Organic solvent—water distribution ratio

Kd = the solute s organic solvent water distribution coefficient. k = chromatographic capacity ratio (k — tr — t0/t0, tr and t0 being solute retention time and mobile phase holdup time, respectively), a and b = coefficients whose magnitudes depend on the LL distribution and RPLC systems. 

The principle of solvent extraction is illustrated in Fig. 1.1. The vessel (a separatory funnel) contains two layers of liquids, one that is generally water (Sa,) and the other generally an organic solvent (S g). In the example shown, the organic solvent is lighter (i.e., has a lower density) than water, but the opposite situation is also possible. The solute A, which initially is dissolved in only one of the two liquids, eventually distributes between the two phases. When this distribution reaches equilibrium, the solute is at concentration [A]a, in the aqueous layer and at concentration [AJ g in the organic layer. The distribution ratio of the solute... 

In contrast to air-water partitioning, the situation may be a little more complicated when dealing with organic solvent-water partitioning of organic acids and bases. As an example, Fig. 8.9 shows the pH dependence of the n-octanol-water distribution ratios, D,ow (HA, A"), of four pesticides exhibiting an acid function ... 

The constant K is termed the distribution or partition coefficient. As a very rough approximation the distribution coefficient may be assumed equal to the ratio of the solubilities in the two solvents. Organic compounds are usually relatively more soluble in organic solvents than in water, hence they may be extracted from aqueous solutions. If electrolytes, e.g., sodium chloride, are added to the aqueous solution, the solubility of the organic substance is lowered, i.e., it will be salted out this will assist the extraction of the organic compound. 

If a substance is soluble in both water and the organic solvent, the result of the extraction depends on the ratio of solubilities if the partition coefficient , e.g. the ratio of the solubility in water to that in ether, is large, correspondingly more ether must be used or the number of extractions must be increased. For this coefficient determines how a substance soluble in two immiscible solvents will distribute itself between them. Whether an aqueous solution should be extracted with a certain amount of ether in one portion or whether it is better to extract several times with smaller portions is... 

Therefore, the distribution ratio of B remains constant only if the ratio of the activity coefficients is independent of the total concentration of B in the system, which holds approximately in dilute solutions. Thus, although solutions of metal chelates in water or nonpolar organic solvents may be quite nonideal, Nernst s law may still be practically obeyed for them if their concentrations are very low (JCchehte< 10" ). Deviations from Nernst s law (constant D ) will in general take place in moderately concentrated solutions, which are of particular importance for industrial solvent extraction (see Chapter 12). 

The particular case of the solubilities of organic solutes in water can be dealt with by rather simple equations, based on a general equation for solvent-dependent properties, apphed to solubilities, distribution ratios, rate constants, chromatographic retention indices, spectroscopic quantities, or heats of association [4] [see Eq. (2.12) for an example of its application]. For the molar solubilities of (liquid) aliphatic solutes B in water at 25°C the equation... 

Several models have been suggested for the estimation of the distribution ratios of nonionic solutes between water and (practically) immiscible organic solvents. One model takes 1-octanol to represent, in general, lipophilic ( fat-liking ) media, which hydrophobic ( water-fearing ) solutes would prefer over water. Such media may be oils, biological lipid membranes, and, somewhat less suitably, hydrocarbon solvents. 

These data allow one to suggest simple correlation equations to predict the distribution ratios of neutial organic molecules between water and [C4CiIm] [PFJ by the distribution ratios measured for conventional solvents ... 

C. Ende have shown that lithium chloride has a tendency to polymerize when dissolved in alcohol and other organic solvents H. C. Jones and co-workers have also shown that there is a tendency to form complexes between the lithium salt and the organic solvent while E. W. Washburn and E. W. Mclnnes calculate that in a JN-aq. soln. of lithium chloride, each mol. of the solute is hydrated with 18 mols. of the solvent. F. G. Donnan and W. E. Garner have studied the distribution ratio of lithium chloride between amyl alcohol and water. [LiCl]Am h[LiCl]Aq =0-0273. 

If a non polar solvent is used as an organic phase, dissociation of the ion pair in this solvent is negligible, whereas it is supposed to be complete in water. Note that in practice, relatively high concentrations are used so that they should be replaced by activities. The distribution ratio of the metal ion is equal to ... 

Nanocapsule/nanosphere size ranges between 200 and 350 nm were observed to be affected by both the oil-ethanol ratio and the oil-monomer ratio [63, 64], It is also influenced by the particular oil, water-miscible organic solvent, and nonionic surfactant in the aqueous phase. The pH of the aqueous phase and the temperature also affect the size distribution. 

Consider, for instance, the use of F ,g milliliters of organic solvent to extract a solution initially containing Xj moles of a solute A dissolved in V milUhters of water. From Equation (23-6) the distribution ratio D may be expressed by... 

In the first two categories the distribution ratio should be relatively independent of the organic solvent, except for the influence of mutual solubility of the organic solvent with water on the distribution ratio. In the last four the organic solvent may play an active role in the extraction process. 

The first observation that the ratio of concentrations of a solute (e.g., I2 or Br2), when distributed between an organic solvent (e.g., CS2 or ether) and water, remained constant even when the volume ratio of the immiscible solvents changed widely, was first reported by Berthelot and Jungfleisch in 1872, as illustrated by Eq. (1) ... 

Equal volumes (20 ml) of the TOPO solution in the organic solvent and uranyl sulphate solution containing sulphuric acid were shaken for 10 min in 50 ml stoppered conical flasks in a thermostatic water-bath at the required temperature. Preliminary experiments showed that equilibration is complete in 10 min. The mixture was centrifuged and separated, and uranium was stripped from the organic phase with 0.5 M ammonium carbonate solution, and then the distribution coefficient (the ratio of the equilibrium concentration of uranium in the organic phase to that in the aqueous phase, [U]org/[U]aq) was obtained. 

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