Selectivity coefficient for ion selective electrode

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. 

One way to estimate selectivity coefficients for ion-selective electrodes is to first equilibrate the electrode in a pure solution of the test ion and measure the potential... 

Umezawa. Y. Umezawa, K. Sato. H. Selectivity coefficients for ion-selective electrodes Recommended methods for reporting K b values. Pure Appl. Chem. 1995. 67, 507-518. 

K. Umezawa, Y. Umezawa Selectivity Coefficients for Ion Selective Electrodes, University of Tokyo Press, Tokyo 1983. 

One problem in assessing true selectivity coefficients for neutral carrier-based ISEs is that although an electrode may exhibit Nemstian response to a secondary ion, once it has been exposed to a solution of the primary ion, it loses that Nemstian response to the secondary ion. Bakker therefore developed a procedure for determining unbiased selectivity coefficients for newly prepared electrodes by first making measurements with the secondary ion, before exposing the electrode to the primary ion (see Refs. 20 and 21). In this manner, data can be obtained about which ionophores are really the most selective for the primary ion. See Ref. 22 for a review of selectivity of ion-selective electrodes. For an excellent review of ionophore-based electrodes, see Ref. 14 by Bakker et al. (a comprehensive 50-page review). 

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,... 

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. 

Most measurements include the determination of ions in aqueous solution, but electrodes that employ selective membranes also allow the determination of molecules. The sensitivity is high for certain ions. When specificity causes a problem, more precise complexometric or titri-metric measurements must replace direct potentiometry. According to the Nernst equation, the measured potential difference is a measure of the activity (rather than concentration) of certain ions. Since the concentration is related to the activity through an appropriate activity coefficient, calibration of the electrode with known solution(s) should be carried out under conditions of reasonable agreement of ionic strengths. For quantitation, the standard addition method is used. 

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. 

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],... 

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. 

Fig. 2.2. Determination of selectivity coefficient of a silver-selective electrode by SSM for indicated ions. (A) Electrode conditioned in 10-3M NaN03. (B) Electrode conditioned in 10-3M AgN03.

The same algorithm was used to calibrate flame photometers and blood analyte analysers for Na, K and Ca determination. The results of calibrating such instruments are also presented in Table 1. Instrument 6 was an ion selective electrode analyser for Na/K/Cl with a 1.5% coefficient of variation at a 95% confidence interval. Finally, instruments 7-10 were flame photometers, validated against monoelement concentration CRMs [5] in accordance with legal metrological regulations. In... 

It is important for the analytical chemist to realize the selectivity coefficient of a particular electrode. Various methods have been suggested for determining the selectivity coefficient, including the fixed-interference method, separate solution method, and the fixed primary ion method (10,11). The most popular fixed interference method involves two solutions, one containing a... 

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