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A glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion. The most common application of ion-selective glass electrodes is for the measurement of pH. The pH electrode is an example of a glass electrode that is sensitive to hydrogen ions. Glass electrodes play an important part in the instrumentation for chemical analysis, and physicochemical studies. The voltage of the glass electrode, relative to some reference value, is sensitive to changes in the activity of a certain type of ions.
The first studies of glass electrodes (GE) found different sensitivities of different glasses to change the medium's acidity (pH), due to the effects of the alkali metal ions.
In 1906, M. Cremer, the father of Erika Cremer, determined that the electric potential that arises between parts of the fluid, located on opposite sides of the glass membrane is proportional to the concentration of acid (hydrogen ion concentration). [1]
In 1909, S. P. L. Sørensen introduced the concept of pH, and in the same year F. Haber and Z. Klemensiewicz reported results of their research on the glass electrode in The Society of Chemistry in Karlsruhe. [2] [3] In 1922, W. S. Hughes showed that the alkali-silicate glass electrodes are similar to hydrogen electrodes, reversible concerning H+. [4]
In 1925, P. M. Tookey Kerridge developed the first glass electrode for analysis of blood samples and highlighted some of the practical problems with the equipment such as the high resistance of glass (50–150 MΩ). [5] During her PhD, Kerridge developed a glass electrode aimed to measure small volume of solution. [6] Her clever and careful design was a pioneering work in the making of glass electrodes.
Glass electrodes are commonly used for pH measurements. There are also specialized ion-sensitive glass electrodes used for the determination of the concentration of lithium, sodium, ammonium, and other ions.
Glass electrodes find a wide diversity of uses in a large range of applications including research labs, control of industrial processes, analysis of foods and cosmetics, monitoring of environmental pollution, or soil acidity measurements... . Micro-electrodes are specifically designed for pH measurements on very small volumes of fluid, or for direct measurements in geochemical micro-environments, or in biochemical studies such as for determining the electrical potential of cell membrane.
Heavy duty electrodes withstanding several tens of bar of hydraulic pressure also allow measurements in water wells in deep aquifers, or to directly determine in situ the pH of pore water in deep clay formations. [7] For long-term in situ measurements, it is critical to minimize the KCl leak from the reference electrode compartment (Ag / AgCl / KCl 3 M), and to use glycerol-free electrodes [8] to avoid fuelling microbial growth, and to prevent unexpected but severe perturbations related to bacterial activity (pH decrease due to sulfate-reducing bacteria, or even methanogen bacteria). [9] [7] [8]
All commercial electrodes respond to single-charged ions, such as H+, Na+, Ag+. The most common glass electrode is the pH-electrode. Only a few chalcogenide glass electrodes are presently known to be sensitive to double-charged ions, such as Pb2+, Cd2+, and some other divalent cations.[ citation needed ]
There are two main types of glass-forming systems:[ citation needed ]
Because of the ion-exchange nature of the glass membrane, it is possible for some other ions to concurrently interact with ion-exchange sites of the glass, and distort the linear dependence of the measured electrode potential on pH or other electrode functions. In some cases, it is possible to change the electrode function from one ion to another. For example, some silicate pPNA[ clarification needed ] electrodes can be changed to pAg function by soaking in a silver salt solution.
Interference effects are commonly described by the semi-empirical Nicolsky-Shultz-Eisenman equation (also known as Nikolsky-Shultz-Eisenman equation), [10] [11] an extension to the Nernst equation. It is given by:
where E is the electromotive force (emf), E0 the standard electrode potential, z the ionic valency including the sign, a the activity, i the ion of interest, j the interfering ions and kij is the selectivity coefficient quantifying the ion-exchange equilibrium between the ions i and j. The smaller the selectivity coefficient, the less is the interference by j.
To see the interfering effect of Na+ to a pH-electrode:
The pH range at constant concentration can be divided into 3 parts:
where F is Faraday's constant (see Nernst equation). [12]
The effect is usually noticeable at pH > 12, and at concentrations of lithium or sodium ions of 0.1 mol/L or more. Potassium ions usually cause less error than sodium ions.
Special electrodes exist for working in extreme pH ranges.
A typical modern pH probe is a combination electrode, which combines both the glass and reference electrodes into one body. The combination electrode consists of the following parts (see the drawing):
The bottom of a pH electrode balloons out into a round thin glass bulb. The pH electrode is best thought of as a tube within a tube. The inner tube contains an unchanging 1×10−7 mol/L HCl solution. Also inside the inner tube is the cathode terminus of the reference probe. The anodic terminus wraps itself around the outside of the inner tube and ends with the same sort of reference probe as was on the inside of the inner tube. It is filled with a reference solution of KCl and has contact with the solution on the outside of the pH probe by way of a porous plug that serves as a salt bridge.
