This article provides an overview of ion selective electrode analysis and its principles, with an analysis of applications, advantages, and disadvantages.
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Electroanalytical techniques use electrical quantities such as charge, current, or potential and measure their relations with chemical parameters.
This field involves studying chemical reaction changes with the passing of electric current and the generation of electrical energy through chemical reactions. The potential across a membrane provides information about the sample composition, termed potentiometry. The electrodes have been historically used to detect ionic species like potassium, hydrogen, fluoride, and calcium. Electroanalysis can only be carried out when the current is conducted by the medium between the two electrodes.
Ion Selective Electrode Analysis and its Principles
Ion selective electrode (ISE) analysis is an electroanalytical technique that uses an electrochemical cell consisting of two electrodes and a sample solution (electrolyte). The electrodes contain an ion permissible membrane that is selective of the ion that passes through it. This membrane separates the sample from the electrode internals. An internal reference electrode present within the ISE is made of silver wire with silver chloride coating and is fixed in a concentrated potassium chloride solution saturated with silver chloride.
A reference electrode like the ISE is also present in the cell, except there is no ion that is required to be measured in the inner electrolyte, and in place of the selective membrane is a porous frit, which forms the liquid junction with the external sample solution. A milli-voltmeter connects the two electrodes, which are immersed in the sample solution and the current is measured. The measured potential is directly proportional to the logarithm (log) of ion concentration.
The selectivity of the electrode is determined by the type of membrane used. Other criteria when selecting the membrane are low electric conductivity and low solubility. There are three types of membranes. Glass membranes are composed of silicate glass and determine pH which can be made sensitive to cations like sodium. Solid-state membranes or crystalline membranes are made up of an insoluble inorganic salt, which creates a potential at the membrane through ion exchange. Mixed crystal membranes can be used to detect Cl- and single crystal LaF3 doped with europium is used to detect F-.
A polymer membrane is seen as an alternative to wet liquid membranes. It is made up of polyvinyl chloride (PVC), a plasticizer, and an exchanger. These electrodes are highly selective and are used to detect ions like K+, Cl-, NO3-, and Ca2+. ISE can be interfered with by responding to other ions. While no membrane is 100% selective of only one ion, the fluoride electrode comes close.
Applications of Ion Selective Electrodes
ISE is used in diagnosing cystic fibrosis by detecting elevated levels of Na+, K+, and Cl+ in sweat. Non-ionic surfactants (NIS) can be determined by liquid membrane electrodes. However, NIS can increase interference by alkali metal (M+) in alkaline earth by partitioning into the membrane and providing sites for M+ binding.
Microelectrodes are also utilized to detect antioxidants in engine oil, which is advantageous since the low current permits voltammetry to be carried out in greatly resistive media. ISE also has its uses in monitoring pollution in natural waters (CN-, F-, S-, Cl-), and food processing (NO3-, NO2- in meat preservatives).
Advantages and Disadvantages
The primary advantage of ISE is that it is a non-destructive technique and can be carried out without the consumption of the analyte. The analysis is also non-contaminating and provides results over a short response time. This enables the monitoring of ion activity with time. Moreover, the response time is in seconds to minutes, making this method useful for industrial applications. The method is not affected by the turbidity or color of the samples and the setup is easy to operate while being inexpensive.
ISE has its own drawbacks. For one, the selective membrane may allow more than one ion since no membrane permits only one ion to pass. Consequently, the passage of unwanted ions affects the measured potential. Since ISE is dependent on its membrane, its use may be limited to one ion only. Furthermore, ion detection can be inaccurate if the solution is concentrated because the mobility of ions is decreased due to the inter-ionic interactions, which lower the concentration near the membrane.
In a paper published in Chemical Society Reviews, the team reviewed pioneering research on functional materials and technologies for constructing solid-contact ISEs (SC-ISEs). With recent developments in material science and processing technology, SC-ISEs made of high-performance functional materials and creative structures have shown great prospects for portable ion detection.
In particular, SC-ISEs have been made more efficient by introducing nanomaterials as ion-to-electron transducers. The researchers also noted that the field has begun to evolve to integrated sensing systems from conventional potentiometric ion sensing, thus enabling broader application scenarios.
Overall, the development of ISEs has paved way for a wide range of applications in determining ions in different mediums. The disadvantages of this method can be quick and easy to eliminate. Although most ISE applications to date have been carried out on aqueous media, numerous applications have been developed with nonaqueous solutions.
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References and Further Reading
Pavan M. V. Raja, Andrew R. Barron, 2021, Ion Selective Electrode Analysis, https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Physical_Methods_in_Chemistry_and_Nano_Science_
Amra Bratovcic, Amra Odobasic, Sead Catic, 2009, The Advantages of the Use of IonSelective Potentiometry in Relation to UV/VIS Spectroscopy, https://citeseerx.ist.psu.edu/
Robert B. Fischer, Ion-Selective Electrodes, https://pubs.acs.org/doi/pdf/10.1021/ed051p387
Vishal Arun Naik, 2016, PRINCIPLE AND APPLICATIONS OF ION SELECTIVE ELECTRODES-AN OVERVIEW, International Journal Of Applied Research In Science And Engineering, https://ijarse.org/images/scripts/201637.pdf
Yuzhou Shao, Yibin Ying, Jianfeng Ping, 2020, Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends, Chemical Society Reviews, https://pubs.rsc.org/en/content/articlelanding/2020/cs/c9cs00587k