Commercially available ion-selective electrodes

A number of ion-selective electrodes are available from laboratory supply houses whilst not intended to be an exhaustive list, Table 15.3 serves to indicate the variety of determinations for which electrodes are available. An indication is also given of the lower limit of detection of the electrodes this figure may vary somewhat according to the source of the electrode but full details are furnished by the manufacturer of the effective range of use of each electrode and of likely interferences. 

Ion Lower limit of detection, pX Ion Lower limit of detection, pX  

Glass membrane Crystalline solid-state membrane  

The response time of an electrode is defined as the time taken for the cell e.m.f. to a reach a value which is 1 millivolt from the final equilibrium value. The response time is obviously affected by the type of electrode, particularly with regard to the nature of the membrane, and is also affected by the presence of interfering ions, and by change in temperature. 

General details for the care and maintenance of ion-selective electrodes are given in Ref. 31. 

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Table Al. Survey of some commercially available ion-selective electrodes... 

TABLE 2.4.1 Typical Commercially Available Ion-Selective Electrodes... 

It is impossible to enumerate the many ions which can be determined with currently commercially available ion-selective electrodes because their number is continually increasing. Those cations or anions which are specifically and directly detected with these electrodes number about 30. In addition, there are those ions which, with the help of these 30, can be determined indirectly through chemical complexation, precipitation or biochemical reactions which serve to change the activity of the indicated ion in the solution equilibrium. Modified electrodes also make it possible to directly indicate gases such as NH3, CO2, NOx, SO2, etc., as well as organic compounds such as amino acids, amygdalin, cholesterol, urea, penicillin, etc. A current summary (Fig. 2) shows the periodic table with shaded boxes for elements which are directly indicated and boxes divided in half for elements which at present can be indicated only indirectly. The second row lists either the form in which the ion is specifically indicated or the chemical reaction with which the indirectly determined ion can be measured. No one has yet attempted to detect the remaining elements with the help of ion-selective electrodes as far as the author is aware (1973). 

Few potentiometric biosensors are commercially available. As shown in Figures 11.16 and 11.17, however, available ion-selective and gas-sensing electrodes may be easily converted into biosensors. Several representative examples are described in Table 11.5, and additional examples can be found in several reviews listed in the suggested readings at the end of the chapter. 

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). 

The classic potentiometric enzyme electrode is a combination of an ion-selective electrode-based sensor and an immobilized (insolubilized) enzyme. Few of the many enzyme electrodes based on potentiometric ion- and gas-selective membrane electrode transducers have been included in commercially available instruments for routine measurements of biomolecules in complex samples such as blood, urine or bioreactor media. The main practical limitation of potentiometric enzyme electrodes for this purpose is their poor selectivity, which does not arise from the biocatalytic reaction, but from the response of the base ion or gas transducer to endogenous ionic and gaseous species in the sample. 

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and... 

Anions may be measured best by ion chromatography, using appropriate anion exchange resin columns that are available commercially. Salts may be diluted for such measurements. Ion-selective electrode methods also yield satisfactory results at trace concentrations. Numerous colorimetric methods are reported in literature. They are susceptible to erroneous results when impurities are present. Many titration methods are available in analytical chemistry. They may be applied successfully to measure certain anions, oxidizing and reducing substances, acids, and bases. 

Ion-selective electrodes for fluoride trace analysis are commercially available and effective down to a level of mg dm [56]. A recent textbook by Cattrall [57] gives background information to the theory... 

These and other ion-selective electrodes are discussed in Chapter 5. A large selection of such electrodes is commercially available, and the biennial reviews of Analytical Chemistry include a section that describes new types of electrodes and their applications. 

Before using the pH electrode, it should be calibrated using two (or more) buffers of known pH. Many standard buffers are commercially available, with an accuracy of 0.01 pH unit. Calibration must be performed at the same temperature at which the measurement will be made care must be taken to match the temperature of samples and standards. The exact procedure depends on the model of pH meter used. Modern pH meters, such as the one shown in Figure 5.8, are microcomputer-controlled, and allow double-point calibration, slope calculation, temperature adjustment, and accuracy to +0.001 pH unit, all with few basic steps. The electrode must be stored in an aqueous solution when not in use, so that the hydrated gel layer of the glass does not dry out. A highly stable response can thus be obtained over long time periods. As with other ion-selective electrodes, the operator should consult the manufacturer s instructions for proper use. Commercial glass electrodes are remarkably... 

