Ion-selective electrodes techniques

Ion Selective Electrodes Technique. Ion selective (ISE) methods, based on a direct potentiometric technique (7) (see Electroanalytical techniques), are routinely used in clinical chemistry to measure pH, sodium, potassium, carbon dioxide, calcium, lithium, and chloride levels in biological fluids. 

Prior to the introduction of ion-selective electrode techniques, in situ monitoring of free copper (II) in seawater was not possible due to the practical limitations of existing techniques (e.g., ligand competition and bacterial reactions). Ex situ analysis of free copper (II) is prone to experimental error, as the removal of seawater from the ocean can lead to speciation of copper (II). Potentially, a copper (II) ion electrode is capable of rapid in situ monitoring of environmental free copper (II). Unfortunately, copper (II) has not been used widely for the analysis of seawater due to chloride interference that is alleged to render the copper nonfunctional in this matrix [288]. 

II. 80) who also used a cupric ion-selective electrode technique and also corrected his data for adsorptive loss of copper. 

For analytical control of pharmaceuticals, most pharmacopeias describe accurate methods, which, however, in some cases are lengthy and difficult. The ion-selective electrode techniques offer several advantages in terms of simplicity and rapidity over official methods. Generally, electrodes of all the types noted above can be used to analyze compounds of pharmaceutical... 

The ion-selective electrode technique has been examined by several workers [ 1, 2,3], but primarily on waters of relatively high purity, and under carefully controlled laboratory conditions. Typical electrode performance in normal laboratory use is shown in Figure 1, and reproducibility of approximately —0.5 mV can be expected over the... 

Attachment of primary amines (resin 84) A 2 m solution of a primary amine in DMSO (5 ml, 66.7 equiv.) was added to the resin 83 (0.5 g, 0.15 mmol, 1 equiv.) and the suspension was shaken (vortexed) at 60 °C for 15 h. Alternatively, comparable results were obtained when the resin 83 was treated twice vtith the same solutions of amines at rt overnight. The resin was subsequently filtered off, washed vtith DMSO (3x7 min), and dried in vacuo overnight. The loading level of secondary amines was approximately 0.30 0.02 mmolg as measured by an ion-selective electrode technique... 

These are indirect methods based around computational procedures, using estimates of the monomeric inorganic aluminium fraction and free and total fluoride concentrations. Normally, free fluoride is determined by direct measurement with an ion-selective electrode. The total fluoride concentration is determined using the same ion-selective electrode technique, but after the addition of a total ionic strength buffer (TISAB). LaZerte [177]... 

A crucial factor affecting analytical measurements, especially in intensive care situations, is speed—clearly an advantage of these electrode systems. The development of the ion-selective electrode technique has reached such a state that flame photometry is likely to disappear from clinical laboratories within a few years. There is no doubt that carrier membrane electrodes based on crown compounds will also become an applicable tool in other fields of chemical analysis. 

The most popular device for fluoride analysis is the ion-selective electrode (see Electro analytical techniques). Analysis usiag the electrode is rapid and this is especially useful for dilute solutions and water analysis. Because the electrode responds only to free fluoride ion, care must be taken to convert complexed fluoride ions to free fluoride to obtain the total fluoride value (8). The fluoride electrode also can be used as an end poiat detector ia titration of fluoride usiag lanthanum nitrate [10099-59-9]. Often volumetric analysis by titration with thorium nitrate [13823-29-5] or lanthanum nitrate is the method of choice. The fluoride is preferably steam distilled from perchloric or sulfuric acid to prevent iaterference (9,10). Fusion with a sodium carbonate—sodium hydroxide mixture or sodium maybe required if the samples are covalent or iasoluble. 

Sources of Error. pH electrodes are subject to fewer iaterfereaces and other types of error than most potentiometric ionic-activity sensors, ie, ion-selective electrodes (see Electro analytical techniques). However, pH electrodes must be used with an awareness of their particular response characteristics, as weU as the potential sources of error that may affect other components of the measurement system, especially the reference electrode. Several common causes of measurement problems are electrode iaterferences and/or fouling of the pH sensor, sample matrix effects, reference electrode iastabiHty, and improper caHbration of the measurement system (12). 

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 sodium hydroxide is titrated with HCl. In a thermometric titration (92), the sibcate solution is treated first with hydrochloric acid to measure Na20 and then with hydrofluoric acid to determine precipitated Si02. Lower sibca concentrations are measured with the sibcomolybdate colorimetric method or instmmental techniques. X-ray fluorescence, atomic absorption and plasma emission spectroscopies, ion-selective electrodes, and ion chromatography are utilized to detect principal components as weU as trace cationic and anionic impurities. Eourier transform infrared, ft-nmr, laser Raman, and x-ray... 

Instmmental methods are useful for the determination of the total silver ia a sample, but such methods do not differentiate the various species of silver that may be present. A silver ion-selective electrode measures the activity of the silver ions present ia a solution. These activity values can be related to the concentration of the free silver ion ia the solution. Commercially available silver ion-selective electrodes measure Ag+ down to 10 flg/L, and special silver ion electrodes can measure free silver ion at 1 ng/L (27) (see Electro analytical techniques). 

Sodium and chloride may be measured using ion-selective electrodes (see Electro analytical techniques). On-line monitors exist for these ions. Sihca and phosphate may be monitored colorimetricaHy. Iron is usually monitored by analysis of filters that have had a measured amount of water flow through them. Chloride, sulfate, phosphate, and other anions may be monitored by ion chromatography using chemical suppression. On-line ion chromatography is used at many nuclear power plants. 

