Selective ion electrode

Renal function Blood urea nitrogen, creatinine 

SOURCE C. C. Young, Evolution of Blood Chemistry Analyzers Based on Ion Selective Electrodes, J. Chem. Ed. 1997, 74, 177. 

Most ion-selective electrodes fall into one of the following classes  

Solid-state electrodes based on inorganic salt crystals 

Liquid-based electrodes using a hydrophobic polymer membrane saturated with a hydrophobic liquid ion exchanger 

The ion-selective membrane is the key component of all potentiometric ion sensors. It establishes the preference with which the sensor responds to the ion of interest in the presence of various other ionic components of the sample. By definition, the ion-selective membrane forms a nonpolarized interface with the solution. If the interface is permeable to only one ion, the potential difference at that interface is expressed by the Nernst equation (6.6). If more than one ion can permeate, the interface can be anything between the liquid junction and the mixed potential. The key distinguishing feature is the absolute magnitude of the total exchange current density. 

It is the same as the equation of general response formulated earlier (1.17), except that the potentiometric transfer function 91 is now given by the Nernst equation. It 

Changes in the reference electrode junction potential result from differences in the composition of die sample and standard solutions (e.g., upon switching from whole blood samples to aqueous calibrants). One approach to alleviate this problem is to use an intermediate salt bridge, with a solution (in the bridge) of ions of nearly equal mobility (e.g., concentrated KC1). Standard solutions with an electrolyte composition similar to that of the sample are also desirable. These precautions, however, will not eliminate the problem completely. Other approaches to address this and other changes in the cell constant have been reviewed (13). 

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. 

Special electrodes can be constructed to measure concentrations of ionic species, such as Na, Ca , and hydronium ions, which are important in biochemical reactions. The potential of a hydrogen electrode is directly 

The glass of a glass electrode is based on lithium silicate doped with heavy-metal oxides it is filled with a phosphate buffer solution containing Cl ions. Conveniently, the electrode has 0 when the external medium is at pH = 7. The electrode is calibrated using solutions of known pH (for example, one of the buffer solutions described in Section 4.11). 

Hg(l) I HgjChls) I Cl (aq), that makes contact with the test solution through a salt bridge. 

Self-test 5.7 ) What range should a voltmeter have (in volts) to display changes of pH from 1 to 14 at 25 C if it is arranged to give a reading of zero when pH = 7  

To calculate the Galvani potential difference for a biphasic system containing three species (for example, a target cation I+, a hydrophilic anion A forming a salt IA in the aqueous phase, and a lipophilic anion X forming a salt IX in the organic phase), we should consider the three respective Nemst equations  

For each species, we should consider the conservation of mass  

Taking into account the electroneutrality condition in each phase, (C, - - c = 0), we have only one equation to solve to calculate the resulting 

When the ratio is large, that is, when IX is in excess versus lA, the 

Galvani potential difference to the distribntion potential of IX is represented by A ( )jjj ijj. Inversely, when the ratio is small, that is, when lA is in excess 

Membrane electrodes are used in the potentiometric methods of analysis where the membrane allows certain kinds of ions to penetrate it while rejecting others. 

Attempts have been made to substitute the aqueous buffer with solid contacts such as those based on the ternary system Li-Ag-I [Kreuer, 1990]. The pH elccuodes with solid contacts extend the operable temperature range to about ISO C. This allows pH measurements at a higher temperature or for those applications requiring sterilization as in fermentors. The issue of hydrolysis of the glass surface is more difficult to solve as the 

Devices called sensors, which are sensitive to physical influences other than electricity and light, like pressure, temperature, chemical concentrations, or magnetic fields, can convert non-electric signals into electrical ones (see, e.g., the review of Janata [108] for chemical sensors). 

Electrochemical sensors play a crucial role in environmental and industrial monitoring, as well as in medical and clinical analysis. The common feature of all electroanalytical sensors is that they rely on the detection of an electrical property (i.e., potential, resistance, current) so that they are normally classified according to the mode of measurement (i.e., potentiometric, conductometric, amperometric). A number of surveys have been published on this immense field. The reader may find the major part of the older and recent bibliography in the comprehensive reviews of Bakker et al. [109-111]. Pejcic and De Marco have presented an interesting survey 

There has been a great deal of work reported in the literature on the analysis of copper(II) in various systems using a pressed pellet of CuS-Ag2S [117], It has been shown that a ternary sulfide called jalpaite (i.e., Ag1.5Cuo.5S) leads to desirable electrochemical and electroanalytical properties for the detection of Cu(II) in natural waters [118]. 

