Ion-selective electrode

An ion-selective electrode , also called ion-specific or ion-sensitive electrode (ISE), serves as a sensor for the concentration or, more precisely, the activity of a specific dissolved ion . For the measurement, the ion-selective electrode and a second electrode, the reference electrode, are immersed in the measurement solution and the voltage between the electrodes is measured. From this you can then determine the concentration you are looking for.

Scheme of the measurement setup with an ion-selective electrode: the voltage is measured against a reference electrode with a sensitive voltmeter.

The measured variable is therefore a concentration- dependent voltage against the reference electrode . According to the Nernst equation, this voltage depends logarithmically on the activity of the ion in question. The best-known ion-selective electrode is the pH electrode , which responds to protons (hydrogen or hydronium ions ). Ion-selective electrodes are used in many areas, e.g. B. in analytical chemistry including environmental analysis , in biochemical and biophysical research and in industrial processes. It is estimated that well over a billion analyzes are performed with ion-selective electrodes in clinical laboratories each year, making medical examinations the most important routine application of ion-selective electrodes today.

advantages

• Measurements with ion-selective electrodes are quick - they only take seconds to a few minutes - and easy, e.g. B. in comparison with titrations .
• The concentration can be measured continuously. Ion-selective electrodes have an advantage in comparison to discontinuous measuring methods whenever there is a need for complete recording and / or rapid regulation.
• Even if the measurement solution may have to be buffered, no reagents are required for the actual analysis, in contrast to titrations or too many photometric processes. This helps to keep the process relatively inexpensive.
• A system consisting of an ion-selective electrode and a reference electrode supplies a voltage as a measured variable; this is well suited for further electronic processing.
• Many measurements can be carried out directly in biological fluids such as sap, blood or urine. In most cases, no separation needs to be made before the analysis, i. H. time-consuming steps such as filtration, distillation or precipitation are not necessary. Cloudiness or coloration is usually not a problem, in contrast to photometric methods.
• The electrodes can get dirty, but otherwise show no wear and tear and can therefore, in the best case, be used for years without maintenance.
• Compared to many automatic analyzers such. B. Titrators are inexpensive ion-selective electrodes.
• Many devices are portable and can be used not only in the laboratory but also in the field.
• The analyzes can be carried out in a very wide concentration range, often e.g. B. from 1 · 10 ⁻⁶ mol / l to 0.1 mol / l, sometimes up to the saturation limit of the ion. There are selective electrodes for at least 12 ions, the detection limit of which is in the range 10 ⁻⁸ mol / l to 10 ⁻¹¹ mol / l or even less.

Important ions that can be determined with ion-selective electrodes

There are ion-selective electrodes for more than 50 ions to be determined. The commercially available ones that are available for the following ions are particularly important for practical application:

Cations

• monovalent cations M ¹⁺ : protons H ⁺ or pH, sodium Na ⁺ , potassium K ⁺ , silver Ag ⁺ , ammonium NH ₄ ⁺
• divalent cations M ²⁺ : copper Cu ²⁺ , lead Pb ²⁺ , calcium Ca ²⁺ , cadmium Cd ²⁺ , barium Ba ²⁺

Anions

• Halide : fluoride F ^- , chloride Cl ^- , bromide Br ^- , iodide I ^- ,
• Others: sulfide HS ^- / S ²⁻ , cyanide CN ^- , nitrate NO ₃ ^- , perchlorate ClO ₄ ^- , fluoroborate BF ₄ ^- , thiocyanate SCN ^-

The clinically important ions that are determined with ion-selective electrodes are H ⁺ , Na ⁺ , K ⁺ , Ca ²⁺ and Cl ^- .

Measurement setup

The central component is the ion-selective membrane, which separates an electrode contained in the electrode housing from the solution to be determined.

Scheme of the measurement setup with an ion-selective electrode, here a fluoride electrode

Membrane types

The most important part of the ion-selective electrode, the ion-selective membrane, has a composition that varies depending on the ion to be determined. The most important membrane types are crystalline or vitreous solids or composites with polymers.

Glass membranes

Glass membranes usually have excellent chemical resistance and are mainly used for pH electrodes and for sodium-selective electrodes.

Crystalline membranes

Crystalline membranes can be polycrystalline or made from a single crystal. Single crystal membranes made from lanthanum fluoride are used for most fluoride electrodes .

Polymer-based membranes

Polymer-based membranes can consist of an ion exchange resin . One example is the potassium-selective electrode, which contains valinomycin as an ion transporter ( ionophore ).

Cross sensitivity

The ideal ion-selective electrode would be ion-specific; that is, it would only respond to the ion to be determined and would not respond to other ions. In practice, ion-selective electrodes are often cross-sensitive to other ions; therefore the IUPAC recommends not to use the term “ion-specific”. For example, many pH electrodes react not only to protons, but also to high concentrations of sodium ions, especially at high pH values. Fluoride electrodes are sensitive to hydroxide ions. An ion-selective electrode is therefore rarely completely ion-specific, and possible cross-influences from other ions must therefore be taken into account for precise analyzes. A quantitative description can be made with the Nicolsky – Eisenman equation:

${\displaystyle E=E^{\circ }+{\frac {RT}{z_{\mathrm {A} }F}}\ln[a_{\mathrm {A} }+\sum _{\mathrm {B} }(K_{\mathrm {A,B} }^{\mathrm {pot} }\ a_{\mathrm {B} }^{\frac {z_{A}}{z_{B}}})]}$;

