Nernst partition law

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. For a solute species A, this distribution is governed by the Nernst partition law... 

Solvent extraction equilibrium or partition equilibria arise when a substance is partitioning between two immiscible solvents in contact with each other. The corresponding law is the Nernst partition law. According to this law of nature, the ratio of concentration (better activity) values in both phases is a constant ... 

To understand the fundamental principles of extraction, the various terms used for expressing the effectiveness of a separation must first be considered. For a solute A distributed between two immiscible phases a and b, the Nernst Distribution (or Partition) Law states that, provided its molecular state is the same in both liquids and that the temperature is constant ... 

The behavioural pattern of two immiscible solvents, say a and ib is essentially nonideal with respect to one another. Now, if a third substance is made to dissolve in a two-phase mixture of the solvents (i.e., a and 3 ), it may behave ideally in either phases provided its concentration in each individual phase is approximately small. Therefore, under these prevailing experimental parameters the ratio of the mole fractions of the solute in the two respective immiscible phases ( a and A) is found to be a constant which is absolutely independent of the quantity of solute present. It is termed as the Nernst Distribution Law or the Partition Law and may be expressed as follows ... 

We have already seen that, within the range of Nernst s law, the solid/liquid partition coefficient differs from the thermodynamic constant by the ratio of the Henry s law activity coefficients in the two phases—i.e.,... 

The equilibrium constant Ki(T,p), which is independent of mole fraction is called the distribution or partition coefficient of the substance i between the solutions 1 and 2. This equation is the generalized form of the Nernst distribution law. 

In other words, the mole fraction ratio of / in the coexisting phases at equilibrium for a given T and P should be constant. This is Nernst s law (cf. Lewis and Randall 1961). K is also called the distribution coefficient, often symbolized by >, and is used in the study of trace element partitioning between coexisting mineral solid solutions. 

DYNAMICS OF DISTRIBUTION The natural aqueous system is a complex multiphase system which contains dissolved chemicals as well as suspended solids. The metals present in such a system are likely to distribute themselves between the various components of the solid phase and the liquid phase. Such a distribution may attain (a) a true equilibrium or (b) follow a steady state condition. If an element in a system has attained a true equilibrium, the ratio of element concentrations in two phases (solid/liquid), in principle, must remain unchanged at any given temperature. The mathematical relation of metal concentrations in these two phases is governed by the Nernst distribution law (41) commonly called the partition coefficient (1 ) and is defined as = s) /a(l) where a(s) is the activity of metal ions associated with the solid phase and a( ) is the activity of metal ions associated with the liquid phase (dissolved). This behavior of element is a direct consequence of the dynamics of ionic distribution in a multiphase system. For dilute solution, which generally obeys Raoult s law (41) activity (a) of a metal ion can be substituted by its concentration, (c) moles L l or moles Kg i. This ratio (Kd) serves as a comparison for relative affinity of metal ions for various components-exchangeable, carbonate, oxide, organic-of the solid phase. Chemical potential which is a function of several variables controls the numerical values of Kd (41). 

The principle of a new technique, SPME, developed in 1990 by Pawlyszin et al., is based on the adsorption of organic compounds present in a small volume of water (2 to 5 ml) on a fused silica fiber coated with a micro-layer of solid phase (7 to 100 /rm thick). The extraction can be described as an equilibrium process in which analytes partition between the aqueous phase and the solid phase immobilized on the fiber, governed by Nernst s law ... 

Liquid-liquid partition is useful when one of the series of metal complexes, ML°, has a zero charge and can partition into an immiscible organic phase according to Nernst s law, p = [ML°]org/[ML°]aq. Equilibration of an aqueous phase with known Mf and Lf with this immiscible organic phase leads to an experimentally determinable distribution ratio ... 

Molecule The smallest part of a substance that is composed of two or more atoms of the same or different type held together by chemical forces Nernst distribution law (partition law) The ratio (constant) at which an analyte will become distributed between two immiscible solvents at... 

SClLLs are limited to biphasic systems, that is, the IL (e.g., [BMIM][0cS04 ) has to be insoluble in the organic phase, for example, in the case of COD hydrogenation in a mixture of the solvent -dodecane with the organic reactant COD and the products COE and cyclooctane (COA). With regard to the effective concentrations of the reactants at the surface of the IL-coated catalyst, concentrations in the IL layer have to be considered, which are governed by the Nernst s law of partition. The Nernst partition coefficient of a substance i (e.g., of COD, COE, or COA),... 

The separation of components in gas chromatographic processes is the result of equilibrium distribution between a mobile gas phase and a liquid or solid stationary phase. The partition of a component between two phases is described by the Nernst distribution law. 

Charge transfer across a phase boundary can be provoked alternatively by an extraction equilibrium (Sect. 2.2.7). If a Uquid immiscible with water is in contact with an aqueous sample solution and certain ions are soluble in both phases, then a concentration partition is estabUshed which can be interpreted by Nernst s law of partition, Eq. (2.44). The establishment of this equilibrium should be accompanied by voltage formation at the interface if the partitioning species is an ion. 

If the substance shared between two solvents can exist in different molecular states in them, the simple distribution law is no longer valid. The experiments of Berthelot and Jungfleiscli, and the thermodynamic deduction show, however, that the distribution law holds for each molecular state separately. Thus, if benzoic acid is shared between water and benzene, the partition coefficient is not constant for all concentrations, but diminishes with increasing concentration in the aqueous layer. This is a consequence of the existence of the acid in benzene chiefly as double molecules (C6H5COOH)2, and if the amount of unpolymerised acid is calculated by the law of mass action (see Chapter XIII.) it is found to be in a constant ratio to that in the aqueous layer, independently of the concentration (cf. Nernst, Theoretical Chemistry, 2nd Eng. trans., 486 Die Verteilnngssatz, W. Hertz, Ahrens h annulling, Stuttgart, 1909). 

Distribution (Nernst) potential — Multi-ion partition equilibria at the -> interface between two immiscible electrolyte solutions give rise to a -> Galvanipotential difference, Af(j> = (j>w- 0°, where 0wand cj>°are the -> inner potentials of phases w and o. This potential difference is called the distribution potential [i]. The theory was developed for the system of N ionic species i (i = 1,2..N) in each phase on the basis of the -> Nernst equation, the -> electroneutrality condition, and the mass-conservation law [ii]. At equilibrium, the equality of the - electrochemical potentials of the ions in the adjacent phases yields the Nernst equation for the ion-transfer potential,... 

The partition of all three types should obey the Nernst law, but in most cases the concentrations of extractable species are affected by chemical equilibria Involving them and other components of the system. These must be taken into account when calculating the optimum conditions for quantitative extraction or separation. 

Partitioning. A substance may have limited solubility in two mutually immiscible solvents, for instance water and oil. This often happens in foods, for example with many flavoring and bactericidal substances. It then is important to know the concentration (or rather activity) in each phase. For low concentration, the partitioning or distribution law of Nernst usually holds ... 

The extent to which the dimer is favored over that of the monomer is determined by the magnitude of This added seeondary equihbrium, this time appearing in the organic phase, is shown in Figure 3.5. The fundamental basis for the partitioning of HOAc between ether and water as introduced by the Nernst law is not violated and still is given by Kj). The measurable concentrations [HOAcjether and [HOAcjaq will definitely differ with this added dimerization reaction. Let us define D for this distribution equilibrium involving weak acid dissociation of HOAc in the aqueous phase and, at the same time, dimerization of HOAc in the ether phase as follows ... 

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