This section describes the functioning of two distinct types of electrodes as one unit which combines both the glass electrode and the reference electrode into one body. It deserves some explanation.
This device is essentially a galvanic cell that can be schematically represented as:
The double "pipe symbols" (||) indicate diffusive barriers – the glass membrane and the ceramic junction. The barriers prevent (glass membrane), or slow down (ceramic junction), the mixing of the different solutions.
In this schematic representation of the galvanic cell, one will note the symmetry between the left and the right members as seen from the center of the row occupied by the "Test Solution" (the solution whose pH must be measured). In other words, the glass membrane and the ceramic junction occupy both the same relative places in each electrode. By using the same electrodes on the left and right, any potentials generated at the interfaces cancel each other (in principle), resulting in the system voltage being dependent only on the interaction of the glass membrane and the test solution.
The measuring part of the electrode, the glass bulb on the bottom, is coated both inside and out with a ~10 nm layer of a hydrated gel. These two layers are separated by a layer of dry glass. The silica glass structure (that is, the conformation of its atomic structure) is shaped so that it allows Na+ ions some mobility. The metal cations (Na+) in the hydrated gel diffuse out of the glass and into solution while H+ from solution can diffuse into the hydrated gel. It is the hydrated gel which makes the pH electrode an ion-selective electrode.
H+ does not cross through the glass membrane of the pH electrode, it is the Na+ which crosses and leads to a change in free energy. When an ion diffuses from a region of activity to another region of activity, there is a free energy change and this is what the pH meter actually measures. The hydrated gel membrane is connected by Na+ transport and thus the concentration of H+ on the outside of the membrane is 'relayed' to the inside of the membrane by Na+.
All glass pH electrodes have extremely high electric resistance from 50 to 500 MΩ. Therefore, the glass electrode can be used only with a high input-impedance measuring device like a pH meter, or, more generically, a high input-impedance voltmeter which is called an electrometer.
The glass electrode has some inherent limitations due to the nature of its construction. Acid and alkaline errors are discussed above. An important limitation results from the existence of asymmetry potentials that are present at glass/liquid interfaces. [13] The existence of these phenomena means that glass electrodes must always be calibrated before use; a common method of calibration involves the use of standard buffer solutions. Also, there is a slow deterioration due to diffusion into and out of the internal solution. These effects are masked when the electrode is calibrated against buffer solutions but deviations from ideal response are easily observed by means of a Gran plot. Typically, the slope of the electrode response decreases over a period of months.
Between measurements any glass and membrane electrodes should be kept in a solution of its own ion. It is necessary to prevent the glass membrane from drying out because the performance is dependent on the existence of a hydrated layer, which forms slowly.
In chemistry, pH, also referred to as acidity or basicity, historically denotes "potential of hydrogen". It is a logarithmic scale used to specify the acidity or basicity of aqueous solutions. Acidic solutions are measured to have lower pH values than basic or alkaline solutions.
An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble salts, acids, and bases, dissolved in a polar solvent like water. Upon dissolving, the substance separates into cations and anions, which disperse uniformly throughout the solvent. Solid-state electrolytes also exist. In medicine and sometimes in chemistry, the term electrolyte refers to the substance that is dissolved.
In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction from the standard electrode potential, absolute temperature, the number of electrons involved in the redox reaction, and activities of the chemical species undergoing reduction and oxidation respectively. It was named after Walther Nernst, a German physical chemist who formulated the equation.
A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions, indicating its acidity or alkalinity expressed as pH. The pH meter measures the difference in electrical potential between a pH electrode and a reference electrode, and so the pH meter is sometimes referred to as a "potentiometric pH meter". The difference in electrical potential relates to the acidity or pH of the solution. Testing of pH via pH meters (pH-metry) is used in many applications ranging from laboratory experimentation to quality control.
Electrophysiology is the branch of physiology that studies the electrical properties of biological cells and tissues. It involves measurements of voltage changes or electric current or manipulations on a wide variety of scales from single ion channel proteins to whole organs like the heart. In neuroscience, it includes measurements of the electrical activity of neurons, and, in particular, action potential activity. Recordings of large-scale electric signals from the nervous system, such as electroencephalography, may also be referred to as electrophysiological recordings. They are useful for electrodiagnosis and monitoring.
In electrochemistry, the standard hydrogen electrode, is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be 4.44 ± 0.02 V at 25 °C, but to form a basis for comparison with all other electrochemical reactions, hydrogen's standard electrode potential is declared to be zero volts at any temperature. Potentials of all other electrodes are compared with that of the standard hydrogen electrode at the same temperature.
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 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.