Examples of the selectivity coefficients can be found in. ${\displaystyle K_{\mathrm {A,B} }^{\mathrm {pot} }}$

Historical

The basic structure of ion-selective electrodes and the concentration dependence of the voltage was already known in the first half of the 20th century: Zygmunt Klemensiewicz had discovered the principle of the pH electrode in Fritz Haber's laboratory and Izaak Kolthoff had examined cells with silver halide membranes. The practical application of the glass electrode came after improved shapes and glasses were used, and after Arnold Orville Beckman developed a sensitive voltage measuring device . The targeted development and application of other ion-selective electrodes did not take place until the 1950s and 1960s, e.g. B. 1957 a sodium electrode was announced. In 1962, the glass manufacturer Corning founded Orion Research to develop new electrodes. In 1966 it presented a calcium and a fluoride electrode and in 1967 had nine different electrodes in its range (for Ag ⁺ , Ca ²⁺ , Cu ²⁺ , F ^- , Br ^- , I ^- , ClO ₄ ^- , NO ₃ ^- and S ₂ ^- ).

Individual evidence

1. ↑ Eric Bakker, Philippe Bühlmann, Ernö Pretsch: Carrier-Based Ion-Selective Electrodes and Bulk Optodes. 1. General Characteristics . In: Chemical Reviews . tape 97 , no. 8 , December 1997, p. 3083-3132 , doi : 10.1021 / cr940394a .
2. ↑ ^{a b} Eric Bakker, Dermot Diamond, Andrzej Lewenstam, Ernö Pretsch: Ion sensors: current limits and new trends . In: Analytica Chimica Acta . tape 393 , no. 1-3 , June 1999, pp. 11-18 , doi : 10.1016 / S0003-2670 (99) 00056-2 .
3. ^ ^{A b} Ernö Pretsch: The new wave of ion-selective electrodes . In: Trends in Analytical Chemistry . tape 26 , no. 1 , January 2007, p. 46–51 , doi : 10.1016 / j.trac.2006.10.006 , PMC 2358928 (free full text).
4. ↑ Entry on ion-selective electrode (ISE) . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.I03244 Version: 2.3.3.
5. ↑ ^{a b} Yoshio Umezawa, Philippe Bühlmann, Kayoko Umezawa, Koji Tohda, Shigeru Amemiya: Potentiometric Selectivity Coefficients of Ion-Selective Electrodes - Part I. Inorganic Cations . In: Pure and Applied Chemistry . tape 72 , no. January 10 , 2000, p. 1851-2082 , doi : 10.1351 / pac200072101851 .
6. ↑ Hendrik Rohler: 2. Potentiometric sensor technology, 2.1 Introduction to potentiometry, 2.1.2 The Nikolski equation. In: Conception and use of ion-selective borehole measuring probes. ETH Zurich, dissertation; Kassel University Press GmbH, 1997, accessed March 18, 2015 . 
7. ↑ Yoshio Umezawa, Kayoko Umezawa, Philippe Bühlmann, Naoko Hamada, Hiroshi Aoki, Jun Nakanishi, Moritoshi Sato, Kang Ping Xiao, Yukiko Nishimura: Potentiometric selectivity selective coefficients of ion-selective electrodes. Part II. Inorganic anions (IUPAC Technical Report) . In: Pure and Applied Chemistry . tape 74 , no. 6 , June 2002, p. 923-994 , doi : 10.1351 / pac200274060923 .
8. ↑ Zygmunt Aleksander Klemensiewicz, Fritz Haber: About electrical phase boundary forces . In: Journal of physical chemistry . tape 67 , 1909, pp. 385-431 .
9. ^ Barbara Marczewska, Krzysztof Marczewski: First Glass Electrode and its Creators F. Haber and Z. Klemensiewicz - On 100th Anniversary . In: Journal of physical chemistry . tape 224 , 2010, p. 795-799 , doi : 10.1524 / zpch.2010.5505 .
10. ^ Izaak M. Kolthoff, HL Sanders: Electric Potentials at Crystal Surfaces, and at Silver Halide Surfaces in Particular . In: Journal of the American Chemical Society (JACS) . tape 59 , no. 2 , 1937, pp. 416-420 , doi : 10.1021 / ja01281a059 .
11. ^ DA MacInnes, Malcolm Dole: The Behavior of Glass Electrodes of Different Compositions . In: Journal of the American Chemical Society (JACS) . tape 52 , no. 1 , 1930, p. 29-36 , doi : 10.1021 / ja01364a005 .
12. ↑ Patent US2058761 : Apparatus for Testing Acidity. Filed October 12, 1934 , published October 27, 1936 , Applicant: National Technical Laboratories, Inventor: Arnold Orville Beckman, Henry E. Fracker. ‌
13. ↑ George Eisenman, Donald O. Rudin, James U. Casby: Glass Electrode for Measuring Sodium Ion . In: Science . tape 126 , no. 3278 , October 1957, p. 831-834 , doi : 10.1126 / science.126.3278.831 .
14. ↑ Martin S. Frant: Historical perspective. History of the early commercialization of ion-selective electrodes . In: Analyst . tape 119 , 1994, pp. 2293-2301 , doi : 10.1039 / AN9941902293 .
15. ↑ Martin S. Frant, James W. Ross Jr .: Electrode for Sensing Fluoride Ion Activity in Solution . In: Science . tape 154 , no. 3756 , December 23, 1966, pp. 1553–1555 , doi : 10.1126 / science.154.3756.1553 .