Selectivity, of an ISE

A commonly adopted approach to measure the selectivity of an ISE is the separate solution method, which is illustrated in Fig. 18a. 13. Here, two separate solutions, which contain the primary ion and interfering ion individually, are measured separately. The selectivity is calculated from... 

The Nicolslcy-Eisenman equation describes the selectivity of an ISE for the ion of interest over interfering ions ... 

There are various approaches for determining the selectivity of an ISE for a primary ion over an interfering ion. A straightforward approach is the separate solution method, where the potential of an ISE is determined in solutions of the primary and interfering ions separately but at equal ionic activities. The selectivity coefficient is then calculated as ... 

Given the reliance of these methods on Eq. (20), it is important to note that deviations from this equation have been reported, particularly for mixtures of ions of different charge (31). Moreover, it has been pointed out that many biased values have been reported using these methods [28]. This has led to a resurgence of interest in the matched potential method [32], which provides a measure of the selectivity of an ISE. Details of the relative merits of each of these methods have been summarized in an excellent review ]28]. 

Figure 32 Selectivity of an ISE for Ag+ over an interfering ion (J+) that does not bind to an electrically nentral ionophore Effect of the molar ratio of the total ionophore concentration to anionic sites. The five different cnrves show the dependence of the selectivity as calculated for ionophores that form complexes of different stoichiometries (a) ionophore does not bind Ag" " at all (b) / AgL = 10 (c) fiA L = 10 (Sa 2 = 10 (d) Pa = 10, Pa,L2 = 10 °, / AgL3 = 10 (e) = 10, SAgL2 = 10 °,...

Two things can be concluded from these. On the one hand, the selectivity of an ISE must depend on the activity ratio of the primary and interfering ions. On the other hand, the selectivity of an electrode is a feature that relates to one primary and one interfering ion. The selectivity of the electrode shows how far the electrode prefers a primary ion over a certain interfering ion. 

The discussion of Section 5-1 clearly illustrates that the most important response characteristic of an ISE is selectivity. Depending on the nature of the membrane material used to impart the desired selectivity, ISEs can be divided into three groups glass, liquid, or sohd electrodes. More than two dozen ISEs are commercially available and are widely used (although many more have been reported in the literature). Such electrodes are produced by firms such as Orion Research, Radiometer, Coming Glass, Beckman, Hitachi, or Sensorex. 

Discuss the significance of the selectivity coefficient of an ISE. How would you determine its value ... 

When using the selectivity constant or coefficient (k) mentioned by ISE suppliers, one must be sure that if the ion under test and the interfering ion have different valence the exponent in the activity term according to Nikolski has been taken into account it has become common practice to mention the interferent concentration that results in a 10% error in the apparent ion concentration these data facilitate the proper choice of an ISE for a specific analytical problem. Often maximum levels for no interference are indicated. 

The selectivity of the fluoride electrode is 1000 1 in the absence of OH , where the selectivity is a measure of how precise the measurement is. Consider an ISE that is monitoring the concentration of the anion X if the response of an ISE is such that nine out of every ten anions it detects is an X ion and the tenth ion is different, then the selectivity is 9 1. Clearly, such a selectivity is dreadful since any measurement can be up to 11% out common sense thus dictates that the selectivity should be maximized. 

We saw earlier in Section 3.5.2.2 how the selectivity of an ion-selective electrode (ISE) can be occasionally problematic, e.g. the activity of fluoride ion when determined by a fluoride ISE can be significantly in error if the solution also contains sufficient hydroxide ion to raise the pH above about 8. 

An ideal ISE would exhibit a specific response to a certain ion J and the effect of interferents would be excluded. Except for the silver sulphide electrode, which is specific for sulphide or silver ions, no ion-selective electrode has this property. The others exhibit selectivity only for a particular ion with respect to the others. The selective behaviour of an ISE follows from (3.1.7). If the activity of the interferent is sufficiently low, i.e. if... 

It has been widely accepted for many years that the LOD of an ISE in an unbuffered solution is at micromolar level. Interestingly, if a com-plexing agent is added into the sample and the concentration of the free primary ions is significantly lowered, the LOD is reduced sometimes to subnanomolar levels [35]. In addition, if halide ions are added to samples in which a silver-selective electrode is immersed, the electrode shows a decrease in potential indicating lowering of the activity of a silver at the sample/membrane phase boundary [36]. Moreover, ionophore-based optodes showed picomolar detection limits [37], even... 

When the conducting polymer is used as ion-to-electron transducer in the form of an intermediate layer between the electronic conductor and the ion-selective membrane it does not significantly influence the sensitivity and selectivity of the ISE, but it allows high potential stability [75]. For example, microfabricated solid-state K+-ISEs with polypyrrole as ion-to-electron transducer was found to show even better long-term potential stability than those based on a hydrogel contact [58]. The potential of the polypyrrole-based K+-ISE was slightly more sensitive to the oxygen concentration of the sample in comparison to... 

Figure 3. Selectivity pattern of an ISE based on ionophore 2. The electrode was exposed to the following anions salicylate (1), thiocyanate (2), nitrite (3), perchlorate (4), iodide (5), benzoate (6), bromide (7), bicarbonate (8), hydrogen phosphate (9), nitrate (10), chloride (11), sulfate (12). (Reproduced with permission from ref. 10. Copyright 1989 Alan R. Liss.)...

The essential part of an ISE is the ion-selective membrane that contains fixed or mobile sites that interact with ions in the solution. The membrane is commonly based on a plasticized polymer, glass, single crystal, or a sparingly soluble salt. The back-side of the membrane is in contact with a liquid- or solid-state ion-to-electron transducer that completes the ISE. New materials, such... 

The ion-selective membrane is the heart of an ISE as it controls the selectivity of the electrode. Ion-selective membranes are typically composed of glass, crystaUine, or polymeric materials. The chemical composition of the membrane is designed to achieve an optimal permselectivity toward the ion of interest. In practice, other ions exhibit finite interaction with membrane sites and will display some degree of interference for, determination of an analytei ion. In chnical practice, if the interference exceeds an acceptable level, a correction is required. 

Potentiometric ion-selective electrodes are passive probes, which in contrast to voltammetric sensors do not convert the analyte in the sample. The response of an ISE depends linearly on the logarithm of the activity (concentration) of a potential determining ion (primary ion) in the presence of other ions. The schematic layout of a complete potentiometric cell including an ion-selective electrode is shown in Figure 2. The electrochemical notation of the cell assembly is given as ... 

The selectivity coefficient of an ISE can be measured in several ways [28], the most popular of which are as follows ... 

Another type of indicator electrodes are the ion-selective electrodes (ISEs). The potential of an ISE follows the Eisenman-Nicolsky equation ... 

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