Diffusion selectivity coefficient

In an aqueous solution of organics, water shows a diffusion selectivity coefficient higher than 1. This is an advantage for hydrophilic membranes which are used to preferentially permeate water, but a disadvantage for organo-phihc membranes, since the diffusivity counteracts the sorption selectivity. 

According to the nine assumptions and approach a) for the diffusion potential inside the membrane the selectivity coefficient Kg , can be expressed by other parameters. Table 3 shows the results for the different kinds of membranes 66). In some cases the expressions for K J i contain ion-mobilities inside the membrane... 

As the complexes of various ions with a single ionophore usually have the same structure (they are isosteric [17, 23, 25]), their mobihties in the membrane are the same and consequently no diffusion potential is formed in the membrane. The selectivity coefficient is then (see p. 35) ... 

If the diffusion potential in the membrane is neglected, this equation yields relationships for the membrane potential, for the ISE potential and for the selectivity coefficient. It is apparent that formation of complexes with various numbers of ions in the membrane does not affect the dependence of the ISE potential on the activities of the determinand and interferent according to the Nikolsky equation. 

Equation (4.19) can be used only when (4.20) is valid. In simple systems (rapid processes at the membrane/electrolyte interface and a simple diffusion potential in the membrane) the apparent selectivity coefficient is a function of theflj/flK ratio alone, whereas in more complicated systems it also depends on the activities of J and K. 

Compound Sorption coefficient S (mg-mg ) Diffusion coefficient D (m -s ) Sorption selectivity Diffusion selectivity Overall permselectivity... 

There are two processes that take place during the interaction of the hydrated layer and the sample the ion exchange and the diffusion of all participating ions. We show that they both contribute to the value of the membrane potential and of the selectivity coefficient. Let us consider a simple case of only two univalent ions Na+ and H+ exchanging between the solution (S) and the hydrated layer (m ) of the membrane, according to the equation ... 

The interfacial pd selectivity coefficient, the factor multiplying a is determined by the ratio K /K, by the activity coefficient ratio, and by the mobility ratio, when the internal diffusion potential contribution is added. Clearly interferences should correlate with the ratio K /1C, which can be determined from salt extraction coefficients K KX/K K for a series of positive drugs, using common anion salts. This result is well documented in the literature (7,8). A curious correlation for N-based drugs studied by us and by Freiser ( ) is a trend in selectivity... 

Table 4.3 shows the permselectivity characteristics of pure, semicrystalline PEO films [76]. The selectivity characteristics for 02/N2 are rather similar to those for silicone rubber and natural rubber shown in Table 4.2. However, the values of permselectivity for C02 relative to the various light gases shown are all much higher than Table 4.2 shows for the rubbery polymers listed there and even for polysulfone except for C02/CH4. Comparison of the data in Tables 4.2 and 4.3 makes it clear that this high permselectivity of PEO stems from its high solubility selectivity for C02 versus other gases this is augmented by modest values of diffusivity selectivity. Data in Table 4.4 for the C02/N2 pair illustrate that this effect can be translated into various block-copolymer structures when the PEO content is high enough to ensure it is the continuous phase. In fact, nearly all these materials have higher permselectivity and solubility selectivity for C02/N2 than does pure PEO (see Table 4.3) however, the diffusion selectivity for these copolymers is much closer to, or even less than, unity than seen for pure PEO. Furthermore, the copolymers all have much higher absolute permeability coefficients than does PEO.

Table 13-5 Partition (KG/f) and diffusion (DF) coefficients of several solvents in a selection of liquid, fatty and solid foods at 23 °C. Mr = relative molecular mass. TB = boiling point.

Calculate the distribution of the concentrations within the lysimeter column taking into account the cation exchange (discretisation and time steps as in the example in chapter 2.2.2.3). Selectivity coefficients are taken from the exemplary data of WATEQ4F.dat data set and an exchange capacity of 0.0011 mol per kg water is assumed. Neither diffusion nor dispersion is considered. Present your results graphically. 

Diffusion selectivity, can also be similarly defined as the ratio of the two diffusion coefficients ... 

Knowledge of the selectivity and diffusion dialysis coefficient can be used to determine the membrane that offers lower resistance to acid diffusion. It also helps in determining the faster-diffusing acid. 

This is a product of the ratio of the mobility of ion A to that of ion B in the membrane phase and the selectivity coefficient of ion A to ion B. In electrodialysis, diffusion boundary layers are formed at membrane surfaces, where the concentration at the membrane surfaces is not the same as that of the bulk solution. To obtain the real PAB of the ion exchange membrane, the diffusion boundary layer should be eliminated by vigorous agitation of the solution, which corresponds to the limiting value at zero current density,... 

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