In some systems, known as continuous-flow analy2ers, the reaction develops as the sample —reagent mixture flows through a conduit held at constant temperature. In such systems, the reaction cuvettes are replaced by optical reading stations called flow cells. In most analy2ers, whether of discrete- or continuous-flow type, deterrnination of electrolyte tests, eg, sodium and potassium levels, is done by a separate unit using the technique of ion-selective electrodes (ISE) rather than optical detection. 

The majoiity of the various analyte measurements made in automated clinical chemistry analyzers involve optical techniques such as absorbance, reflectance, luminescence, and turbidimetric and nephelometric detection means. Some of these ate illustrated in Figure 3. The measurement of electrolytes such as sodium and potassium have generally been accomphshed by flame photometry or ion-selective electrode sensors (qv). However, the development of chromogenic ionophores permits these measurements to be done by absorbance photometry also. 

Perhaps the most precise, reHable, accurate, convenient, selective, inexpensive, and commercially successful electroanalytical techniques are the passive techniques, which include only potentiometry and use of ion-selective electrodes, either direcdy or in potentiometric titrations. Whereas these techniques receive only cursory or no treatment in electrochemistry textbooks, the subject is regularly reviewed and treated (19—22). Reference 22 is especially recommended for novices in the field. Additionally, there is a journal, Ion-Selective Electrode Reviews, devoted solely to the use of ion-selective electrodes. 

According to the definition, a passive technique is one for which no appHed signal is required to measure a response that is analytically usehil. Only the potential (the equiHbrium potential) corresponding to zero current is measured. Because no current flows, the auxiHary electrode is no longer needed. The two-electrode system, where the working electrode may or not be an ion-selective electrode, suffices. 

Many double-charged anions, such as sulfate, hydrophosphate, oxalate etc., are highly widespread in natural sources and at the same time lack any convenient technique for their determination. Therefore, development of ion-selective electrodes (ISEs), responsive to these anions, is of great practical importance. However, for a long time all attempts directed toward creation of such electrodes were unsuccessful (except for carbonate ISEs based on trifluoroacetylbenzene derivatives), and only in recent years this field has shown significant progress. 

Ion-selective electrodes are a relatively cheap approach to analysis of many ions in solution. The emf of the selective electrode is measured relative to a reference electrode. The electrode potential varies with the logarithm of the activity of the ion. The electrodes are calibrated using standards of the ion under investigation. Application is limited to those ions not subject to the same interference as ion chromatography (the preferred technique), e.g. fluoride, hydrogen chloride (see Table 10.3). 

Electrochemical analytical techniques are a class of titration methods which in turn can be subdivided into potentiometric titrations using ion-selective electrodes and polarographic methods. Polarographic methods are based on the suppression of the overpotential associated with oxygen or other species in the polarographic cell caused by surfactants or on the effect of surfactants on the capacitance of the electrode. One example of this latter case is the method based on the interference of anionic surfactants with cationic surfactants, or vice versa, on the capacitance of a mercury drop electrode. This interference can be used in the one-phase titration of sulfates without indicator to determine the endpoint... 

Situation Suppose a (monovalent) ionic species is to be measured in an aqueous matrix containing modifiers direct calibration with pure solutions of the ion (say, as its chloride salt) are viewed with suspicion because modifier/ion complexation and modifier/electrode interactions are a definite possibility. The analyst therefore opts for a standard addition technique using an ion-selective electrode. He intends to run a simulation to get a feeling for the numbers and interactions to expect. The following assumptions are made ... 

Figure 4.24. Estimated concentration of ion using the standard addition technique with an ion-selective electrode. The simulated signal traces are for DVM resolutions of 1, 0.1, 0.01, resp. 0.001 mV (left to right). For each resolution the added volume V2 is varied from 2.4 to 2.55 ml in increments of V2 = 10 /rl. The ordinate marks indicate the 95-105% SLs. The expanded traces for 0.1. .. 0.001 mV resolution are also given. The simulation was run for five different values of 0 = 300 + RND [mV]. The vertical drops (e.g., A B) occur at...

De Marco R, Pejcic B, Prince K, van Riessen A (2003) A multi-technique surface study of the mercury(ll) chalcogenide ion-selective electrode in saline media. Analyst 128 742-749 Pejcic B, De Marco R (2004) Characterization of an AgBr-Ag2S-As2S3-Hgl2ion-selective electrode membrane a X-ray photoelectron and impedance spectroscopy approach. Appl Surf Sci 228 378-400... 

Various types of research are carried out on ITIESs nowadays. These studies are modeled on electrochemical techniques, theories, and systems. Studies of ion transfer across ITIESs are especially interesting and important because these are the only studies on ITIESs. Many complex ion transfers assisted by some chemical reactions have been studied, to say nothing of single ion transfers. In the world of nature, many types of ion transfer play important roles such as selective ion transfer through biological membranes. Therefore, there are quite a few studies that get ideas from those systems, while many interests from analytical applications motivate those too. Since the ion transfer at an ITIES is closely related with the fields of solvent extraction and ion-selective electrodes, these studies mainly deal with facilitated ion transfer by various kinds of ionophores. Since crown ethers as ionophores show interesting selectivity, a lot of derivatives are synthesized and their selectivities are evaluated in solvent extraction, ion-selective systems, etc. Of course electrochemical studies on ITIESs are also suitable for the systems of ion transfer facilitated by crown ethers and have thrown new light on the mechanisms of selectivity exhibited by crown ethers. 

It was reported that neutral ionophore naphtho-15-crown-5 (N15C5, see Fig. 1) gives an excellent selective response to ion over Na+ ion when it is doped into an ion-selective electrode [2,3]. This result is very interesting considering the hole size of the crown. Ishibashi and coworkers studied the mechanisms of that selectivity using the technique of solvent extraction into 1,2-dichloroethane (DCE) where picrate anion was used as a counterion [4]. Their result is also interesting because Na+ is extracted as a... 

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