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. 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. This device will have an input signal, a property that we wish to quantify, and an output signal, a quantity we can register. 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.

General Theory of Ion-Selective Electrodes 
When using ion-selective electrodes, a scientist wants to compare the signal of an analyte to the electrochemical potential shown by the ISE. Different types of electrodes can be used to do this, as described in the sections below. 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.

 

All ISE measurements are made with a comparison to an internal reference electrode with a known concentration of the analyte being measured. The external reference electrode is the part of the system that is exposed to the solution. The potential is measured using the following formula:

Eise includes the potential of the internal reference electrode and the ion-selective membrane potential, Em. The Eise is governed by analyte activity in the internal solution whereas Em is governed by the activity of the analyte on each side of the selective membrane. 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.

Reference Electrodes 
The most common types of reference electrodes used in analytical chemistry include the standard hydrogen electrode, the saturated calomel electrode, and the Ag/AgCl electrode.

The standard hydrogen electrode (SHE) is the primary reference electrode that has a potential of 0 volts at all temperatures and a pressure of 1 atm. 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. The wire is immersed in an acidic solution with an H2 (g) outlet pumping gaseous hydrogen into the solution. On the surface of the Pt electrode, a half-reaction occurs:

2H^+{}_{(aq)} + 2e^- <=> H_2{}_{(g)}

The cell notation is as follows with a single line denoting a phase boundary and a double line representing a salt bridge: Pt | H_2 (1 atm) | H^+(1M) ||

In fieldwork, the SHE is inconvenient, making the Saturated Calomel Electrode (SCE) the second most used reference. However, it contains mercury, making it the less preferred choice of measurement. The electrode, as shown on the right, is connected to an electrical lead. A platinum wire in a paste of Hg/Hg2Cl2 is placed in a saturated 3M KCl solution. 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. The overall half-reaction is:

Hg2Cl2 + 2e^- <-> 2Hg + 2Cl^-

The notation for the cell is: Hg | Hg2Cl2 | KCl(xM) ||

Given its toxicity, the silver chloride electrode is most frequently used even over the SCE. Within the reference electrode, an Ag/AgCl wire is immersed in a KCl-filling solution. A frit at the bottom of the reference electrode plays the role of a salt bridge. The overall half-reaction is:

AgCl + e^-<=> Ag +Cl^-

The notation for the cell is: Ag | AgCl | KCl(xM)||

Types of ion-selective membrane
There are four main types of ion-selective membrane used in ion-selective electrodes (ISEs): glass,  solid state, liquid based, and compound electrode.

Glass membranes

Glass membranes are made from an ion-exchange type of glass (silicate or chalcogenide). This type of ISE has good selectivity, but only for several single-charged cations; mainly H+, Na+, and Ag+. Chalcogenide glass also has selectivity for double-charged metal ions, such as Pb2+, and Cd2+. The glass membrane has excellent chemical durability and can work in very aggressive media. A very common example of this type of electrode is the pH glass electrode.

Crystalline membranes
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. This is the major difference between this type of electrodes and the glass membrane electrodes. The lack of internal solution reduces the potential junctions. Selectivity of crystalline membranes can be for both cation and anion of the membrane-forming substance. An example is the fluoride selective electrode based on LaF3 crystals.

Ion-exchange resin membranes
Ion-exchange resins are based on special organic polymer membranes which contain a specific ion-exchange substance (resin). This is the most widespread type of ion-specific electrode. Usage of specific resins allows preparation of selective electrodes for tens of different ions, both single-atom or multi-atom. They are also the most widespread electrodes with anionic selectivity. 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.

Enzyme electrodes
Enzyme electrodes are not true ion-selective electrodes, but are usually considered to be within the ion-selective electrode scope. Such an electrode has a "double reaction" mechanism - an enzyme reacts with a specific substance, and the product of this reaction (usually H+ or OH−) is detected by a true ion-selective electrode, such as a pH-selective electrodes. All these reactions occur inside a special membrane, which covers the true ion-selective electrode. This is why enzyme electrodes are sometimes considered ion-selective. An example is a glucose selective electrode.

Alkali metal ISE

Electrodes specific for each alkali metal ion, Li+, Na+, K+, Rb+ and Cs+ have been developed. The principle on which these electrodes are based is that the alkali metal ion is encapsulated in a molecular cavity whose size is matched to the size of the ion. For example, an electrode based on Valinomycin may be used for the determination of potassium ion concentration.

See also
Fluoride selective electrode
Ion transport number
Solvated electron
Electrochemical hydrogen compressor

References

External links
 Ion-selective electrodes
 Nico 2000 - Student Learning Guide (Beginners Guide to ISE Measurement: nico2000.net)
 ION-Selective electrodes analysers

Electrodes