Determination of the Selectivity Coefficient

As is the case with any analytical technique, interferences can arise when working with ion-selective electrodes. These interferences can generally be classified as interferences due to direct indication of other ions or interferences due to chemical influences on the measured ion, such as complexation, which only alter the activity of free measured ion indicated by the electrode. This second type of interference is not due to the electrode itself, but rather to the solution chemistry of the corresponding measured ion in the particular matrix employed, and as such will not be considered in any further detail here. 

Constructing a calibration curve for any particular interfering ion usually results in a picture qualitatively the same as that shown in Fig. 52. With such a graph there are two methods for determining the selectivity coefficient. 

At the same measured and interfering ion activities can be calculated from the following equation  

It is easy to see from Fig. 52 that the selectivity coefficient itself is a function of the absolute measured and interfering ion concentrations. The first procedure is suitable for quick rough measurements since only two measurements and two solutions are required. Both procedures are unredistic in that in practice, one is dealing with a mixture of measured and interfering ions. The lUPAC [455] has recommended a method in which a constant interfering ion activity and a variable measured ion activity are employed. The measured EMF values are used to construct a calibration curve as usual (Fig. 53). To evaluate one first determines the intersection point of the two 

In addition the selectivity coefficient depends on the method of determination, therefore this information should also be included. Because some electrodes show a considerable EMF drift and different slopes when exposed to pure interfering ion solutions, the lUPAC method is strongly recommended. 

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Plot of cell potential versus the log of the analyte s concentration In the presence of a fixed concentration of Interferent, showing the determination of the selectivity coefficient. 

The above description may be used to explain the improvement of selectivity coefficients obtained on a crystallized membrane, as compared with glass membranes. The size of the conduction sites is nonhomogeneous in such amorphous materials, while it is well defined in size in a crystalline material. The mobility of interfering ions is therefore lower in the latter type of stracture, which will favor a higher selectivity (see Equation [10.32]), This is well illustrated by NASICON, where the selectivity coefficient for alkali ions is improved by a factor of at least ten. - The main interfering ions of common membranes are indicated in Table 10.1. The methods for the determination of the selectivity coefficients are described in specific papers. ... 

It should be obvious by now that one of the key tasks in ion exchange experiments is the accurate determination of the selectivity coefficient. In principle, this is straightforward the zeolite, initially in the B-form, must be equilibrated in solution using an increasing ratio cjc and, after equilibrium has been attained, the concentrations of A and B in the zeolite and also in the solution are measured. 

Fig. 52. Determination of the selectivity coefficients according to the method of separately measured solutions...

Discuss the significance of the selectivity coefficient of an ISE. How would you determine its value ... 

Some of the elements of thermodynamics of irreversible processes were described in Sections 2.1 and 2.3. Consider the system represented by n fluxes of thermodynamic quantities and n driving forces it follows from Eqs (2.1.3) and (2.1.4) that n(n +1) independent experiments are needed for determination of all phenomenological coefficients (e.g. by gradual elimination of all the driving forces except one, by gradual elimination of all the fluxes except one, etc.). Suitable selection of the driving forces restricted by relationship (2.3.4) leads to considerable simplification in the determination of the phenomenological coefficients and thus to a complete description of the transport process. 

Implicit in this equation is the assumption of constancy of the selectivity coefficient K, and the assumption of constancy of the term Vq. Those assumptions put this otherwise useful equation in the category of empirical relationships, the same as the Nikolskij-Eisenman equation. An example of such a response is shown in Fig. 6.36 in which WF CHEMFET with doped polyaniline/camphorsulfonic/ionic liquid gate was exposed to stepwise changes of ammonia concentration. In this case ammonia acts as an electron donor, thus lowering the work function of the selective layer. The value of 5g determined from the slope of (6.101) was found to be 0.6. 

Selectivity coefficients were obtained by the modified separate solution method (MSSM), which allows for the determination of unbiased selectivity coefficients [3]. The selectivity coefficients for Cs I were first... 

Third, the stationary phase should permit separate elution of the substances into the mobile phase and the selectivity toward samples of interest has to be sufficient to lead to separations with good resolution. The selectivity of solvent systems can be estimated by determination of the partition coefficients for each substance. The batch partition coefficients D are calculated as the ratio of the component concentration in the organic phase to that in the aqueous phase. The dynamic partition coefficients of compounds are determined from an experimental elution curve [7]. Several solvent systems for organic separation were investigated [4-8]. The most efficient evolution usually occurs when the value of the partition coefficient is equal 1. However, in some CCC schemes, the best results are obtained with lower partition coefficient values ofO.3-0.5 [1,4]. 

The selectivity coefficient can be determined experimentally by adding a certain amount of resin material to a solution with known concentrations of X- and HCOf. The resulting concentration of the exchanged ions is determined in the mobile and stationary phase, respectively, after equilibrium is achieved. To precisely calculate the selectivity coefficient, the activities a have to be used instead of the concentrations Cj. As a prerequisite, the determination of the activity coefficient/j according to Eq. (35) is required, which is difficult to perform in the matrix of an ion-exchange resin. 

One area of material science where ET-IR imaging has proved to be of extraordinary importance, in terms of scientific and practical aspects, is that of polymer analysis and polymer physics. In order to illustrate the broad range of appUcability in these disciplines, we will now discuss some selected examples in detail, ranging from phase separation in biopolymer blends, the use of polarized radiation to produce anisotropy images of inhomogeneously deformed polymer films, and determination of the diffusion coefficient of D2O in an aliphatic polyamide. 

Amperometric selectivity coefficients are reported for the validation of amperometric sensors. The ratio between the analyte and interferent must be 1 10, and the mixed solution must be used in the determination of these selectivity coefficients. Only under these conditions will the results of the selectivity test be validated, and only then will the method attain the status of validated or not validated. 

Several electrodes of differing selectivity are commercially available at present for measurements of most common ions. Preliminary information may be obtained on the basis of tabulated values of the selectivity coefficients nevertheless, it must be remembered that their values depend on the method of determination, on the manner in which the electrodes are maintained, and on the life of the electrode. Therefore it is a good habit to check the selectivity under standard conditions. After long periods of use the selectivity of the electrode may also change, giving rise to erratic results. For reliable measurements the potentiometric selectivity coefficient should not be greater than 10 for ions of the same charge and for comparable concentration levels of the analyte and interferent. 

On the basis of the dependence of the selectivity coefficients (a) of the tested substances on temperature In a =f[l/T), it is possible to determine the selectivities of liquid crystals in relation to the substances of molecules that are different in shape. However, on the basis of the... 

Several methods have been proposed for the experimental determination of ion-selectivity coefficient. They can be listed in the two groups separate solution methods and mixed solution methods. Applying both methods, the cell voltages (emf) are measured in a cell when the reference electrodes and the ISEs are dipped in solutions of known composition. When the separate solution method is used, pure primary ion solutions or pure solutions of the tested interfering ion are introduced into the measuring cell, while in mixed solution methods, both the primary and the selected interfering ions are introduced. 

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