The melting point of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium; the melting point of a substance depends on pressure and is specified at a standard pressure such as 1 atmosphere or 100 kPa. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point or crystallization point; because of the ability of some substances to supercool, the freezing point is not considered as a characteristic property of a substance. When the "characteristic freezing point" of a substance is determined, in fact the actual methodology is always "the principle of observing the disappearance rather than the formation of ice", that is, the melting point. For most substances and freezing points are equal. For example, the melting point and freezing point of mercury is 234.32 kelvins. However, certain substances possess differing solid-liquid transition temperatures.
For example, agar melts at 85 °C and solidifies from 31 °C. The melting point of ice at 1 atmosphere of pressure is close to 0 °C. In the presence of nucleating substances, the freezing point of water is not always the same as the melting point. In the absence of nucleators water can exist as a supercooled liquid down to −48.3 °C before freezing. The chemical element with the highest melting point is tungsten, at 3,414 °C; the often-cited carbon does not melt at ambient pressure but sublimes at about 3,726.85 °C. Tantalum hafnium carbide is a refractory compound with a high melting point of 4215 K. At the other end of the scale, helium does not freeze at all at normal pressure at temperatures arbitrarily close to absolute zero. Many laboratory techniques exist for the determination of melting points. A Kofler bench is a metal strip with a temperature gradient. Any substance can be placed on a section of the strip, revealing its thermal behaviour at the temperature at that point. Differential scanning calorimetry gives information on melting point together with its enthalpy of fusion.
A basic melting point apparatus for the analysis of crystalline solids consists of an oil bath with a transparent window and a simple magnifier. The several grains of a solid are placed in a thin glass tube and immersed in the oil bath; the oil bath is heated and with the aid of the magnifier melting of the individual crystals at a certain temperature can be observed. In large/small devices, the sample is placed in a heating block, optical detection is automated; the measurement can be made continuously with an operating process. For instance, oil refineries measure the freeze point of diesel fuel online, meaning that the sample is taken from the process and measured automatically; this allows for more frequent measurements as the sample does not have to be manually collected and taken to a remote laboratory. For refractory materials the high melting point may be determined by heating the material in a black body furnace and measuring the black-body temperature with an optical pyrometer. For the highest melting materials, this may require extrapolation by several hundred degrees.
The spectral radiance from an incandescent body is known to be a function of its temperature. An optical pyrometer matches the radiance of a body under study to the radiance of a source, calibrated as a function of temperature. In this way, the measurement of the absolute magnitude of the intensity of radiation is unnecessary. However, known temperatures must be used to determine the calibration of the pyrometer. For temperatures above the calibration range of the source, an extrapolation technique must be employed; this extrapolation is accomplished by using Planck's law of radiation. The constants in this equation are not known with sufficient accuracy, causing errors in the extrapolation to become larger at higher temperatures. However, standard techniques have been developed to perform this extrapolation. Consider the case of using gold as the source. In this technique, the current through the filament of the pyrometer is adjusted until the light intensity of the filament matches that of a black-body at the melting point of gold.
This establishes the primary calibration temperature and can be expressed in terms of current through the pyrometer lamp. With the same current setting, the pyrometer is sighted on another black-body at a higher temperature. An absorbing medium of known transmission is inserted between this black-body; the temperature of the black-body is adjusted until a match exists between its intensity and that of the pyrometer filament. The true higher temperature of the black-body is determined from Planck's Law; the absorbing medium is removed and the current through the filament is adjusted to match the filament intensity to that of the black-body. This establishes a second calibration point for the pyrometer; this step is repeated to carry the calibration to hi
Théophile-Jules Pelouze was a French chemist. He was born at Valognes, died in Paris, his father, Edmond Pelouze, was the author of several technical handbooks. The son, after spending some time in a pharmacy at La Fère acted as laboratory assistant to Gay-Lussac and Jean Louis Lassaigne at Paris from 1827 to 1829. In 1830 he was appointed associate professor of chemistry at Lille, but returning to Paris next year became repetiteur, subsequently professor at the École polytechnique, he held the chair of chemistry at the Collège de France, in 1833 became assayer to the mint and in 1848 president of the Commission des Monnaies. He resigned all his public positions in 1852. After the coup d'état in 1851 he resigned his appointments, but continued to conduct an experimental laboratory-school he had started in 1846. There he worked with other nitrosulphates, his student Ascanio Sobrero was the discoverer of nitroglycerin, another student, Alfred Nobel, was to take that discovery on to great heights in the form of commercial explosives including dynamite.
