Distribution coefficients, course

The most widely used descriptor for the hydrophobicity term in toxicology is the distribution coefficient between octanol and water, log Pq< - (the environmental scientists would rather call it log The bulk solvent octanol is of course a... 

Of course, the shape of the lipophilicity profiles becomes much more complicated for molecules with multiple ionized forms. Nevertheless, the partition coefficients may be calculated from the distribution coefficient using the general equation [113] ... 

Of course, the first question is, where must the system go This is formally answered by chemical thermodynamics in terms of equilibrium-distribution coefficients or Henry s Law constants. For our purposes these are well described from the elementary viewpoint by MacDougall (8) and from the viewpoint of practical application by Norman (9). 

The value of A ex is, of course, fixed for any given cation-extracting agent-solvent system. Analytically important distribution coefficients, D, can be varied by changing the concentration of ligand relative to that of the metal and more significantly by adjusting the pH for 50% extraction D = 1 and equation (21) reduces to... 

Of course, redox reactions do not occur in isolation but are coupled through complexation reactions to other species. For example, Eq. 2.30 could include terms for nitrate and amine complexes, in addition to those for free nitrate, nitrite, and ammonium ions, if a typical soil solution were under consideration. The calculation of nitrogen speciation then would proceed just as described above. Indeed, redox reactions introduce no new mathematical elements into a speciation computation, any more than would the consideration of, for example, C02 reactions. The only new item brought in is an additional variable, the pE value, which, like the partial pressure of C02(g), must be specified in order to solve mole balance equations for distribution coefficients. 

The concentration of the weak acid or of the weak base can be determined by distribution between water and another solvent, such as benzene or chloroform the partition coefficient of the acid or base between the water and the other solvent must, of course, be known. The degree of hydrolysis may then be calculated from the concentration of the salt and the determined concentration of the weak acid or base. An example of such a salt is aniline hydrochloride. This is partially hydrolysed into aniline and hydrogen chloride. On shaking the aqueous solution with benzene the aniline will distribute itself between the water and benzene in the ratio of the distribution coefficient. The initial concentration of aniline hydrochloride is known, the concentration of the free aniline in the aqueous solution can be computed from that found in the benzene solution, and from this the total concentration of aniline, produced by hydrolysis, is deduced. Sufficient data are then available for the calculation of the degree of hydrolysis. 

The distribution coefficient values compiled for different ions in the course of their sorption from seawater on different clinoptilolite sources are presented in Table 2 [54]. The clinoptilolite content in all of the samples was approximately the same (about a mass % of 70). All experiments were carried out at t = 20°C on clinoptilolite in the NH form. The distribution coefficient was defined as follows ... 

The major factor in achieving a successful simple extraction is the proper choice of solvent. The solvent must, of course, be insoluble in the original solution and must give a reasonably favorable distribution coefficient. As a general rule, one chooses a solvent in which the compound is quite soluble. Commonly used solvents are ether, benzene, carbon tetrachloride, and chloroform when extracting a compound from an aqueous solution. Not only are these good solvents for most organic compounds, but they may also be easily removed from the solute. 

The term on the left hand side of equation (5.28) is constant at a given temperature and pressure, so it follows that aja = constant and, of course aja = constant. These constants are the partition coefficients or distribution coefficients, P. If the solute forms an ideal solution in both solvents, activities can be replaced by concentration, so that... 

The other common representation of the uptake studies is in form of percentage of uptake (pH) or distribution coefficient (pH) curves at constant initial concentration of the adsorbate and solid to liquid ratio. The typical course of the percentage of uptake (pH) curves for cations and anions is presented in Fig. 4.5-4.7. In a few publications the dissolved fraction rather than uptake is plotted as a function of pH. In such a representation anion type curves look like the cation type curves shown in Fig. 4,5 (A) and vice versa. The typical uptake curves for cations (A) and anions (B) (for low concentration of the solute) are shown in Fig. 4.5. The uptake... 

From the data and from the experimental procedure described in the patent the overall distribution coefficient based on total acetate is calculated as 0.004. This is an extremely small number, which, of course, is a consequence of the small amount of free acid available in solution at the high pFI of the solution. Other authors (Yates, 1981 Busche, Shimshick, and Yates, 1982) address the problem of incomplete extraction of organic acids from high-pH solutions using CO2. The authors suggest that the raffinate from the extractor can be recycled to the fermentor. 

The above simple model of a steric exclusion mechanism was considered by several authors attempting to describe quantitatively the gel chromatographic separation process. Distribution coefficients were expressed on the basis of the model considerations of the dimensions of both the separated molecules and the pores of gel, as well as of the stochastic model approaches (for reviews see e.g.. Refs. 1, 3-6), and also of the thermodynamic reasoning on the changes of conformational entropy of macromolecules due to their transfer from the interstitial volume into the pores in the course of separation [7]. However, besides the steric exclusion from the pores, at least two other size-based mechanisms are operative in the ideal gel chromatography ... 

