Nemst distribution coefficient

Consider two phases a and /J containing octahedral sites available for cation occupancy. The distribution of cations between these sites will depend on the relative values of the CFSE for the two sites. If CFSE > CFSE 1, then the Nemst distribution coefficient, D, expressing the concentration ratio of transition element, M, between the two phases... 

The distribution of trace elements between phases may be described by a distribution coefficient or partition coefficient (Mclntire, 1963). The Nemst distribution coefficient is used extensively in trace element geochemistry and describes the equilibrium distribution of a trace element between a mineral and a melt. The Nemst distribution coefficient is defined by ... 

The distribution coefficients are not constant. They strongly depend on the concentration. For the system water-ethanol-cyclohexane the distribution coefficients for ethanol are shown in Figure 5.69. It can be seen that the largest distribution coefficients are obtained at infinite dilution. These values at infinite dilution are called Nemst distribution coefficients. 

K Nemst distribution coefficient Vs Volume of the stationary phase Volume of the mobile phase Cs Solute concentration in the stationary phase Cm Solute concentration in the mobile phase... 

The capacity factor, k, is the product of the phase ratio (P between stationary and mobile phases in the separator column and the Nemst distribution coefficient, K, as shown in Eq. (7) ... 

For a successful separation, all that is required is basically an immiscible solvent pair in which the components of the mixture have different partition coefficients according to the Nemst distribution law. Details about the instrumentation as well as numerous applications, including the separation of aroma precursors, can be found in the literature cited (23-28). 

Insofar as the distribution coefficient, Kp- or Dp is constantf and not a function of composition, then Nemst s law [35] is obeyed, especially for nomeactive, simple systems. The lack of a composition dependence may be taken as an indication that a particnlar system is not reactive. In snch cases, the distribution of solute between the two phases is primarily determined by its solnbility in each phase hence Henry s law and/or the Lewis and Randall rule apply. 

Octanol is quite insoluble in water and vice versa hence the partition coefficients are not strongly temperature dependent, being mostly in the order of 0.001 to 0.01 log units per K (Lyman, 1990b). Even the low mutual solubility - at equilibrium the aqueous phase contains 4.5 x 10" mol/11-octanol and the organic phase contains 2.3 mol/1 water (Lyman, 1990b) - reveals so that the 1-octanol/water partition coefficient does not equal the ratio of the compounds solubilities in the individual solvents. This miscibility of the solvent phases, as well as ionization and association phenomena, limits the stringent validity of the presumed Nemst distribution, which, moreover, requires infinite dilution. Hence, at concentrations > 0.01 mol/1, is frequently observed to be dependent on the solute concentration. 

In the final stages of the process, the system separates into layers, ultimately forming a biphasic oligomer-water system. The dependence between concentrations (in mass percents) at equilibrium of phenol, water (C,, )e, and oligomer (Q) is nonlinear [3]. The distribution coefficient of phenol, K, in the oligomer-water system is determined by the Nemst equation ... 

In this respect attention is drawn to the limitations of the conventional Berthelot-Nemst and Henderson-Kracek distribution coefficients in geochemistry. Particular attention is paid to the thermodynamics of gas-solid reactions since these are becoming increasingly important as evidence accrues for fractionation of rare-earth elements by solid-gas reactions prior to meteorite formation. 

We now show that Nemst s distribution law is valid for solutions that obey Henry s law and obtain a formula for the distribution coefficient. The chemical potential of the solute in the two phases is given by... 

Essentially, extraction of an analyte from one phase into a second phase is dependent upon two main factors solubility and equilibrium. The principle by which solvent extraction is successful is that like dissolves like . To identify which solvent performs best in which system, a number of chemical properties must be considered to determine the efficiency and success of an extraction [77]. Separation of a solute from solid, liquid or gaseous sample by using a suitable solvent is reliant upon the relationship described by Nemst s distribution or partition law. The traditional distribution or partition coefficient is defined as Kn = Cs/C, where Cs is the concentration of the solute in the solid and Ci is the species concentration in the liquid. A small Kd value stands for a more powerful solvent which is more likely to accumulate the target analyte. The shape of the partition isotherm can be used to deduce the behaviour of the solute in the extracting solvent. In theory, partitioning of the analyte between polymer and solvent prevents complete extraction. However, as the quantity of extracting solvent is much larger than that of the polymeric material, and the partition coefficients usually favour the solvent, in practice at equilibrium very low levels in the polymer will result. 

However, this is an important issue in the determination of a diffusion coefficient from experimental data. It seems that the charge transfer resistance, necessary to determine the reaction kinetics, is less affected by a 2D current distribution, and so a simpler model might be used in the kinetic studies. Even simple models based on the Nemst diffusion layer are often used in kinetic studies [180] because of their simplicity. 

Nemst pointed out that distribution law is applicable only where the solute exists as normal mi ecules in both the solvents. If a solute were to dissociate into simple molecules or associate to more complex molecules then the distribution law is not valid. In such cases, the concentration terms in the distribution law should refer to the concentrations of the particular species common to both. Thus if a substance A dissolves in one solvent as normal molecules and with partial association (or dissociation) in the other, then the partition coefficient for the distribution if given by the ratio of the total concentration of A in the first solvent and the concentration of the unassociated (or dissociated) molecules in the second. 

A distribution ratio is often quoted as a measure of how extractable a species is. The distribution ratio, called Nemst s partition coefficient (Walther Nemst, see box), is equal to the content of a solute i in the first liquid phase 1 (usually the extracting agent) divided by its content in the second liquid phase (phase 2), usually the raffinate ... 

Solvent extraction is governed by the Nemst DisMbutioii or Partition Law. This states that at constant temperature and pressure, a solute, S, will always be distributed in the same proportions between two particular immiscible solvents. The ratio of the equilibrium concentrations (strictly their activities (Topic Cl)) in the two phases defines a distribution or partition coefficient, Xj, given by the expression... 

Few measurements of important parameters are as simple to make as the partition coefficient of a solute between water and octanol. The principles of the shake-flask measurements remain the same as in the Berthelot and Jungfleisch reports of 1872 and the further analysis by Nemst. Yet the interplay of solvation forces in both the water and octanol phases are so involved and complex that current molecular mechanics and quantum chemical calculations can dispel very little of the empiricism that now dominates logP estimation. It is difficult to predict how soon ab initio calculations will give us dependable information regarding the conformation, the tautomeric form, and the electron distribution of complex solutes, not just as they exist in a vacuum or crystal, but also as they exist in the aqueous phase as well as the water-saturated octanol phase. At present the quantum chemical methods that look the most promising for the complex solutes seem to fail when applied to the simplest hydrophobic organic structures of all the aliphatic hydrocarbons. 

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