As mentioned in a previous chapter, the art of manufactiu-ing ion-selective electrodes (ISEs) consists in searching for the proper method to prepare the sensor siuface in such a way that a Galvani potential difference results which should depend selectively on the activity of only one type of ion, if possible. The best option would be a specific electrode, but at the very least it should be selective. 

Galvanic cells can be set up with solid electrolytes rather than electrolytic solutions. Such a cell is the basis for a well-known potentiometric gas sensor, the lambda probe. The latter is designed to determine the oxygen content of combustion gases, e.g. in motor vehicles. The lambda probe can operate in two different modes, either potentiometrically or amperometrically. 

Different kinds of solid-membrane ISEs can be defined by considering how they make an electric connection between membrane and measuring instrmnent (Fig. 7.4). A most versatile embodiment is the connection via an internal reference electrode (Fig. 7.4, left). Such an electrode can hardly be realized in miniature form. An internal cavity is filled with a solution that develops 

Examples of Solid-Membrane ISEs. In the 1960s, enthusiastic expectations subsided in connection with ISEs. Countless studies were devoted to new ISE types. Later, when a more realistic consideration arose, it proved that only a few ISE types were really useful in analytical practice. Some examples of well-tried sensors are given in Table 7.2, which lists examples in order of decreasing practicability. 

The functional principle of solid-membrane ISEs can be explained best by considering them as a special sort of electrode of the second kind. In principle, every second-kind electrode can be used as a potentiometric sensor. Starting with a simple metal/metal ion electrode, e.g. the silver/silver ion electrode, the potential can be written as follows  

Glass electrodes sensitive to proton concentration were first introduced in 1909 and have long been the generally accepted way of determining pH. Similar electrodes which respond selectively to other ions are a much more recent development dating back only to the mid 1960s even so, ion-selective electrodes now have many applications in water and environmental analysis, e.g. the determination of pH, F , CN, NHj and total hardness (Ca -t-Mg ). 

The cell for the determination of the concentration of an ion, M , with an ion-selective electrode is shown in Figs 12.4 and 12.5, although in many cases the ion-selective electrode and the external reference electrode are mounted in a single (combination) probe. The measurement of the potential between the two reference electrodes allows the determination of the ion M between the analyte and the internal reference solution. For application in analysis, the membrane potential m should be given by  

The success of an analysis with an ion-selective electrode is almost totally dependent on the properties of the membrane. The response of the membrane to changes in the concentration of M should be given by equation (12.1), but on the other hand, it should be completely independent of the concentration of 

Internal electrolyte Reference electrode PIPE body 

Several quite different types of membranes have been used in the construction of ion-selective electrodes, namely (1) glasses (2) solid-state membranes (3) heterogeneous membranes and (4) liquid membranes. 

Various types of membrane electrodes have been developed in which the membrane potential is selective toward a given ion or ions, just as the potential of the glass membrane of a conventional glass electrode is selective toward hydrogen ions. These electrodes are important in the measurement of ions, especially in small concentrations. Generally, they are not poisoned by the presence of proteins, as some other electrodes are, and so they are ideally suited to measurements in biological media. This is especidly true for the glass membrane ion-selective electrodes. 

None of these electrodes is specific for a given ion, but each will possess a certain selectivity toward a given ion or ions. So they are properly referred to as ion-selective electrodes (ISEs). 

See www.nico2Q00.net for an excellent tutorial (12,000-word beginners guide) on principles of pH and ion-selective electrodes, calibration, and measuring procedures. 

The glass membrane pH electrode is the ultimate ion-selective electrode. 

pH type. This is the conventional pH glass electrode, and it has a selectivity order of H Na K , Rb, Cs . . . Ca . The response to ions other than H is the alkaline error we talked about above. 

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. 

One of the more interesting intrinsic properties of ion-selective electrodes is that they respond logarithmically to the activity of an ion of interest that is, in the case where the reference-electrode and liquid-junction potentials are constant, for a cation electrode 

One interesting result of this property is that the relative concentration error for direct potentiometric measurements is theoretically independent of the actual concentration. Unfortunately, the error is rather large—approximately 4n% per mV uncertainty in measurement, perhaps the most serious limitation of ISEs. Since potential measurements are seldom better than 0.1 mV total uncertainty, the best measurements for monovalent ions under near-ideal conditions are limited to about 0.5% relative concentration error. For divalent ions, this error would be doubled and in particularly bad cases where, for example, liquid-junction potentials may vary by 5 to 10 mV (as in high or variable ionic-strength solutions), the relative concentration error may be as high as 507o- This limitation may be overcome, however, by using ISEs as endpoint indicators in potentiometric titrations (Sec. 2.6). At the cost of some extra time, accuracies and precisions on the order of 0.1% or better are possible. 

