Solvent solute distribution between two

Figure 11.56. Equilibrium diagram solute distributed between two solvents...

The distribution coefficient w of a solute distributed between two solvents, usually between an organic phase and an aqueous phase, is the ratio of its concentrations in the two phases ... 

The constant Kp is called the partition coefficient or distribution coefficient for the solute distributed between two solvents at a given temperature. The partition or distribution law states that at a fixed temperature a solute distributes itself between two immiscible solvents so that the ratio of the concentrations of solute in each layer is constant. 

The simplest system of four components occurs when two solutes distribute between two solvents, e.g., the distribution of formic and acetic acids between the partly soluble solvents water and carbon tetrachloride. Complete display of such equilibria requires a three-dimensional graph [72], which is difficult to work with, but frequently we can simplify this to the distribution xy) curves, one for each solute, such as that for a single solute in Fig. 10.3 More than four components cannot be conveniently dealt with graphically. 

Another type of multicomponent separation involves one or more solutes distributed between two solvents that are immiscible and which form two different phases. A third type of multicomponent separation involves a gas phase and a liquid phase such that the solvent, which is the dominant... 

The isopiestic method is based upon the equality of the solvent chemical potentials and fugacities when solutions of different solutes, but the same solvent, are allowed to come to equilibrium together. A method in which a solute is allowed to establish an equilibrium distribution between two solvents has also been developed to determine activities of the solute, usually based on the Henry s law standard state. In this case, one brings together two immiscible solvents, A and B, adds a solute, and shakes the mixture to obtain two phases that are in equilibrium, a solution of the solute in A with composition. vA, and a solution of the solute in B with composition, a . 

The distribution of the solute, called the extractable complex or species, between the two immiscible solvents. This step can be quantitatively described by Nernst s distribution law, which states that the ratio of the concentrations of a solute distributing between two essentially immiscible solvents at constant temperature is a constant, provided that the solute is not involved in chemical interactions in either solvent phase (other than solvation). That is,... 

Application of Nernsfs distribution law. If a solute is distributed between two solvents a and fi, then in place of equation (8 38) Mre have... 

Solute distribution between two partially-miscible solvents... 

Liquid-liquid extraction is a process for separating components in solution by their distribution between two immiscible liquid phases. Such a process can also be simply referred to as liquid extraction or solvent extraction however, the latter term may be confusing because it also applies to the leaching of a soluble substance from a solid. 

Solvent extraction, sometimes called liquid-liquid extraction, involves the selective transfer of a substance from one liquid phase to another. Usually, an aqueous solution of the sample is extracted with an immiscible organic solvent. For example, if an aqueous solution of iodine and sodium chloride is shaken with carbon tetrachloride, and the liquids allowed to separate, most of the iodine will be transferred to the carbon tetrachloride layer, whilst the sodium chloride will remain in the aqueous layer. The extraction of a solute in this manner is governed by the Nernstpartition or distribution law which states that at equilibrium, a given solute will always be distributed between two essentially immiscible liquids in the same proportions. Thus, for solute A distributing between an aqueous and an organic solvent,... 

If a quantity of a solute A is distributed between two immiscible solvents, for example I2 between carbon tetrachloride and water, then at equUibrium the chemical potentials or escaping tendencies of the solute are the same in both phases thus, for A(in solvent a) = A(in solvent b)... 

ACTIVITY OF A SOLUTE FROM DISTRIBUTION BETWEEN TWO IMMISCIBLE SOLVENTS... 

Since consideration of thermodynamics demand that the activity (or chemical potential) of a solute should be equal in the two phases at equilibrium, a distribution coefficient of other than unity implies that the solute must have different activity coefficients in the two phases. The origin of such a difference usually resides in the degree of interaction between the solute and the two solvents. 

Difficult separations can often be effected by liquid-liquid solvent extraction, which depends on differences in the distribution of solute species between two immiscible or partially immiscible phases.4,9 For a solute species A, this distribution is governed by the Nemst partition law... 

