Selectivity coefficient ion-selective electrode

Y. Umezawa, Ed., Handbook of Ion-Selective Electrodes Selectivity Coefficients, CRC Press, Boca Raton, FL, 1990. 

CRC Handbook tf Ion-Selective Electrodes Selectivity Coefficients Umezawa, Y.. Ed. CRC Press Florida. 1990. 

Umezawa Y (ed.) (1990) Handbook of Ion-selective Electrodes Selectivity Coefficients. Boca Raton, PL CRC Press. 

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. 

Selectivity As described earlier, most ion-selective electrodes respond to more than one analyte. For many ion-selective electrodes, however, the selectivity for the analyte is significantly greater than for most interfering ions. Published selectivity coefficients for ion-selective electrodes (representative values are found in Tables 11.1 through 11.3) provide a useful guide in helping the analyst determine whether a potentiometric analysis is feasible for a given sample. 

Creager and colleagues designed a salicylate ion-selective electrode using a PVC membrane impregnated with tetraalkylammonium salicylate. To determine the ion-selective electrode s selectivity coefficient for benzoate,... 

The main chemico-analytical properties of the designed ionoselective electrodes have been determined. The work pH range of the electrodes is 1 to 5. The steepness of the electrode function is close to the idealized one calculated for two-charged ions (26-29 mV/pC). The electrode function have been established in the concentration range from 0.1 to 0.00001 mole/1. The principal advantage of such electrodes is the fact that thiocyanate ions are simultaneously both complexing ligands and the ionic power. The sensitivity (the discovery limits), selectivity (coefficient of selectivity) and the influence of the main temporal factors (drift of a potential, time of the response, lifetime of the membranes) were determined for these electrodes. 

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... 

Selectivity coefficients values for K - and Na -ISFETs with the optimized ion-sen-sing membranes encapsulating valinomycin and bis(12-crown-4) are summarized in Fig. 9. The selectivity coefficient for with respect to Na in the K -ISFET is 2 x 10 " and that for Na with respect to in the Na -ISFET is 3 x 10. The selectivity coefficient values are similar to those for the ISFETs and ion-selective electrodes with the previous membrane materials containing the same neutral carriers. The high sensitivity and selectivity for the neutral-carrier-type ISFETs based on sol-gel-derived membranes can last for at least 3 weeks. 

Thermodynamics describes the behaviour of systems in terms of quantities and functions of state, but cannot express these quantities in terms of model concepts and assumptions on the structure of the system, inter-molecular forces, etc. This is also true of the activity coefficients thermodynamics defines these quantities and gives their dependence on the temperature, pressure and composition, but cannot interpret them from the point of view of intermolecular interactions. Every theoretical expression of the activity coefficients as a function of the composition of the solution is necessarily based on extrathermodynamic, mainly statistical concepts. This approach makes it possible to elaborate quantitatively the theory of individual activity coefficients. Their values are of paramount importance, for example, for operational definition of the pH and its potentiometric determination (Section 3.3.2), for potentiometric measurement with ion-selective electrodes (Section 6.3), in general for all the systems where liquid junctions appear (Section 2.5.3), etc. 

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

E. Bakker, Determination of improved selectivity coefficients of polymer membrane ion-selective electrodes by conditioning with a discriminated ion. J. Electrochem. Soc. 43, L83—L85 (1996). 

Eisenman [98] verified his relationship for the selectivity coefficient (3.4.1) using a glass electrode selective for both potassium and sodium ions. First he found... 

Liquid membrane electrodes are not capable of being specific for only one ion in solution. There is always some interference from other ions in solution with the given analyte. The selectivity coefficient provides an indication of the ability of an electrode to measure a particular ion in the presence of another ion. The response of an electrode to an interfering ion can be included in the Nernstian equation ... 

Van Staden reported a rapid, reliable automated method for direct measurement of the chloride content in milk based on the principles of flow injection analysis and the use of a dialyser to remove interferents. Dialysed chloride was measured by means of a coated tubular chloride ion-selective electrode. Potential changes arising from the interference of casein were thus avoided and baseline stability ensured. The results obtained for chloride in milk compared well with those provided by standard recommended methods. The linear range for chloride was 250-5000 pg/mL for 30 pL of sample, and the coefficient of variation was better than 0.5%. The throughput was ca. 120 samples/h [132],... 

Umezawa, Y., Umezawa, K., Sato, H. Pure Appl. Chem. 1995, 67, 507 Umezawa, Y., Buhlmann, P., Umezawa, K., Tohda, K., Amemiya, S. Pure Appl. Chem. 2000, 72, 1851 Umezawa, Y. (Ed.) Handbook of Ion-Selective Electrodes Sensitivity Coefficients, CRC Press, Boca Raton, FL, 1990. 

The selectivity of an electrode for other ions can be variable. For each interfering ion, selectivity is defined by a coefficient specific ion/interfering ion- For example, Br/ci = 2.5 x 10-3 indicates that the selectivity of the electrode for the bromide ion will be 1 /2.5 x 10 3, hence 400 times greater for bromide than for chloride. 

An electrode intended to measure ion A also responds to ion X. The selectivity coefficient gives the relative response of the electrode to different species with the same charge ... 

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