Net distribution coefficient

When the relationship between the distribution coefficient of a solute and solvent composition, or the corrected retention volume and solvent composition, was evaluated for aqueous solvent mixtures, it was found that the simple relationship identified by Purnell and Laub and Katz et al. no longer applied. The suspected cause for the failure was the strong association between the solvent and water. As a consequence, the mixture was not binary in nature but, in fact, a ternary system. An aqueous solution of methanol, for example, contained methanol, water and methanol associated with water. It follows that the prediction of the net distribution coefficient or net retention volume for a ternary system would require the use of three distribution coefficients one representing the distribution of the solute between the stationary phase and water, one representing that between the stationary phase and methanol and one between the stationary phase and the methanol/water associate. Unfortunately, as the relative amount of association varies with the initial... 

Thus, k" , the net distribution coefficient of acetic acid, is considerably increased from kio. 

To determine the net distribution coefficient of the metal between the two phases, defined by... 

Equation (1) can be viewed in an over-simplistic manner and it might be assumed that it would be relatively easy to calculate the retention volume of a solute from the distribution coefficient, which, in turn, could be calculated from a knowledge of the standard enthalpy and standard entropy of distribution. Unfortunately, these properties of a distribution system are bulk properties. They represent, in a single measurement, the net effect of a large number of different types of molecular interactions which, individually, are almost impossible to separately identify and assess quantitatively. 

The semi-empirical descriptions of adsorbate/solid interactions are based on net changes in system composition and, unlike surface complexation models, do not explicitly identify the details of such interactions. Included in this group are distribution coefficients (Kp) and apparent adsorbate/proton exchange stoichiometries. Distribution coefficients are derived from the simple association reaction... 

The net amount of analyte extracted depends on the value of the distribution coefficient. 

Recently, kinetic models have been combined with the equilibrium data of the interfacial processes, taking into account that soils and rocks are heterogeneous and consequently have different sites. These models are called nonequilibrium models (Wu and Gschwend 1986 Miller and Pedit 1992 Pedit and Miller 1993 Fuller et al. 1993 Sparks 2003 Table 7.2). These models describe processes when a fast reaction (physical or chemical) is followed by one or more slower reactions. In these cases, Fick s second law is expressed—that the diffusion coefficient is corrected by an equilibrium thermodynamic parameter of the fast reaction (e.g., by a distribution coefficient), that is, the fast reaction is always assumed to be in equilibrium. In this way, the net processes are characterized by apparent diffusion coefficients. However, such reactions can be equally well described using Equation 1.126. 

The sorption of 85Sr2+ion was studied on bentonites from the Carpathian basin (Table 3.4 Nemes et al. 2006). Similar to the sorption of Cs-137, the distribution coefficients of the mineral components (Table 3.10) were calculated from the net... 

The dissociation constant of the carboxyl or amine group, the separation factor of the cation or anion of the amino acid and the distribution coefficient of zwitterion with zero net charge are considered to be approximately constant. 

Selectivity, a, is a ratio of the net retention volumes of adjacent compounds. It also may be thought of as a ratio of the distribution coefficients. The desire is to get a to be large, so a short column can be used. 

The phase capacity factor (kf) is the net retention time relative to the nonsorbed time (to) and is therefore directly related to the distribution coefficient of a solute between two phases ... 

The net retention volume, Vn, (definition (7.1.99])), for species i can also be expressed in terms of Pf- and yf. Consider the distribution coefficient kh first and assume ideal gas behavior ... 

A radial distribution function can be determined by setting up a histogram for various distances and then looking at all pairs of molecules to construct the diagram. Diffusion coefficients can be obtained by measuring the net distances... 

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