He was a major inspiration for both students. Though Pelouze made no discovery of outstanding importance, he was a busy investigator, his work including researches on salicin, on beetroot sugar, on various organic acids, on oenanthic ether, on the nitrosulphates, on guncotton, on the composition and manufacture of glass, he carried out determinations of the atomic weights of several elements, with E. Fremy, published Traité de chimie générale, his son Eugène-Philippe Pelouze married Marguerite Wilson, a rich heiress, in 1857. The couple purchased the Château de Chenonceau in 1864. Marguerite continued to live there until 1888, when she was forced to sell, his name is one of the 72 names inscribed on the Eiffel Tower. This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed.. "Pelouze, Théophile Jules". Encyclopædia Britannica. 21. Cambridge University Press. Works by Théophile-Jules Pelouze at Open Library
A carbohydrate is a biomolecule consisting of carbon and oxygen atoms with a hydrogen–oxygen atom ratio of 2:1 and thus with the empirical formula Cmn. This formula holds true for monosaccharides; some exceptions exist. The carbohydrates are technically hydrates of carbon; the term is most common in biochemistry, where it is a synonym of saccharide, a group that includes sugars and cellulose. The saccharides are divided into four chemical groups: monosaccharides, disaccharides and polysaccharides. Monosaccharides and disaccharides, the smallest carbohydrates, are referred to as sugars; the word saccharide comes from the Greek word σάκχαρον, meaning "sugar". While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides often end in the suffix -ose, as in the monosaccharides fructose and glucose and the disaccharides sucrose and lactose. Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as structural components; the 5-carbon monosaccharide ribose is an important component of coenzymes and the backbone of the genetic molecule known as RNA.
The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, preventing pathogenesis, blood clotting, development, they are found in a wide variety of processed foods. Starch is a polysaccharide, it is abundant in cereals and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in human diet as table sugar, lactose and fructose, both of which occur in honey, many fruits, some vegetables. Table sugar, milk, or honey are added to drinks and many prepared foods such as jam and cakes. Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible, insoluble dietary fiber helps to maintain a healthy digestive system by easing defecation. Other polysaccharides contained in dietary fiber include resistant starch and inulin, which feed some bacteria in the microbiota of the large intestine, are metabolized by these bacteria to yield short-chain fatty acids.
In scientific literature, the term "carbohydrate" has many synonyms, like "sugar", "saccharide", "ose", "glucide", "hydrate of carbon" or "polyhydroxy compounds with aldehyde or ketone". Some of these terms, specially "carbohydrate" and "sugar", are used with other meanings. In food science and in many informal contexts, the term "carbohydrate" means any food, rich in the complex carbohydrate starch or simple carbohydrates, such as sugar. In lists of nutritional information, such as the USDA National Nutrient Database, the term "carbohydrate" is used for everything other than water, fat and ethanol; this includes chemical compounds such as acetic or lactic acid, which are not considered carbohydrates. It includes dietary fiber, a carbohydrate but which does not contribute much in the way of food energy though it is included in the calculation of total food energy just as though it were a sugar. In the strict sense, "sugar" is applied for sweet, soluble carbohydrates, many of which are used in food.
The name "carbohydrate" was used in chemistry for any compound with the formula Cm n. Following this definition, some chemists considered formaldehyde to be the simplest carbohydrate, while others claimed that title for glycolaldehyde. Today, the term is understood in the biochemistry sense, which excludes compounds with only one or two carbons and includes many biological carbohydrates which deviate from this formula. For example, while the above representative formulas would seem to capture the known carbohydrates and abundant carbohydrates deviate from this. For example, carbohydrates display chemical groups such as: N-acetyl, carboxylic acid and deoxy modifications. Natural saccharides are built of simple carbohydrates called monosaccharides with general formula n where n is three or more. A typical monosaccharide has the structure H–x–y–H, that is, an aldehyde or ketone with many hydroxyl groups added one on each carbon atom, not part of the aldehyde or ketone functional group. Examples of monosaccharides are glucose and glyceraldehydes.