Another intrinsic property of ISEs is that they measure activities—the thermodynamic free concentration of an ion—although they can be made to determine concentrations by appropriate calibration procedures. Activity measurements may be more valuable in certain cases because activities or free concentrations 

Interferences. Broadly speaking, interferences may be classed into two general categories chemical interferences in solution, such as complexation, and electrode interferences due to less than perfect electrode specificity. An ion-selective electrode will respond, more or less strongly, to ions other than the one for which it is nominally designed. 

A significant development in the methodology of potentiometry that paved the way for its utility in bioanalysis was the discovery of the ion selective electrode (ISE). Conceptually, the ISE involves the measurement of a membrane potential. The response of the electrochemical cell is therefore based on an interaction between the membrane and the analyte that alters the potential across the membrane. The selectivity of the potential response to the analyte depends on the specificity of the membrane interaction for the analyte. 

A representative ISE is shown schematically in Fig. 1. The electrode consists of a membrane, an internal reference electrolyte of fixed activity, (ai)i , ai and an internal reference electrode. The ISE is immersed in sample solution that contains analyte of some activity, (ajXampie and into which an external reference electrode is also immersed. The potential measured by the pH/mV meter (Eoe,) is equal to the difference in potential between the internal (Eraf.int) and external (Eref.ext) reference electrodes, plus the membrane potential (E emb), plus the liquid junction potential 

If the membrane is permselective for a i rticular ion (i), a potential develops ross the membrane which depends on the ratio of activities of the ion on either side of the membrane as describ l by the Nemst equation 

The half-cell potentials of the two reference electrodes are constant sample solution conditions can often be controlled so that E,j is effectively constant and the composition of the internal solution can be maintained so that (ai)i , ai is fixed. Consequently Eq. (3) can be simplified to give 

Since membranes respond to a certain degree to ions other than the analyte (i.e., interferents), a more general expr sion than Eq. (4) is 

The glass pH electrode is an example of a solid-state ion-selective electrode whose operation depends on (1) an ion-exchange reaction of H between the glass surface and analyte solution and (2) transport of Na across the glass membrane. We now examine several ion-selective electrodes. 

Ion Concentration range (M) Membrane crystal pH range Interfering species 

The constant value of [F ]jnside is incorporated into the constant term in Equation 15-7. 

A fluoride electrode immersed in standard solutions gave the following potentials  

SOLUTION (a) Our strategy is to fit the calibration data with Equation 15-7 and then to substitute the concentration of F into the equation to find the potential  

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Initial attempts at developing precipitation titration methods were limited by a poor end point signal. Finding the end point by looking for the first addition of titrant that does not yield additional precipitate is cumbersome at best. The feasibility of precipitation titrimetry improved with the development of visual indicators and potentiometric ion-selective electrodes. 

Finding the End Point Potcntiomctrically Another method for locating the end point of a precipitation titration is to monitor the change in concentration for the analyte or titrant using an ion-selective electrode. The end point can then be found from a visual inspection of the titration curve. A further discussion of potentiome-try is found in Chapter 11. 

When first developed, potentiometry was restricted to redox equilibria at metallic electrodes, limiting its application to a few ions. In 1906, Cremer discovered that a potential difference exists between the two sides of a thin glass membrane when opposite sides of the membrane are in contact with solutions containing different concentrations of H3O+. This discovery led to the development of the glass pH electrode in 1909. Other types of membranes also yield useful potentials. Kolthoff and Sanders, for example, showed in 1937 that pellets made from AgCl could be used to determine the concentration of Ag+. Electrodes based on membrane potentials are called ion-selective electrodes, and their continued development has extended potentiometry to a diverse array of analytes. 

The potential of the indicator electrode in a potentiometric electrochemical cell is proportional to the concentration of analyte. Two classes of indicator electrodes are used in potentiometry metallic electrodes, which are the subject of this section, and ion-selective electrodes, which are covered in the next section. 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

Membrane Potentials Ion-selective electrodes, such as the glass pH electrode, function by using a membrane that reacts selectively with a single ion. figure 11.10 shows a generic diagram for a potentiometric electrochemical cell equipped with an ion-selective electrode. The shorthand notation for this cell is... 