Extraction is a process for separating components in solution by their distribution between two immiscible phases. Such a process can also be called liquid extraction or solvent extraction. The former term may be confusing because it also applies to extraction by solid solvents. Since extraction involves the transfer of mass from one phase into a second immiscible phase, the process can be carried out in many ways. The simplest example involves the transfer of one component from a binary mixture into a second immiscible phase — extraction of an impurity from wastewater into an organic phase. In some cases, a chemical reaction can be used to enhance the transfer, e.g., the use of an aqueous caustic solution to remove phenolics from a hydrocarbon stream. 

When two immiscible solvents are placed in contact, any substance soluble in both of them will distribute or partition between the two phases in a definite proportion. According to the Nernst partition isotherm, the following relationship holds for a solute partitioning between two phases a and b ... 

A solute distributes itself between two immiscible (substances that do not mix) solvents so that the ratio of its concentrations in dilute solutions in the two solvents is constant, regardless of the actual concentration in either solvent. The solvents will form layers, since they do not mix. In this situation, both concentrations are assumed to be on the same volumetric basis (e.g., mol/L). 

When a solute is distributed at equilibrium between two solvents, its vapor pressure, which is a measure of its escaping tendency, must be equal above the two solvents ... 

SOLVENT extraction (liquid-liquid extraction) is the separation and/or concentration of the components of a solution by distribution between two immiscible liquid phases. A particularly valuable feature is its power to separate mixtures into components according to their chemical type. Solvent extraction is widely used in the chemical industry. Its applications range from hydrometallurgy, e.g., reprocessing of spent nuclear fuel, to fertilizer manufacture and from petrochemicals to pharmaceutical products. Important factors in industrial extraction are the selection of an appropriate solvent and the design of equipment most suited to the process requirements. 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

The Partition of a Solute between Two Solvents. If a solution of iodine in water is shaken with chloroform, most of the iodine is transferred to the chloroform. The ratio of concentrations of iodine in the two phases, called the distribution ratio, is a constant in the range of small concentrations of the solute in each solution. For iodine in chloroform and water the value of this ratio at room temperature is 250 hence whenever a solution of iodine in chloroform is shaken with water or a solution in water is shaken with chloroform until equilibrium is reached the iodine concentration in the chloroform phase is 250 times that in the water phase. 

It is seen on. consideration of the various equilibria that the distribution ratio of a solute between two solvents is equal to the ratio of the solubilities of the solute (as a crystalline or liquid phase) in the two solvents, provided that the solubilities are small. Moreover, the distribution ratio of a gaseous solute between two -solvents is proportional to the ratio of its two Henry s-law constants (its solubilities in the two solvents). 

Let us take the case of a single solute, A, which can be distributed between two immiscible solvents. If at equilibrium with equal volumes of the two phases, half of the solute is found in each phase, then the... 

In liquid-liquid extraction, a solvent is added to a liquid matrix (feed) to remove selectively transition components by the formation of two coexisting, immiscible liquid phases. The selected solvent (receiver phase) must be capable of preferentially dissolving the solutes to be extracted and be either immiscible or only partly miscible with the carrier (release phase). This process is, therefore, based on the different affinities of the solute distributing between the two coexisting liquid phases. Of the two phases, the solvent-rich solution containing the extracted solute is the extract and the solvent-lean, residual feed mixture is the raffinate. In the case of a closed miscibility gap, the correlation of the solute mole fraction in the extract and the raffinate phase is called the distribution coefficient (partition coefficient) K ... 

Prove that if a solute is distributed between two immiscible solvents (I and II), the ratio of the activities in the two solvents, i.e., ai/an, should be constant at constant temperature and pressure. Show that this result is the basis of the Nernst distribution law, i.e., ci/cii (or mi/mn) is constant for dilute solutions. 

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