However, some biological substances called "monosaccharides" do not conform to this formula and there are many chemicals that do conform to this formula but are not considered to be monosaccharides. The open-chain form of a monosaccharide coexists with a closed ring form where the aldehyde/ketone carbonyl group carbon and hydroxyl group react forming a hemiacetal with a new C–O–C bridge. Monosaccharides can be linked togeth
Redox is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes. Redox reactions include all chemical reactions; the chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. It can be explained in simple terms: Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Reduction is a decrease in oxidation state by a molecule, atom, or ion; as an example, during the combustion of wood, oxygen from the air is reduced, gaining electrons from carbon, oxidized. Although oxidation reactions are associated with the formation of oxides from oxygen molecules, oxygen is not included in such reactions, as other chemical species can serve the same function; the reaction can occur slowly, as with the formation of rust, or more in the case of fire.
There are simple redox processes, such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane, more complex processes such as the oxidation of glucose in the human body. "Redox" is a portmanteau of the words "reduction" and "oxidation". The word oxidation implied reaction with oxygen to form an oxide, since dioxygen was the first recognized oxidizing agent; the term was expanded to encompass oxygen-like substances that accomplished parallel chemical reactions. The meaning was generalized to include all processes involving loss of electrons; the word reduction referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. In other words, ore was "reduced" to metal. Antoine Lavoisier showed. Scientists realized that the metal atom gains electrons in this process; the meaning of reduction became generalized to include all processes involving a gain of electrons. Though "reduction" seems counter-intuitive when speaking of the gain of electrons, it might help to think of reduction as the loss of oxygen, its historical meaning.
Since electrons are negatively charged, it is helpful to think of this as reduction in electrical charge. The electrochemist John Bockris has used the words electronation and deelectronation to describe reduction and oxidation processes when they occur at electrodes; these words are analogous to protonation and deprotonation, but they have not been adopted by chemists worldwide. The term "hydrogenation" could be used instead of reduction, since hydrogen is the reducing agent in a large number of reactions in organic chemistry and biochemistry. But, unlike oxidation, generalized beyond its root element, hydrogenation has maintained its specific connection to reactions that add hydrogen to another substance; the word "redox" was first used in 1928. The processes of oxidation and reduction occur and cannot happen independently of one another, similar to the acid–base reaction; the oxidation alone and the reduction alone are each called a half-reaction, because two half-reactions always occur together to form a whole reaction.
When writing half-reactions, the gained or lost electrons are included explicitly in order that the half-reaction be balanced with respect to electric charge. Though sufficient for many purposes, these general descriptions are not correct. Although oxidation and reduction properly refer to a change in oxidation state — the actual transfer of electrons may never occur; the oxidation state of an atom is the fictitious charge that an atom would have if all bonds between atoms of different elements were 100% ionic. Thus, oxidation is best defined as an increase in oxidation state, reduction as a decrease in oxidation state. In practice, the transfer of electrons will always cause a change in oxidation state, but there are many reactions that are classed as "redox" though no electron transfer occurs. In redox processes, the reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or reducing agent loses electrons and is oxidized, the oxidant or oxidizing agent gains electrons and is reduced.
The pair of an oxidizing and reducing agent that are involved in a particular reaction is called a redox pair. A redox couple is a reducing species and its corresponding oxidizing form, e.g. Fe2+/Fe3+ Substances that have the ability to oxidize other substances are said to be oxidative or oxidizing and are known as oxidizing agents, oxidants, or oxidizers; that is, the oxidant removes electrons from another substance, is thus itself reduced. And, because it "accepts" electrons, the oxidizing agent is called an electron acceptor. Oxygen is the quintessential oxidizer. Oxidants are chemical substances with elements in high oxidation states, or else electronegative elements that can gain extra electrons by oxidizing another substance. Substances that have the ability to reduce other substances are said to be reductive or reducing and are known as
Glyoxylic acid or oxoacetic acid is an organic compound. Together with acetic acid, glycolic acid, oxalic acid, glyoxylic acid is one of the C2 carboxylic acids, it is a colourless solid that occurs and is useful industrially. Glyoxylic acid is described with the chemical formula OCHCO2H, i.e. containing an aldehyde functional group. The aldehyde in fact is not observed as a solid; as seen for many other aldehydes, it exists most as the hydrate. Thus, the formula for glyoxylic acid is 2CHCO2H, described as the "monohydrate." This geminal diol exists in equilibrium with the dimeric hemiacetal in solution: Henry's law constant of glyoxylic acid is KH = 1.09 × 104 × exp. 2 2CHCO2H ⇌ O2 + H2O The conjugate base of glyoxylic acid is known as glyoxylate and is the form that the compound exists in solution at neutral pH. Glyoxylate is the byproduct of the amidation process in biosynthesis of several amidated peptides. Glyoxylic acid was prepared from oxalic acid electrosynthetically: In organic synthesis, lead dioxide cathodes were applied for the production of glyoxylic acid from oxalic acid in a sulfuric acid electrolyte.
