An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a

transducer A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and contr ...

(or

sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

) that converts the activity of a specific

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...

dissolved in a solution into an electrical

potential Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple re ...

. The voltage is theoretically dependent on the

logarithm In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number to the base is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 o ...

of the ionic activity, according to the

Nernst equation In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction (half-cell or full cell reaction) from the standard electrode potential, absolute tempera ...

. Ion-selective electrodes are used in analytical chemistry and

biochemical Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...

biophysical Biophysics is an interdisciplinary science that applies approaches and methods traditionally used in physics to study biological phenomena. Biophysics covers all scales of biological organization, from molecular to organismic and populations. ...

research, where measurements of

ic concentration in an

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be ...

are required.

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 Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) o ...

membranes are made from an ion-exchange type of glass ( silicate or

chalcogenide : 220px, Cadmium sulfide, a prototypical metal chalcogenide, is used as a yellow pigment. A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements ...

). This type of ISE has good

selectivity Selectivity may refer to: Psychology and behaviour * Choice, making a selection among options * Discrimination, the ability to recognize differences * Socioemotional selectivity theory, in social psychology Engineering * Selectivity (radio), a m ...

, but only for several single-charged

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s; mainly H⁺, Na⁺, and Ag⁺.

Chalcogenide glass Chalcogenide glass (pronounced hard ''ch'' as in ''chemistry'') is a glass containing one or more chalcogens (sulfur, selenium and tellurium, but excluding oxygen). Such glasses are covalently bonded materials and may be classified as covalent net ...

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 An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...

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

and

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

of the membrane-forming substance. An example is the

fluoride selective electrode A fluoride selective electrode is a type of ion selective electrode sensitive to the concentration of the fluoride ion. A common example is the lanthanum fluoride electrode. Lanthanum fluoride electrode In the lanthanum fluoride electrode, the se ...

based on LaF₃ crystals.

Ion-exchange resin membranes

Ion-exchange resin An ion-exchange resin or ion-exchange polymer is a resin or polymer that acts as a medium for ion exchange. It is an insoluble matrix (or support structure) normally in the form of small (0.25–1.43 mm radius) microbeads, usually white or ye ...

s are based on special organic

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

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 Potassium selective electrodes are a type of ion selective electrode used in biochemical and biophysical research, where measurements of potassium concentration in an aqueous solution are required, usually on a real time basis. These electrodes are ...

, based on

valinomycin Valinomycin is a naturally occurring dodecadepsipeptide used in the transport of potassium and as an antibiotic. Valinomycin is obtained from the cells of several ''Streptomyces'' species, '' S. fulvissimus'' being a notable one. It is a member ...

as an ion-exchange agent.

Enzyme electrodes

Enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...

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 Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...

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 Valinomycin is a naturally occurring dodecadepsipeptide used in the transport of potassium and as an antibiotic. Valinomycin is obtained from the cells of several ''Streptomyces'' species, '' S. fulvissimus'' being a notable one. It is a member ...

may be used for the determination of potassium ion concentration.

References

External links

Ion-selective electrodes

(Beginners Guide to ISE Measurement: nico2000.net) {{DEFAULTSORT:Ion Selective Electrode Electrodes