An ion-selective electrode based on a glass membrane in which the potential develops from an ion-exchange reaction on the membrane s surface. 

V Representative Examples of Glass Membrane r Ion-Selective Electrodes ... 

An ion-selective electrode based on a sparingly soluble inorganic crystalline material. 

If a mixture of an insoluble silver salt and Ag2S is used to make the membrane, then the membrane potential also responds to the concentration of the anion of the added silver salt. Thus, pellets made from a mixture of Ag2S and AgCl can serve as a Ck ion-selective electrode, with a cell potential of... 

Membranes fashioned from a mixture of Ag2S with CdS, CuS, or PbS are used to make ion-selective electrodes that respond to the concentration of Cd +, Cu +, or Pb +. In this case the cell potential is... 

The membrane potential for a E ion-selective electrode results from a difference in the solubility of LaE3 on opposite sides of the membrane, with the potential given by... 

One advantage of the E ion-selective electrode is its freedom from interference. The only significant exception is OH (kip-zon- = 0-1), which imposes a maximum pH limit for a successful analysis. 

Schematic diagram of a Ca + liquid-based ion-selective electrode.

An ion-selective electrode in which a chelating agent is incorporated into a hydrophobic membrane. 

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. 

One example of a liquid-based ion-selective electrode is that for Ca +, which uses a porous plastic membrane saturated with di-(n-decyl) phosphate (Figure 11.13). As shown in Figure 11.14, the membrane is placed at the end of a nonconducting cylindrical tube and is in contact with two reservoirs. The outer reservoir contains di-(n-decyl) phosphate in di- -octylphenylphosphonate, which soaks into the porous membrane. The inner reservoir contains a standard aqueous solution of Ca + and a Ag/AgCl reference electrode. Calcium ion-selective electrodes are also available in which the di-(n-decyl) phosphate is immobilized in a polyvinyl chloride... 

The properties of several representative liquid-based ion-selective electrodes are presented in Table 11.3. An electrode using a liquid reservoir can be stored in a dilute solution of analyte and needs no additional conditioning before use. The lifetime of an electrode with a PVC membrane, however, is proportional to its exposure to aqueous solutions. For this reason these electrodes are best stored by covering the membrane with a cap containing a small amount of wetted gauze to... 

The change in the concentration of H3O+ is monitored with a pH ion-selective electrode, for which the cell potential is given by equation 11.9. The relationship between the concentration of H3O+ and CO2 is given by rearranging the equilibrium constant expression for reaction 11.10 thus... 

An electrode that responds to the concentration of a substrate by reacting the substrate with an immobilized enzyme, producing an ion that can be monitored with an ion-selective electrode. 

Potcntiomctric Biosensors Potentiometric electrodes for the analysis of molecules of biochemical importance can be constructed in a fashion similar to that used for gas-sensing electrodes. The most common class of potentiometric biosensors are the so-called enzyme electrodes, in which an enzyme is trapped or immobilized at the surface of an ion-selective electrode. Reaction of the analyte with the enzyme produces a product whose concentration is monitored by the ion-selective electrode. Potentiometric biosensors have also been designed around other biologically active species, including antibodies, bacterial particles, tissue, and hormone receptors. 

Analyte Reaction in Inner Solution Inner Solution Ion-Selective Electrode... 

The concentration of Ca + in a sample of sea water is determined using a Ca ion-selective electrode and a one-point standard addition. A 10.00-mL sample is transferred to a 100-mL volumetric flask and diluted to volume. A 50.00-mL aliquot of sample is placed in a beaker with the Ca ion-selective electrode and a reference electrode, and the potential is measured as -0.05290 V. A 1.00-mL aliquot of a 5.00 X 10 M standard solution of Ca + is added, and a potential of -0.04417 V is measured. What is the concentration of Ca + in the sample of sea water ... 

Free Ions Versus Complexed Ions In discussing the ion-selective electrode, we noted that the membrane potential is influenced by the concentration of F , but not the concentration of HF. An analysis for fluoride, therefore, is pH-dependent. Below a pH of approximately 4, fluoride is present predominantly as HF, and a quantitative analysis for total fluoride is impossible. If the pH is increased to greater than 4, however, the equilibrium... 

Representative Method Ion-selective electrodes find application in numerous quantitative analyses, each of which has its own unique considerations. The following procedure for the analysis of fluoride in toothpaste provides an instructive example. 

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