Glyoxylic acid can be formed by organic oxidation of glyoxal with hot nitric acid, the main side product being oxalic acid. However, this reaction is exothermic and prone to thermal runaway. Ozonolysis of maleic acid is effective. Glyoxylate is an intermediate of the glyoxylate cycle, which enables organisms, such as bacteria and plants to convert fatty acids into carbohydrates; the glyoxylate cycle is important for induction of plant defense mechanisms in response to fungi. The glyoxylate cycle is initiated through the activity of isocitrate lyase, which converts isocitrate into glyoxylate and succinate. Research is being done to co-opt the pathway for a variety of uses such as the biosynthesis of succinate. Glyoxylate is produced via two pathways: through the oxidation of glycolate in peroxisomes or through the catabolism of hydroxyproline in mitochondria. In the peroxisomes, glyoxylate is converted into glycine by AGT1 or into oxalate by glycolate oxidase. In the mitochondria, glyoxylate is converted into glycine by AGT2 or into glycolate by glycolate reductase.
A small amount of glyoxylate is converted into oxalate by cytoplasmic lactate dehydrogenase. In addition to being an intermediate in the glyoxylate pathway, glyoxylate is an important intermediate in the photorespiration pathway. Photorespiration is a result of the side reaction of Rubisco with O2 instead of CO2. While at first considered a waste of energy and resources, photorespiration has been shown to be an important method of regenerating carbon and CO2, removing toxic phosphoglycolate, initiating defense mechanisms. In photorespiration, glyoxylate is converted from glycolate through the activity of glycolate oxidase in the peroxisome, it is converted into glycine through parallel actions by SGAT and GGAT, transported into the mitochondria. It has been reported that the pyruvate dehydrogenase complex may play a role in glycolate and glyoxylate metabolism. Glyoxylate is thought to be a potential early marker for Type II diabetes. One of the key conditions of diabetes pathology is the production of advanced glycation end-products caused by the hyperglycemia.
AGEs can lead to further complications of diabetes, such as tissue damage and cardiovascular disease. They are formed from reactive aldehydes, such as those present on reducing sugars and alpha-oxoaldehydes. In a study, glyoxylate levels were found to be increased in patients who were diagnosed with Type II diabetes; the elevated levels were found sometimes up to three years before the diagnosis, demonstrating the potential role for glyoxylate to be an early predictive marker. Glyoxylate is involved in the development of a key cause of nephrolithiasis. Glyoxylate is both a substrate and inductor of sulfate anion transporter-1, a gene responsible for oxalate transportation, allowing it to increase sat-1 mRNA expression and as a result oxalate efflux from the cell; the increased oxalate release allows the buildup of calcium oxalate in the urine, thus the eventual formation of kidney stones. The disruption of glyoxylate metabolism provides an additional mechanism of hyperoxaluria development. Loss of function mutations in the HOGA1 gene leads to a loss of the 4-hydroxy-2-oxoglutarate aldolase, an enzyme in the hydroxyproline to glyoxylate pathway.
The glyoxylate resulting from this pathway is stored away to prevent oxidation to oxalate in the cytosol. The disrupted pathway, causes a buildup of 4-hydroxy-2-oxoglutarate which can be transported to the cytosol and converted into glyoxylate through a different aldolase; these glyoxylate molecules can be oxidized into oxalate increasing its concentration and causing hyperoxaluria. Glyoxylic acid is about 10x stronger acid than acetic acid, with an acid dissociation constant of 4.7 × 10−4: OCHCO2H ⇌ OCHCO2− + H+With base, glyoxylic acid disproportionates: 2 OCHCO2H + H2O → HOCH2CO2H + 2Glyoxylic acid gives heterocycles upon condensation with urea and 1,2-diaminobenzene. Its condensation with phenols is versatile; the immediate product is 4-hydroxymandelic acid. This species reacts with ammonia to give a precursor to the drug amoxicillin. Reduction of the 4-hydroxymandelic acid gives 4-hydroxyphenylacetic acid, a precursor to the drug atenolol. Condensations with guaiacol in place of phenol provides a route to a net formylation.
