Using Ion-Selective Electrodes

Unlike ion-selective electrodes using glass membranes, crystalline solid-state ion-selective electrodes do not need to be conditioned before use and may be stored dry. The surface of the electrode is subject to poisoning, as described earlier for a Ck ISE in contact with an excessive concentration of Br. When this happens, the electrode can be returned to its original condition by sanding and polishing the crystalline membrane. 

This experiment describes the preparation and evaluation of two liquid-membrane Na+ ion-selective electrodes, using either the sodium salt of monensin or a hemisodium ionophore as ion exchangers incorporated into a PVG matrix. Electrodes prepared using monensin performed poorly, but those prepared using hemisodium showed a linear response over a range of 0.1 M to 3 X 10 M Na+ with slopes close to the theoretical value. 

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,... 

Determine the parts per million of F in the tap water, (b) For the analysis of toothpaste a 0.3619-g sample was transferred to a 100-mL volumetric flask along with 50.0 mL of TISAB and diluted to volume with distilled water. Three 20.0-mL aliquots were removed, and the potential was measured with an L ion-selective electrode using a saturated calomel electrode as a reference. Live separate 1.00-mL additions of a 100.0-ppm solution of L were added to each, measuring the potential following each addition. 

Example 5-4 A student calibrated a Mg2+ ion-selective electrode using two standard solutions at 25°C and constant ionic strength. He obtained the following results ... 

Fig. 18a. 16. EMF of an ion-selective electrode used in the nanomolar determination of PbJ ions in tap water at pH 4. Measurement of the same sample via ICP-MS is indicated by an arrow. From Ref. [72] with permission.

In the previous section, the role of solvent extraction was limited to preparing the analyte for subsequent analysis. A large majority of procedures that use solvent extraction in chemical analysis are used in this fashion. However, the extraction itself, or rather the distribution ratio characterizing it, may provide an appropriate measured signal for analysis. Examples of this use of solvent extraction are found in spectroscopy, isotope dilution radiometry, and ion-selective electrodes using liquid membranes. In the latter case, electrochemical determinations are possible by controlling the local concentration of specific ions in a solution by extraction. 

L has some ability to bind other ions besides C so those other ions interfere to some extent with the measurement of C. An ion-selective electrode uses a ligand with a strong preference to bind the desired ion. 

The size, cost, and accuracy requirements for successful sensors place a substantial burden on the processes used to manufacture them. Typically, today s sensors are manufactured by processes that are specific to one sensor or, at best, to a limited class of sensors. This process specificity has occurred because each type of sensor is usually made of unique materials in unique configurations that best convert the quantities to be measured into electrical signals. For example, pressure transducers may use piezoelectric or piezoresistive materials on thin diaphragms, whereas ion-selective electrodes use ion-conductive glasses or polymers around electrodes. Unfortunately, this situation implies large developmental costs. 

Semipermeable membrane electrode membrane electrode with membrane permeable only for limited type of ionic species. Its potential is equal to the -> Donnan potential. The electrode with the membrane permeable only for one type of ionic species is called -> ion selective electrode used in -> potentiometry. 

The gas-sensing configuration described above forms a very useful basic unit for potentiometric measurements of biologically important species. In principle, the immobilized or insolubilized biocatalyst is placed on a conventional ion-selective electrode used to measure the decrease in the reactants or the increase in products of the biochemical reaction. The biocatalyst include... 

The most important ion selective electrode using a solid membrane is the glass electrode for pH measurement. Despite their outstanding selectivity for H+ ions glass electrodes are used only seldom in enzyme electrodes because their sensitivity is affected by the buffer capacity of the measuring solution. 

Potentiometric sensors for analysis of solutions were enabled by the development of the glass electrode, and its coupling to a volt meter for measuring pH. This work was later extended to ion-selective electrodes using membrane materials such as Lanthanum Fluoride and valinomycin/Polyvinyl Chloride. 

