Selectivity coefficient of ISE

The selectivity coefficients () of ISEs with polyfvinyl chloride)... 

Table 1 tabulates the normal measuring ranges of some common cations in blood, and required maximum selectivity coefficients of ISEs used to assess these ions in undiluted and diluted blood samples. 

Tab. 1 Normal concentration range of some cations in blood and required maximum selectivity coefficients of ISEs for their reliable assessment by direct potentiometry in...

The selectivity coefficients of ISEs based on immiscible organic liquid ion-exchangers can be correlated with data for liquid extraction systems (24-27) by computing the degree of saturation of the organic phase by the ion to be measured from the extraction constants or distribution coefficients. Since the ion transference occurs through the membrane and associated diffusion layers, it is probable that both equilibrium ion-exchange and relative... 

Membrane compositions and selectivity coefficients of ISEs based on commercially available neutral ionophores and their complexation constants... 

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

A survey of selectivity coefficients for ISEs with liquid membranes is found in the Appendix. 

Calcium activities as low as 5 x 10 7 M can be measured, with selectivity coefficients of AlCa,Mg and KCa,k of 0.02 and 0.001, respectively. Such potential response is independent of the pH over the pH range 5.5-11.0. Above pH 11, Ca(OH)+ is formed, while below 5.5, protons interfere. Because of its attractive response characteristics, the calcium ISE has proved to be a valuable tool for the determination of calcium ion activity in various biological fluids. 

The selectivity coefficient of an ISE can be measured in several ways [28], the most popular of which are as follows ... 

Table 3.4 Selectivity coefficients of PVC calcium ISEs based on calcium bis di-[4-(l, 1, 3,3-tetramethylbutyl) phenyl] phosphate and various mediators (17)... 

Table 3.5 Extraction coefficients of divalent cations from 4M sodium nitrate into di(2-ethyl-hexyl) phosphoric acid in benzene, and selectivity coefficients of PVC ISEs with dialkylphosphate sensors (24)...

Table 3.7 CHEMICAL SENSORS Selectivity coefficients of the PVC ISEs in Table 3.6 (14) ... 

ORGANIC SENSOR MATERIALS FOR ION-SELECTIVE ELECTRODES Table 3.9 Selectivity coefficients of some PVC calcium ISEs (15)... 

Kj/i is the selectivity coefficient of the ISE for an interfering ion S. This is a factor that describes the influence of the interfering ion in relationship to the ion to be measured (i). These selectivity coefficients are known for the most important interfering ions for an ISE, and therefore, a simple estimation can be made to estimate if an interfering ion present in the sample solution will influence the measured value or not. 

The selectivity coefficient of an ISE is an important property. One can guess knowing the AT g values of an electrode that whether the electrode is applicable for solving the planned analytical problem or not. Eor this only, the ion activity range of the analytes in the sample and that of the interfering ions in the matrix have to be known or guessed. However, the selectivity coefficient is not a physical constant. Its value depends very much on the experimental condition used in its determination. 

The weighting factors are the selectivity coefficients of the membrane toward ions j relative to ion i. The smaller the /fl", the smaller are the contributions of the interfering ions to the total cell potential. The selectivity coefficients can be determined potentiometrically and are often listed in textbooks. Their value is used as a guideline when designing ISE-based potentiometric experiments. 

Although rum ammonia levels are not routinely measured, it is a useful indicator of Reye s syndrome and should be monitored in newborns at risk of developing hyperammonemia Ammonia is produced in many analytically useful enzyme reactions and the ammonium ISE has been used as the base sensor in several enzyme electrodes (see next section). In addition to valinomycin, other antibiotics such as the nonactin homalogs and gramicidins also behave as ionophores. The nonactin homolo were originally studied for their ability to selectively bind potassiiun ions It was then discovered that ammonium ions were preferred over potassium ions, and the selectivity coefficient Knh+ = 0.12 was reported. Since ammonia is present at fairly low levels in serum, this selectivity is not sufficient to to accurately measure NH4 in the presence of K. An extra measure of selectivity can be gained by using a gas permeable membrane to separate the ammonia gas from the sample matrix... 

To appreciate that an ISE must be selective if its measurements are to be useful, and how the Nernst equation can be adapted to take account of the ISE selectivity by using the concept of a selectivity coefficient (or ratio ). To leam how the problems of selectivity can be overcome by using a standard-addition method, such as drawing a Gran plot - one of the more useful of such methods. 

It will be shown below that the selectivity coefficient is frequently a function of the composition of the analyte nonetheless, it provides an indication of the effect of interfering ions on the determination of the test ion. It can be seen that the ISE potential yields the activity of the test component, not its concentration. The activity can, however, be used to calculate the concentration if the activity coefficient is known. 

The Nikolsky equation (3.1.7) frequently corresponds to the ise versus logflj dependence only in those regions where (3.1.8) or (3.1.10) is valid and is thus of ten only an operational formula with the same asymptotes as the actual ise versus log aj dependence. Nonetheless, the selectivity coefficient obtained from (3.1.7) is important in estimating the ISE selectivity, although this equation cannot be used where the igE versus log aj dependence is clearly curved. In the intercept of the asymptotes to (3.1.7), log aj has value log further discussion the operational value of the... 

Introduction of ionophores into the membrane should lead to a considerable increase in the ISE selectivity (in the absence of an ionophore, the selectivity coefficient would be given by constant/Texch alone, cf. (3.3.12)) however, as (3.3.10) contains only the square root of the stability constant ratio, this increase in selectivity is not extremely high. Better results are obtained in the second system, where the membrane contains an excess constant ionophore concentration. 

This equation contains selectivity coefficient for cation J with respect to anion B consequently, activity a j ( 1) has a negative exponent. The behaviour of the ISE in fig. 3.4 can be explained in this manner. 

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. 

The selectivity coefficient was defined in chapter 3 and several theoretical relationships were given for this quantity for various ISE systems. Several methods have been proposed [38, 120,123, 135] for the determination of selectivity coefficients two basic methods were recommended by the lUPAC Commission for Analytical Nomenclature [138],... 

The temperature coefficient of the ISE potential has received relatively little attention. As follows from (3.1.7), the constant term (the ISE standard potential), the determinand and interferent activity coefficients and the selectivity coefficient, liquid-junction potentials and, of course, also the RTIZfF coefficient, depend on the temperature [118]. When the internal reference electrode and the reference electrode in the test solution are identical, the interferent activity sufficiently low and the liquid-j unction potentials negligible, then the constant term depends on the determinand activity in the electrode internal solution alone and thus the temperature coefficient of the measured EMV depends only on the temperature coefficient of the determinand activity coefficient and on the/ 77z,F coefficient. Measuring instruments are usually... 

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