Selectivity coefficient with membrane electrodes

If we compare the above-described electrodes with classic sensors containing polymer membranes with associates in the form of chlorates [72], it can be stated that classic ibuprofen electrodes have similar analytical parameters determined based on a calibration curve (the slope of linear response of 60 mV/decade, linearity range of 10 -10" mol L, detection limit of 4.0-6.0 x 10"mol L ) and similar values of selectivity coefficients. The BMSA electrode containing TOA-IBU has, however, a longer lifetime. All classic electrodes can function properly for approx. 3 months. 

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,... 

A second surface modification has been reported by Yamamoto et al. These workers added stearic acid to their carbon paste mixture. This produced an electrode which was relatively insensitive to ascorbic acid and DOPAC relative to dopamine. It is theorized that this electrode works because of electrostatic repulsion of the anionic ascorbate and DOPAC by surface stearate groups. Ionic repulsion has also been employed by covering the surface of the working electrode with an anionic polymer membrane. Gerhardt et al. used Nafion, a hydrophobic sulfonated perfluoro-polymer, to make a dopamine selective electrode. This electrode exhibited selectivity coefficients as large as 250 1 for dopamine and norepinephrine over ascorbic acid, uric acid, and DOPAC. 

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. 

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. 

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). 

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 ... 

Here, the potentiometric selectivity coefficient is given with respect to the hydroxyl ion. Single-crystal lanthanum fluoride is a wide bandgap semiconductor in which the electrical conductivity is due only to the hopping mobility of fluoride ions through the defects in the crystal. It does not respond to the La3+ ion because of the slow ion exchange of that ion. Hydroxyl ion is the only other ion that has appreciable mobility, and is the only known interference. For this reason, the measurements with a fluoride electrode are always done below pH 7, which circumvents this interference. As shown later, the consideration of ionic and/or electronic conductivity of the membrane plays a critical role also in the design of the internal contact in nonsymmetric potentiometric sensors. 

A polyvinyl chloride membrane electrode for heroin based on an ion-pair complex with tetraphenylborate is described (328). The sensor shows a near-Nernstian response over the heroin concentration range 0.01 M to 0.1 mM, with good selectivity for heroin in the presence of a number of adultercuits and base compounds present in illicit heroin powders. A cocaine sensor proposed by Zeng (329) uses a cocaine picrylaminate membrane. The calibration curve is linear in the range 0.01 M to 10 iM, with a coefficient of variation of less than... 

In measurements with ion-selective electrodes, interference by other ions is expressed by selectivity coefficients as in Eq. (17). If the nature of the ion-selective membrane is known, these interferences may easily be estimated. For example, in the determination of chloride with a Cl -selective electrode containing AgCl as the electroactive component in its membrane, concentrations of bromides or iodides (generally X ) must be controlled because they form less soluble silver salts than AgCl the solubility products of corresponding silver halides are used in Eq. (20) to estimate the selectivity coefficient ... 

Liquid membrane electrodes are subject to interferences from ions other than that of prime selectivity. For example, the Ca-ISE is also responsive to Mg2+ and Ba2+, the selectivity coefficients being approximately 0.01 for each ion. This indicates that the electrode is only 100 times more sensitive to Ca than to these ions, and this is normally much more important with respect to Mg than to Ba where food analysis is concerned. There are techniques which can be used to minimise the interference of Mg2+. 

Quaternary Ammonium Ions. In a recent study (17), 1200 EW Nafion has been used to construct a membrane ion selective electrode. The electrode was placed in both the tetrabutylammonium ion and cesium ion forms, and the response characteristics of each form were measured. These electrodes show Nernstian responses, and the tetrabutylammonium ion electrode has no interference from inorganic cations such as Na" ", K" ", and Ca2" ". However, this electrode shows a marked interference with decyltri-methylammonium ion. In addition the cesium ion electrode response is sensitive to the presence of tetrabutylammonium ion and especially dodecyltrimethylammonium ion. Although membrane electrode sensitivities are not in general proportional to thermodynamic selectivity coefficients, the results do indicate that these large, hydrophobic cations are preferred over smaller inorganic cations by the polymer. The authors suggest that the surfactant character of the two asymmetric tetraalkylammonium ions may lead to non-electrostatic interactions with the fluorocarbon regions of the polymer, which would enhance their affinities (17). 

E. Bakker, Determination of Improved Selectivity Coefficients of Polymer Membrane Ion-Electrodes by Conditioning with a Discriminating Ion, J. Elec-trochem. Soc., 143(4) (1996) L83. 

We have formulated this problem in a manner that considers only Na and as active species. Glass membranes also respond to other ions, such as Li, K", Ag, and NH4. The relative responses can be expressed through the corresponding potentiometric selectivity coefficients (see Problem 2.16 for some typical numbers), which are influenced to a great extent by the composition of the glass. Different types of electrodes, based on different types of glass, are marketed. They are broadly classified as (a) pH electrodes with a selectivity order >>> Na" > K, Rb", Cs >> Ca , (b) sodium-sensitive electrodes with the order Ag > > Na >> K, Li >> Ca ", and (c) a more... 

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