Ion-selective electrodes used in conjunction with immobilized enzymes can serve as the basis of electrodes that are selective for specific enzyme substrates. Enzymes are proteins that catalyze specific reactions to a high degree of specificity. The reactants are the substrates. Enzymes and their properties are discussed in more detail in Chapter 24. 

The sulfide content in a pulp plant effluent is determined with a sulfide ion-selective electrode using the method of standard additions for calibration. A 10.0-mL sample is diluted to 25.0 mL with water and gives a potential reading of —216.4 mV. A similar 10.0-mL sample plus 1.00 mL of 0.030 M sulfide standard diluted to 25.0 mL gives a reading of —224.0 mV. Calculate the concentration of sulfide in the sample. 

Cyanides can also be determined by membrane ion-selective electrodes using direct potentiometry (argentoiodide electrode or an electrode with a mixture of Agl-AgjS). The principle of the response to cyanides in the case of the most frequently used ion-selective electrodes for CN ions consists in the dissolution of Agl according to the following reaction ... 

Z. Gao and M. Lu, Flow Injection Analysis by Ion-Selective Electrodes. Use of a Solid Membrane Chloride Ion-Selective Electrode for Determination of Chloride in Soil-Water Extracts [in Chinese]. Huanjing Kexue, 2 (1981) 376. 

Example 5-4 A student calibrated a ion-selective electrode using two... 

A 14-crown-4 derivative has also been incorporated in a PVC membrane to construct a coated-wire lithium ion-selective electrode used for the determination of lithium in blood sera, after on-line dialysis separation in a FIA system [6]. An estimation of the overall selectivity of the method over the sodium content, including that of the dialysis membrane and the electrode, resulted in a selectivity coefficient k of 1/50. This selectivity is not sufficient to overcome interferences from relatively large fluctuations in sodium contents, and corrections based on a simultaneous evaluation of the sodium content are required under such circumstances. 

Ion-selective electrodes use membranes which are permeable only to the ion being measured. To the extent that this can be done, the specificity of the electrode can be very high. One way of overcoming a lack of specificity for certain electrodes is to make multiple simultaneous measurement of several ions which include the most important interfering ones. A simple algorithm can then make corrections for the interfering effects. This technique is used in some commercial electrolyte analyzers. A partial list of the ions that can be measured with ion-selective electrodes includes H+ (pH), Na, K, Lff, Ca +, Cl , F , NHt, and CO. ... 

Ion-selective electrodes using a sulfur dioxide gassensing membrane probe have been developed into commercial models for the determination of sul-fur(IV) oxo-species. A typical procedure for the determination of free sulfur dioxide requires the probe to be immersed in a stirred solution at pH 1, whereas total sulfur dioxide requires the sample to be treated with strong alkali prior to acidification and analysis at pH 1. Molecular sulfur dioxide passes through the membrane into a solution of hydrogen sulfite ion and the pH of the SO2 H20/HS03 buffer can be determined with a glass electrode. 

Ion-selective electrodes used in whole blood or undiluted plasma or serum respond to the electrochemical activity of the ion. At the high ionic strengths of 1x0.16 mol 1 encountered in these biological samples, the molality tUi can no longer be substituted for active molality mi. Free ion molality may be converted to the active molality thi by multiplying with the molal activity coefficient... 

The ion-selective electrodes used in analysis are based on a system that comprises the ion-selective electrode membrane and a reference electrode. A variety of electrode types can be used for the analysis of species in water and these are described below, but the most important is undoubtedly the glass electrode. The glass electrode is used primarily for the determination of the pH of water samples and is widely used in water analysis. 

There are several types of electroactive polymers which have found application in chemical sensors. Thus, a large group of ion selective electrodes uses polymer matrices which host specific binding sites, called ionophores or ion exchangers. There is a selective partitioning of ions... 

What is the function of the Ion Selective Electrode used in this lab ... 

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