Ion-selective electrode
An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a simple membrane-based potentiometric device which measures the activity of ions in solution.
[1] It is a transducer (or sensor) that converts the change in the concentration of a specific ion dissolved in a solution into an electrical potential.
ISE is a type of sensor device that senses changes in signal based on the surrounding environment through time.
In this case, ion selective electrode are electrochemical sensors that give potentiometric signals.
The voltage is theoretically dependent on the logarithm of the ionic activity, according to the Nernst equation.
Analysis with ISEs expands throughout a range of technological fields such as biology, chemistry, environmental science and other industrial workplaces like agriculture.
Ion-selective electrodes are used in analytical chemistry and biochemical/biophysical research, where measurements of ionic concentration in an aqueous solution are required.
[2] When using ion-selective electrodes, a scientist wants to compare the signal of an analyte to the electrochemical potential shown by the ISE.
As shown in the general schematic, an ion-selective membrane (consisting of glass, crystalline, liquid, or polymers) selectively allows specific types of ions to travel through, or in other words, is selectively permeable.
The external reference electrode is the part of the system that is exposed to the solution.
Furthermore, the Eref or external reference portion of the cell is dependent on the half-reaction of the electrode and the liquid junction potential Ej.
The figure on the left highlights the platinum (Pt) wire electrode which is not a part of the reaction (it’s a catalyst) and can serve as either the anode or cathode.
In fieldwork, the SHE is inconvenient, making the Saturated Calomel Electrode (SCE) the second most used reference.
A small hole of asbestos wire is located on the bottom of the internal electrode.
A ceramic frit, acting as the salt bridge, is located on the bottom of the reference electrode.
A frit at the bottom of the reference electrode plays the role of a salt bridge.
This type of ISE has good selectivity, but only for several single-charged cations; mainly H+, Na+, and Ag+.
The glass membrane has excellent chemical durability and can work in very aggressive media.
Crystalline membranes are made from mono- or polycrystallites of a single substance.
They have good selectivity, because only ions which can introduce themselves into the crystal structure can interfere with the electrode response.
Selectivity of crystalline membranes can be for both cation and anion of the membrane-forming substance.
Usage of specific resins allows preparation of selective electrodes for tens of different ions, both single-atom or multi-atom.
However, such electrodes have low chemical and physical durability as well as "survival time".
An example is the potassium selective electrode, based on valinomycin as an ion-exchange agent.
All these reactions occur inside a special membrane, which covers the true ion-selective electrode.
Electrodes specific for each alkali metal ion, Li+, Na+, K+, Rb+ and Cs+ have been developed.
For example, an electrode based on Valinomycin may be used for the determination of potassium ion concentration.
