Role of Ion-Selective Electrodes in Potentiometry

Potentiometry, or potentiometric titration, characterises and measures the potential of an analyte. The importance of potentiometry lies in describing an acid without using an indicator. In this technique, two electrodes measure the potential of the analyte: an indicator electrode and another reference electrode. Another type of electrode known as an ion-selective electrode (ISE) or specific ion electrode (SIE) works as a sensor during potentiometric titrations. A potentiometric ion-selective electrode is a simple chemical sensor.

Definition of A Potentiometric Ion-Selective Electrode

Ion-selective electrodes are sensors used in potentiometry. It uses a selective membrane as an electrode to allow the movement of only particular ions. It allows only a specific ion to cross the membrane barrier, and the ion can be a cation or an anion. Here a reference solution is separated by an analyte solution through a membrane. 

The internal reference electrode is dipped in the reference solution and connected to the external reference electrode dipped in the analyte solution. No redox reaction occurs here. The transport of ions across the membrane leads to potential generation.

The Function of Ion-Selective Electrode in Potentiometry

• Ion-selective electrodes are used as sensors in potentiometry to measure the potential of a solution when no current is flowing through the circuit.
•  These electrodes are highly selective due to the presence of a selective membrane. 
• This membrane allows only specific ions to travel across it.
•  The primary function of an ion-selective electrode in potentiometry is to analyse the ions in a solution and determine the potential of that solution through the ions present in it. 

Types of Membrane

The membrane used in the ion-selective electrode can be of 4 types as follows.

1. Glass membrane electrodes- This type of electrode forms from silicate glass sensitive to cations. The most common example of this type of electrode is the pH metre. We can also use chalcogenide glass for cations carrying a double positive charge.
2. Crystalline membrane electrodes- This type of electrode comes from inorganic salts. We can use it both for anions and cations. Here ions that can form a crystalline structure are the ones that interact with the electrode and can lead to any changes in the reading.
3. Liquid membrane electrodes- This electrode type is also known as ion-exchange electrodes or ionophores. Here the ion-exchanger or the ionophore is responsible for the penetration of ions into the membrane.
4. Polymer membrane electrodes- This electrode type comes from polyvinyl chloride, plasticiser, and an exchanger. It can readily work as an alternative to the wet liquid membrane electrodes. They are highly selective.

Why are Electrodes Important in Potentiometry?

There are two types of electrodes used in potentiometry, and each has its function in the process.

• Indicator electrode- this electrode is responsible for determining the potential of a sample solution by the concentration of ions present in the solution.
• Reference electrode- this electrode is responsible for providing a stable potential so the indicator electrode can analyse the change in potential due to the ions.

Applications of Ion-selective Electrodes (ISE)

• Ion-selective electrodes are used widely in clinical laboratories to analyse the samples and determine the specific ions’ activity. For example, indirect ISE is used to measure the total plasma sample that is highly diluted.
• They are used in pH meters to calculate the pH of a solution based on the presence of H+ ions in the solution.
• We can also use it to analyse different ions like chloride ions, calcium ions, carbonate ions, and many more such ions.
• Ion-selective electrodes can help analyse drugs in the pharmaceutical industry.
• We can use it to measure the amount of potassium present in food materials.
• A sodium ion-selective electrode can measure the salt content of food material.

Conclusion

Potentiometry, also regarded as potentiometric titration, determines a solution’s concentration in chemistry. A particular type of electrode used is the ion-selective electrode. The definition of the ion-selective electrode is very well understood from its name only, an electrode that is selective towards ions. The important thing about this type of electrode is that apart from being sensitive toward ions, it functions in the absence or minimal current. The electrode is selective due to a particular membrane that forms these types of electrodes. The membrane can be further divided into different classes to respond to different kinds of ions. The primary application of ion-selective electrodes is in the clinical and pharmaceutical industries.