Sensitivity ion selective electrodes

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. 

Hazemoto et al (1+0) developed an ion-selective electrode sensitive to saccharin, by establishing an ion association between Fe2+-bathophenanthroline chelate and saccharin in nitrobenzene. The electrode developed could measure saccharin ion in presence of other sweetening agents e.g., sucrose, glucose, sodium cyclamate and sorbitol in the concentration range of 10 - - to 10 M. 

Enzymes are substances that react very selectively with a substrate in a very specific reaction. Their immobilization on a membrane which is then placed over an electrode in a solution together with the substrate to be determined leads to reaction products that can be detected at the electrode covered by the membrane. An example is the degradation of urea by urease with an internal sensor element (i.e. ion-selective electrode) sensitive to ammonium ion ... 

Analytical schemes concerned with the determination of blood ions and gases can be divided into two categories analyses done in vivo and those done in vitro. By far the most common method of determining blood ions in vitro involves atomic spectroscopy. Atomic absorption and flame emission have both been used although the latter is the most popular. In the clinical lab nearly all of the remaining determinations (both in vivo and in vitro) are performed with ion-selective (for ions, NH3 and CO2) or amperometric electrodes (O2 and H2). Two important characteristics of ion-selective electrodes, sensitivity and selectivity, should be mentioned. The applicability of a specific electrode in any particular situation can be determined by considering, on one hand, the ionic constituents of the solution to be measured and, on the other hand, the sensitivity and specificity of the electrode in question. Proper consideration of these points will allow an investigator to determine the accuracy and validity of the measurement. 

An ion selective electrode operates on exactly the same principles as a pH electrode (see Chapter 1). In fact, a pH electrode is a type of ion selective electrode sensitive to hydrogen ions. Just like a pH electrode, the electrode body contains a reference solution and an internal reference electrode. On to this electrode body is sealed an ion selective membrane which acts as... 

One such research direction is development of new ion-selective electrodes sensitive to medical substances, among which is the group of non-narcotic, analgetic drugs known as nonsteroidal anti-inflammatory drugs (NSAIDs), which are widely used around the world. Many recent works on ion-selective electrodes were associated with pharmaceutical analysis [1-4]. 

Methods for iodine deterrnination in foods using colorimetry (95,96), ion-selective electrodes (94,97), micro acid digestion methods (98), and gas chromatography (99) suffer some limitations such as potential interferences, possibHity of contamination, and loss during analysis. More recendy neutron activation analysis, which is probably the most sensitive analytical technique for determining iodine, has also been used (100—102). 

The other method is less accurate but more rapid and involves direct Nessleri2ation of the sample for colorimetric deterrnination. Other colorimetric indicators with more sensitivity, such as indophenol, have been used in place of Nessler s reagent. Ion-selective electrodes have also found use in analysis for trace ammonia (93). 

Potentiometric Titrations. If one wishes to analyze electroactive analytes that are not ions or for which ion-selective electrodes are not available, two problems arise. First, the working electrodes, such as silver, platinum, mercury, etc, are not selective. Second, metallic electrodes may exhibit mixed potentials, which may arise from a variety of causes. For example, silver may exchange electrons with redox couples in solution, sense Ag" via electron exchange with the external circuit, or tarnish to produce pH-sensitive oxide sites or Ag2S sites that are sensitive to sulfide and haUde. On the other... 

NEW COMPLEX CHALCOGENIDES AS SENSITIVE MATERIALS FOR ION SELECTIVE ELECTRODES... 

The sensor is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme mease (Figme 6-11) (22). The generated ammonium ions are detected after 30-60 s to reach a steady-state potential. Alternately, the changes in the proton concentration can be probed with glass pH or other pH-sensitive electrodes. As expected for potentiometric probes, the potential is a linear function of the logarithm of the urea concentration in the sample solution. 

Instead of using an indicator, an ion-sensitive electrode can be used. An aqueous solution is titrated potentiometrically against 0.04 N hyamine 1622 solution using a nitrate ion-selective electrode and a silver/silver chloride reference electrode [106]. 

Legin AV, Vlasov YG, Rudnitskaya AM, Bychkov EA (1996) Cross-sensitivity of chalcogenide glass sensors in solutions of heavy metal ions. Sens Actuators B 34 456 61 De Marco R, Shackleton J (1999) Cahbration of the Hg chalcogenide glass membrane ion-selective electrode in seawater media. Talanta 49 385-391... 

Studies of the polarized IBTILE provide a fundamental knowledge that makes it possible to explain phenomena occurring at the membranes of ion-selective electrodes. In addition, the rates of ion transfer and assisted ion transfer reactions are proportional to concentrations, which is a basis of an amperometric ion-selective (sensitive) electrode. 

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. 

Controlled potential methods have been successfully applied to ion-selective electrodes. The term voltammetric ion-selective electrode (VISE) was suggested by Cammann [60], Senda and coworkers called electrodes placed under constant potential conditions amperometric ion-selective electrodes (AISE) [61, 62], Similarly to controlled current methods potentiostatic techniques help to overcome two major drawbacks of classic potentiometry. First, ISEs have a logarithmic response function, which makes them less sensitive to the small change in activity of the detected analyte. Second, an increased charge of the detected ions leads to the reduction of the response slope and, therefore, to the loss of sensitivity, especially in the case of large polyionic molecules. Due to the underlying response mechanism voltammetric ISEs yield a linear response function that is not as sensitive to the charge of the ion. 

The properties of a pH electrode are characterized by parameters like linear response slope, response time, sensitivity, selectivity, reproducibility/accuracy, stability and biocompatibility. Most of these properties are related to each other, and an optimization process of sensor properties often leads to a compromised result. For the development of pH sensors for in-vivo measurements or implantable applications, both reproducibility and biocompatibility are crucial. Recommendations about using ion-selective electrodes for blood electrolyte analysis have been made by the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) [37], IUPAC working party on pH has published IUPAC s recommendations on the definition, standards, and procedures... 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

Direct determination of surfactants in complex matrices can also be carried out using ion-selective electrodes. Depending on the membranes and additives used, the detergent electrodes are optimized for the detection of anionic surfactants [81], cationic surfactants [82], and even nonionic surfactants [83]. The devices are sensitive to the respective group of surfactants but normally do not exhibit sufficient stability and reproducibility for their use in household appliances. With further optimization of membrane materials, plasticizers and measurement technology, surfactant-selective electrodes offer high potential for future applications. 

An ion-selective electrode is a half-cell that is sensitive to a particular ion like the pH electrode is... 

The membrane of the glass electrode used for pH measurements is selectively permeable to hydrogen ions and from this basic concept a whole range of ion-selective electrodes have been developed. Varying the composition of the glass membrane can change the permeability of the glass and several cation-sensitive... 

Apart from interference the greatest problem in the use of ion-selective electrodes is that of contamination. Any insoluble material deposited on the surface of the electrode will significantly reduce its sensitivity and oil films or protein deposits must be removed by frequent and thorough washing. It is possible to wipe membranes with soft tissue but they can be easily damaged. Solid-state membranes are more robust but they must not be used in any solution which might react with the membrane material. 

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