Potentiometric measurements are simple the redox potential is measured compared to a reference electrode. For pH measurements, commercially available electrodes, comprising the working (glass electrode) and reference electrodes, can be used. For redox potential measurements, the working electrode is usually an inert (e.g., platinum) electrode, and the reference electrode can be a hydrogen electrode, calomel, or other electrodes. Ion-selective electrodes are also based on potential measurements. 

The role of C104 as a ligand in solution has been reviewed by Johansson.144 Perchlorate-ion-selective electrodes prepared using liquid ion-exchangers in PVC are said145 to exhibit approximately the same characteristics as the commercially available electrode the selectivity of the new electrode was claimed to be superior. The ion-exchanger was mixed with PVC dissolved in THF and the mixture dried as a membrane disc or used to coat a Pt electrode the useful life ranged from 2 weeks, for a wire electrode, up to a month. 

Table 21-2 lists some liquid-membrane electrodes available from commercial sources. The anion-sensitive electrodes shown make use of a solution containing an anion-exchange resin in an organic solvent. Liquid-membrane electrodes in which the exchange liquid is held in a polyvinyl chloride gel have been developed for Ca-, K", NOj, and BF4. These have the appearance of crystalline electrodes, which are considered in the following section. A homemade liquid-membrane ion-selective electrode is described in Feature 21-1. 

The concentration of free metal ions in water can be determined directly by use of an ion-selective electrode (ISE), or by anodic stripping voltammetry (ASV). A major limitation of ISE is its rather low sensitivity furthermore, only a few commercial electrodes are available (e.g., Cu, Pb, Cd +, Ca ). In both methods, electrode response is affected by pH, ionic strength, and sorption of organics on the electrode surface (Brezonik et al., 1976 Blutstein and Smith, 1978 Greter et al., 1979). 

There are over 300 enzymes available commercially, and many of these can be used in one way or another for analytical purposes. One of methods of use involves the determination of an analyte or substrate by means of the enzyme which reacts specifically with that substrate. Examples are 1. Glucose and glucose oxidase, O2 released, measurement by O2-ISE 2. Urea and urease, NHs and CO2 released, measurement by an NH4 - or CO2-ISE 3. Pectin and pectin-esterase, titration of H+ released. In chnical work, the opposing approach is often used, and the amount of an enzyme determined by adding the proper substrate to a solution of the enzyme and measuring with an ion-selective electrode a product of the reaction. Early work in this area, and in that of immobihsed enzyme electrodes, was carried out by Katz and Rechnitz [15,16], Guilbault and his co-workers [17] and Guilhault and Montalvo [18]. 

Table 2.4.1 is a listing of typical commercial ion-selective electrodes, the pH and concentration ranges over which they operate, and typical interferences. Selectivity coefficients for many of these electrodes are available (55, 57). 

Ion-selective membranes attain their permselectivity from ion-exchange, dissolution, or complexation phenomena. Different types of membranes are available for the construction of ion-selective electrodes glass and other solid state rods (crystals), liquid or polymer ion ecchangers, or dissolved ionophores. Many electrodes are commercially available with selec-tivities for different ions, mainly H, alkali metal cations, heavy metal ions, and halides or pseudohalides. Also gas-sensing electrodes may be constructed from an ion-selective electrode and a gas-permeable membrane [182]. Ion selective electrodes and gas-selective electrodes... 

Commercial silver/silver-chloride reference electrodes are available in a variety of styles and sizes. They are often used as the internal reference electrodes in glass pH and other ion-selective electrodes. Ag/AgCl microelectrodes formed from very thin silver wire have found extensive use, for example, in biomedical applications such as in vivo studies of biological fluids and intracellular measurements, because of the miniaturization possible with these electrodes. 

Within about the last 10 years a wide variety of commercial and homemade ion-selective electrodes (ISEs) has become available they respond more or less selectively to a wide range of ions in solution. 

Given the vride variety of ion-selective electrodes already commercially available and the many more specialized ones that can be fabricated, titrations involving the precipitation or complexation of ions are widely used. Halides, cyanide, thiocyanate, sulfide, chromate, and thiols can be titrated with silver nitrate, using the appropriate... 

Commercial solid-state potential measuring devices based on the type of op-amp described are often called pH or plon meters and are designed to work with glass pH electrodes, ion selective electrodes, and other indicator electrodes described earlier. Research quality plon meters have built-in temperature measurement and compensation, autocalibration routines for a three-point (or more) calibration curve, recognition of electrodes (so you do not try measuring fluoride ion with your pH electrode ), and the ability to download data to computer data collection programs. The relative accuracy of pH measurements with such a meter is about +0.005 pH units. Meters are available as handheld... 

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