry, which is a method applied without any separation procedure, was used to determine the partition coefficients of three N-monodemethylated phenothiazine drugs, such as CPZ, triflupromazine and promazine, between the phosphatidylcholine bilayers of small unilamellar vesicles (SUVs) and water, in order to evaluate their affinity to biomembranes. The second-derivative spectra exhibited distinct isobestic points, confirming the entire elimination of the residual background signal effects of the SUVs which were observed in the conventional absorption spectra [161]. 

In determining the partition coefficient and the solute diffusion coefficient in the stationary polymer phase, either moment analysis or Fourier domain fitting was used. The two techniques are described below. 

Overview. The capillary column IGC technique was used to determine the partition coefficients and diffusion coefficients of a number of solvents (methanol, acetone, methyl acetate, ethyl acetate, propyl acetate, benzene, toluene, and ethylbenzene) in poly(methyl methacrylate). Measurements below the glass transition temperature were obtained for the PMMA/methanol system. 

To establish extraction efficiencies for a substance A one first needs to know, or experimentally determine, the partition coefficient, Kp. This constant relates the concentration of solute A in the extraction solvent to the concentration of the solute in the extracted phase, at equilibrium (Eq. 10.35). 

The binding of synthetic ion channels and pores to lipid bilayer membranes often causes a change in intra- or intermolecular self-organization that is visible in sufficiently sensitive methods such as fluorescence (e.g. tryptophan emission) [14] or circular dichroism spectroscopy and can be used to determine the partition coefficient. Convenient methods of detection under relevant conditions are fluorescence resonance energy transfer (FRET) or fluorescence depth quenching (FDQ) [3, 4, 6]. Many fluorescent probes for the labeling of both synthetic ion channels/... 

Many different experimental methods have been used to determine the partition coefficient of solutes. In this chapter it is not possible to present aU the different methods, but we try to give a representative selection of the most-used methods. The methods have been classified into the following main groups (1) solubility, (2) properties of the surfactants, (3) spectroscopic methods, (4) separation methods, (5) thermodynamic functions. 

Basically two nuclear magnetic resonance (NMR) techniques can be used to determine the partition coefficient. The first is based on the Fourier transform NMR pulsed-gradient spin echo (FT-PGSE) self-diffusion techiuque, the other by the NMR paramagnetic relaxation technique. In both techniques the fraction of solute in the micelle, a, is determined and can thus be calculated through Equation 6.10. 

The experimental techniques vary in the sense that some will determine the partition coefficients directly and some will determine the fraction of solute solubilized in the micellar phase. The data are easily converted if the surfactant and solute concentrations are given. 

The rejection R depends mainly on the ratio of solute size to pore size, A, which determines the partition coefficient K, and on the ratio v/k, which determines the concentration polarization effect. From Eqs. (30.52) and (30.57)... 

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