Thus the distribution coefficient again becomes linear in as 9 tends to h, its maximum value according to Eq. 4.10. The slope of the line is /9o/ i and its x intercept is, of course, b. If adsorption data are plotted as in... 

Other techniques that can be used to measure distribution coefficients of a guest molecule between the bulk aqueous phase and a micellar or vesicular aggregate include gel filtration, electromotive force measurements, solnbility and vapor pressure measurements, muon spin rotation experiments and headspace gas chromatography. Theoretical models are, of course, also helpful, as, for example, in stndies of the enveloping of charged proteins by lipid bilayers. 

In Section 11.4, it was shown how suitable solvents can be selected with the help of powerful predictive thermodynamic models or direct access to the DDB using a sophisticated software package. A similar procedure for the selection of suitable solvents was also realized for other separation processes, such as physical absorption, extraction, solution crystallization, supercritical extraction, and so on. In the case of absorption processes or supercritical extraction instead of a g -model, for example, modified UNIFAC, of course an equation of state such as PSRK or VTPR has to be used. For the separation processes mentioned above instead of azeotropic data or activity coefficients at infinite dilution, now gas solubility data, liquid-liquid equilibrium data, distribution coefficients, solid-liquid equilibrium data or VLE data with supercritical compounds are required and can be accessed from the DDB. 

Based on the concept mentioned above, Shibukawa et al. [ref. 31] have proposed a new model regarding the distribution of ionic solutes in practical exclusion chromatography, where the distribution of sample ion is assumed to be dependent not only on its own physicochemical properties but also on those of the counterion and coion in the eluent. The background eluent ion effect on the distribution coefficients of ionic solutes on neutral hydrophilic gels can be understood on the basis of the ion partition model presented. If there is not any specific interaction such as complex formation between the sample ion s (hereafter sample ion is represented by cation, but, of course, the expressions... 

Consider a distribution system that consists of a gaseous mobile phase and a liquid stationary phase. As the temperature is raised the energy distribution curve in the gas moves to embrace a higher range of energies. Thus, if the column temperature is increased, an increasing number of the solute molecules in the stationary phase will randomly acquire sufficient energy (Ea) to leave the stationary phase and enter the gas phase. As a result, the distribution coefficient with respect to the stationary phase of all solutes will be reduced as the temperature rises and it will be seen in due course that this will cause the band velocity of all the solutes to be increased. 

Separation of Acids and Bases. The separation of organic acids and bases is another important and extensive use of the extraction method. The distribution coefficients of organic acids and bases are affected by pH when one of the solvents is water. An organic acid that is insoluble in neutral water (pH 7) becomes soluble when the water is made basic with an aqueous sodium hydroxide solution. The acid and the sodium hydroxide quickly react to form a sodium carboxy-late salt, RC02 Na". The salt is, of course, ionic and therefore it readily dissolves in the water. Thus, the add-base reaction reverses the solubilLty characteristics of a water-insoluble organic add. 

As rule of thumb, it can be stated that the purification step sweating is more suitable for the static mode compared to the dynamic mode, because the achieved improvements in distribution coefficient are less than those for static mode. This rule depends, of course, on the mixture (the substances) used. In summary, the possible advantages of sweating are [7,9]... 

The extent of solubilization of benzoic acid by several n-hexadecylpolyoxy-ethylene surfactants of varying ethylene oxide chain length showed an apparent increase with increasing temperature [163]. However, benzoic acid has an appreciable water solubility which also increases with temperature and this is, of course, a contributing factor towards the overall increase in the amount of benzoic acid taken into solution. Allowance for this effect was made by expressing the solubilization data in terms of the micelle/water distribution coefficient, P ,. Table 5.21 shows a minimum in at about 300 K. The decrease in below this... 

In the above figures, Lau and La refer to content ratio of Au Ag in matte phase and slag phase. The experiments have shown distribution coefficients of Au Ag decrease repetitively with the increases of matte grade and Cu in slag in the course of matte smelting by using copper... 

An important factor in determining the course of uptake, transport, and distribution of xenobiotics is their polarity. Compounds of low polarity tend to be lipophilic and of low water solubility. Compounds of high polarity tend to be hydrophilic and of low fat solubility. The balance between the lipophilicity and hydrophilicity of any compound is indicated by its octanol-water partition coefficient (K J, a value determined when equilibrium is reached between the two adjoining phases ... 

The resulting equations for heterogeneous polymers assume the same general form, but numerical evaluation of the second coefficient, A 2 or F2, involves formidable summations over the entire distribution. Molecular weights M occurring in the first term of the osmotic expressions must, of course, be replaced by number averages, Mn- Dilute solutions of two chemically different polymer species also have been treated. ... 

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