Nafion is a brand name for a sulfonated tetrafluoroethylene based fluoropolymer-copolymer synthesized in 1962 by Dr. Donald J. Connolly at the DuPont Experimental Station in Wilmington Delaware. Additional work on the polymer family was performed in the late 1960s by Dr. Walther Grot of DuPont. Nafion is a brand of the Chemours company. It is the first of a class of synthetic polymers with ionic properties that are called ionomers. Nafion's unique ionic properties are a result of incorporating perfluorovinyl ether groups terminated with sulfonate groups onto a tetrafluoroethylene (PTFE) backbone. Nafion has received a considerable amount of attention as a proton conductor for proton exchange membrane (PEM) fuel cells because of its excellent chemical and mechanical stability in the harsh conditions of this application.
A reference electrode is an electrode that has a stable and well-known electrode potential. The overall chemical reaction taking place in a cell is made up of two independent half-reactions, which describe chemical changes at the two electrodes. To focus on the reaction at the working electrode, the reference electrode is standardized with constant concentrations of each participant of the redox reaction.
A silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For environmental reasons it has widely replaced the saturated calomel electrode. For example, it is usually the internal reference electrode in pH meters and it is often used as reference in reduction potential measurements. As an example of the latter, the silver chloride electrode is the most commonly used reference electrode for testing cathodic protection corrosion control systems in sea water environments.
An ion-sensitive field-effect transistor (ISFET) is a field-effect transistor used for measuring ion concentrations in solution; when the ion concentration (such as H+, see pH scale) changes, the current through the transistor will change accordingly. Here, the solution is used as the gate electrode. A voltage between substrate and oxide surfaces arises due to an ion sheath. It is a special type of MOSFET (metal–oxide–semiconductor field-effect transistor), and shares the same basic structure, but with the metal gate replaced by an ion-sensitive membrane, electrolyte solution and reference electrode. Invented in 1970, the ISFET was the first biosensor FET (BioFET).
Redox potential is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respectively. Redox potential is expressed in volts (V). Each species has its own intrinsic redox potential; for example, the more positive the reduction potential, the greater the species' affinity for electrons and tendency to be reduced.
The sucrose gap technique is used to create a conduction block in nerve or muscle fibers. A high concentration of sucrose is applied to the extracellular space, which prevents the correct opening and closing of sodium and potassium channels, increasing resistance between two groups of cells. It was originally developed by Robert Stämpfli for recording action potentials in nerve fibers, and is particularly useful for measuring irreversible or highly variable pharmacological modifications of channel properties since untreated regions of membrane can be pulled into the node between the sucrose regions.
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis cell. The cell consists of a feed (dilute) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes. In almost all practical electrodialysis processes, multiple electrodialysis cells are arranged into a configuration called an electrodialysis stack, with alternating anion and cation-exchange membranes forming the multiple electrodialysis cells. Electrodialysis processes are different from distillation techniques and other membrane based processes in that dissolved species are moved away from the feed stream, whereas other processes move away the water from the remaining substances. Because the quantity of dissolved species in the feed stream is far less than that of the fluid, electrodialysis offers the practical advantage of much higher feed recovery in many applications.
Electroanalytical methods are a class of techniques in analytical chemistry which study an analyte by measuring the potential (volts) and/or current (amperes) in an electrochemical cell containing the analyte. These methods can be broken down into several categories depending on which aspects of the cell are controlled and which are measured. The three main categories are potentiometry, amperometry, coulometry.
The alkali–silica reaction (ASR), also commonly known as concrete cancer, is a deleterious internal swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive amorphous silica found in many common aggregates, given sufficient moisture.
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.
Jesse Francis McClendon was an American chemist, zoologist, and physiologist known for the first pH measurement of human stomach in situ.
A metal ion in aqueous solution or aqua ion is a cation, dissolved in water, of chemical formula [M(H2O)n]z+. The solvation number, n, determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for most elements in periods 3 and 4 of the periodic table. Lanthanide and actinide aqua ions have higher solvation numbers (often 8 to 9), with the highest known being 11 for Ac3+. The strength of the bonds between the metal ion and water molecules in the primary solvation shell increases with the electrical charge, z, on the metal ion and decreases as its ionic radius, r, increases. Aqua ions are subject to hydrolysis. The logarithm of the first hydrolysis constant is proportional to z2/r for most aqua ions.
A metal-ion buffer provides a controlled source of free metal ions in a manner similar to the regulation of hydrogen ion concentration by a pH buffer A metal-ion buffer solution contains the free (hydrated) metal ion along with a complex compound formed by the association of the ion with a ligand in excess. The concentration of free metal ion depends on the total concentration of each component as well as on the stability constant of the complex. If the ligand can undergo protonation, the concentration of the free metal ion depends also on solution pH.