Glyoxylic acid is a component of the Hopkins Cole reaction, used to check for the presence of tryptop
Acetic acid, systematically named ethanoic acid, is a colourless liquid organic compound with the chemical formula CH3COOH. When undiluted, it is sometimes called glacial acetic acid. Vinegar is no less than 4% acetic acid by volume, making acetic acid the main component of vinegar apart from water. Acetic acid has pungent smell. In addition to household vinegar, it is produced as a precursor to polyvinyl acetate and cellulose acetate, it is classified as a weak acid since it only dissociates in solution, but concentrated acetic acid is corrosive and can attack the skin. Acetic acid is the second simplest carboxylic acid, it consists of a methyl group attached to a carboxyl group. It is an important chemical reagent and industrial chemical, used in the production of cellulose acetate for photographic film, polyvinyl acetate for wood glue, synthetic fibres and fabrics. In households, diluted acetic acid is used in descaling agents. In the food industry, acetic acid is controlled by the food additive code E260 as an acidity regulator and as a condiment.
In biochemistry, the acetyl group, derived from acetic acid, is fundamental to all forms of life. When bound to coenzyme A, it is central to the metabolism of fats; the global demand for acetic acid is about 6.5 million metric tons per year, of which 1.5 Mt/a is met by recycling. Vinegar is dilute acetic acid produced by fermentation and subsequent oxidation of ethanol; the trivial name acetic acid is the most used and preferred IUPAC name. The systematic name ethanoic acid, a valid IUPAC name, is constructed according to the substitutive nomenclature; the name acetic acid derives from acetum, the Latin word for vinegar, is related to the word acid itself. Glacial acetic acid is a name for water-free acetic acid. Similar to the German name Eisessig, the name comes from the ice-like crystals that form below room temperature at 16.6 °C. A common symbol for acetic acid is AcOH, where Ac is the pseudoelement symbol representing the acetyl group CH3−C−. To better reflect its structure, acetic acid is written as CH3–COH, CH3−COH, CH3COOH, CH3CO2H.
In the context of acid-base reactions, the abbreviation HAc is sometimes used, where Ac in this case is a symbol for acetate. Acetate is the ion resulting from loss of H+ from acetic acid; the name acetate can refer to a salt containing this anion, or an ester of acetic acid. The hydrogen centre in the carboxyl group in carboxylic acids such as acetic acid can separate from the molecule by ionization: CH3CO2H ⇌ CH3CO2− + H+Because of this release of the proton, acetic acid has acidic character. Acetic acid is a weak monoprotic acid. In aqueous solution, it has a pKa value of 4.76. Its conjugate base is acetate. A 1.0 M solution has a pH of 2.4, indicating that 0.4% of the acetic acid molecules are dissociated. However, in dilute solution acetic acid is >90% dissociated. In solid acetic acid, the molecules form chains, individual molecules being interconnected by hydrogen bonds. In the vapour at 120 °C, dimers can be detected. Dimers occur in the liquid phase in dilute solutions in non-hydrogen-bonding solvents, a certain extent in pure acetic acid, but are disrupted by hydrogen-bonding solvents.
The dissociation enthalpy of the dimer is estimated at 65.0–66.0 kJ/mol, the dissociation entropy at 154–157 J mol−1 K−1. Other carboxylic acids engage in similar intermolecular hydrogen bonding interactions. Liquid acetic acid is a hydrophilic protic similar to ethanol and water. With a moderate relative static permittivity of 6.2, it dissolves not only polar compounds such as inorganic salts and sugars, but non-polar compounds such as oils as well as polar solutes. It is miscible with polar and non-polar solvents such as water and hexane. With higher alkanes, acetic acid is not miscible, its miscibility declines with longer n-alkanes; the solvent and miscibility properties of acetic acid make it a useful industrial chemical, for example, as a solvent in the production of dimethyl terephthalate. At physiological pHs, acetic acid is fully ionised to acetate; the acetyl group, formally derived from acetic acid, is fundamental to all forms of life. When bound to coenzyme A, it is central to the metabolism of fats.
Unlike longer-chain carboxylic acids, acetic acid does not occur in natural triglycerides. However, the artificial triglyceride triacetin is a common food additive and is found in cosmetics and topical medicines. Acetic acid is produced and excreted by acetic acid bacteria, notably the genus Acetobacter and Clostridium acetobutylicum; these bacteria are found universally in foodstuffs and soil, acetic acid is produced as fruits and other foods spoil. Acetic acid is a component of the vaginal lubrication of humans and other primates, where it appears to serve as a mild antibacterial agent. Acetic acid is produced industrially both synthetically and by bacterial fermentation. About 75% of acetic acid made for use in the chemical industry is made by the carbonylation of methanol, explained below; the biological route accounts for only a