Ascorbic acid
Overview
 
Ascorbic acid is a naturally occurring organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 with antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

 properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbic acid is one form ("vitamer
Vitamer
A vitamer of a particular vitamin is any of a number of chemical substances, each of which shows vitamin activity. Very commonly each "vitamin" is not a single chemical, but rather multiple chemical substances called vitamers, each of which is defined by its different biological activity.For...

") of vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...

. The name is derived from a- (meaning "no") and scorbutus (scurvy
Scurvy
Scurvy is a disease resulting from a deficiency of vitamin C, which is required for the synthesis of collagen in humans. The chemical name for vitamin C, ascorbic acid, is derived from the Latin name of scurvy, scorbutus, which also provides the adjective scorbutic...

), the disease caused by a deficiency of vitamin C. Being derived from glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

, many animals are able to produce it, but humans require it as part of their nutrition.
Encyclopedia
Ascorbic acid is a naturally occurring organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 with antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

 properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbic acid is one form ("vitamer
Vitamer
A vitamer of a particular vitamin is any of a number of chemical substances, each of which shows vitamin activity. Very commonly each "vitamin" is not a single chemical, but rather multiple chemical substances called vitamers, each of which is defined by its different biological activity.For...

") of vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...

. The name is derived from a- (meaning "no") and scorbutus (scurvy
Scurvy
Scurvy is a disease resulting from a deficiency of vitamin C, which is required for the synthesis of collagen in humans. The chemical name for vitamin C, ascorbic acid, is derived from the Latin name of scurvy, scorbutus, which also provides the adjective scorbutic...

), the disease caused by a deficiency of vitamin C. Being derived from glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

, many animals are able to produce it, but humans require it as part of their nutrition. Whereas most vertebrates can produce ascorbic acid, some groups such as primates —including humans—, guinea pigs, teleost fishes, bats and birds cannot and require it as a dietary micronutrient (that is, a vitamin).

History

From the middle of the 18th century, it was noted that lemon juice could prevent the sailors from getting scurvy
Scurvy
Scurvy is a disease resulting from a deficiency of vitamin C, which is required for the synthesis of collagen in humans. The chemical name for vitamin C, ascorbic acid, is derived from the Latin name of scurvy, scorbutus, which also provides the adjective scorbutic...

. At first it was supposed that the acid properties were responsible for this benefit; however, it soon became clear that other dietary acids, such as vinegar, had no benefits. In 1907, two Norwegian physicians reported an essential disease-preventing compound in foods that was distinct from the one that prevented Beriberi
Beriberi
Beriberi is a nervous system ailment caused by a thiamine deficiency in the diet. Thiamine is involved in the breakdown of energy molecules such as glucose and is also found on the membranes of neurons...

. The physicians were investigating dietary deficiency diseases using the new animal model of guinea pig
Guinea pig
The guinea pig , also called the cavy, is a species of rodent belonging to the family Caviidae and the genus Cavia. Despite their common name, these animals are not in the pig family, nor are they from Guinea...

s, which are susceptible to scurvy
Scurvy
Scurvy is a disease resulting from a deficiency of vitamin C, which is required for the synthesis of collagen in humans. The chemical name for vitamin C, ascorbic acid, is derived from the Latin name of scurvy, scorbutus, which also provides the adjective scorbutic...

. The newly discovered food-factor was eventually called vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...

.

From 1928 to 1932, the Hungarian research team led by Albert Szent-Györgyi
Albert Szent-Györgyi
Albert Szent-Györgyi de Nagyrápolt was a Hungarian physiologist who won the Nobel Prize in Physiology or Medicine in 1937. He is credited with discovering vitamin C and the components and reactions of the citric acid cycle...

, as well as that of the American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 worker Charles Glen King
Charles Glen King
Charles Glen King was an American biochemist who was a pioneer in the field of nutrition research and who isolated vitamin C at the same time as Albert Szent-Györgyi...

, identified the antiscorbutic factor as a particular single chemical substance. At the Mayo clinic, Szent-Györgyi had isolated the chemical hexuronic acid from animal adrenal glands. He suspected it to be the antiscorbutic factor, but could not prove it without a biological assay. This assay was finally conducted at the University of Pittsburgh in the laboratory of King, which had been working on the problem for years. In late 1931, King's lab obtained adrenal hexuronic acid indirectly from Szent-Györgyi and proved that it is vitamin C by early 1932. This was the last of the compound from animal sources, but, later that year, Szent-Györgyi's group discovered that paprika pepper, a common spice in the Hungarian diet, is a rich source of hexuronic acid, so he sent some of the now-more-available chemical to Walter Norman Haworth, a British sugar chemist. In 1933, working with the then-Assistant Director of Research (later Sir) Edmund Hirst
Edmund Hirst
Sir Edmund Langley Hirst CBE FRS FRSE , was a British chemist.He held the Forbes Chair of Organic Chemistry at Edinburgh University and was head of department there from 1959 to 1964....

 and their research teams, Haworth deduced the correct structure and optical-isomeric nature of vitamin C, and in 1934 reported the first synthesis of the vitamin. In honor of the compound's antiscorbutic properties, Haworth and Szent-Györgyi now proposed the new name of "a-scorbic acid" for the compound. It was named L-ascorbic acid by Haworth and Szent-Györgyi when its structure was finally proven by synthesis.

In 1937, the Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...

 for chemistry was awarded to Norman Haworth for his work in determining the structure of ascorbic acid (shared with Paul Karrer
Paul Karrer
Paul Karrer was a Swiss organic chemist best known for his research on vitamins. He and Walter Haworth won the Nobel Prize for Chemistry in 1937.-Early years:...

, who received his award for work on vitamin
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...

s), and the prize for Physiology or Medicine that year went to Albert Szent-Györgyi
Albert Szent-Györgyi
Albert Szent-Györgyi de Nagyrápolt was a Hungarian physiologist who won the Nobel Prize in Physiology or Medicine in 1937. He is credited with discovering vitamin C and the components and reactions of the citric acid cycle...

 for his studies of the biological functions of L-ascorbic acid.
The American physician Fred R. Klenner
Fred R. Klenner
Frederick Robert Klenner, was an American medical researcher and doctor in general practice in Reidsville, North Carolina. From the 1940s on he experimented with the use of vitamin C megadosage as a therapy for a wide range of illnesses, most notably polio. He authored 28 research papers during...

 M.D. promoted vitamin C as a cure for many diseases in the 1950s by elevating the dosages largely. Tens of grams vitamin C daily by injections were no exception. From 1967 on, Nobel prize winner Linus Pauling
Linus Pauling
Linus Carl Pauling was an American chemist, biochemist, peace activist, author, and educator. He was one of the most influential chemists in history and ranks among the most important scientists of the 20th century...

 recommended high doses of ascorbic acid (he himself took 18 grams daily) as a prevention against cold and cancer.
The results of Klenner have been controversial as yet, since his investigations do not meet the modern methodologic standards.

Reactions

Ascorbic acid resembles the sugar from which it is derived, being a ring containing many oxygen-containing functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

s. The molecule exists in equilibrium with two ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

 tautomer
Tautomer
Tautomers are isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond...

s, which are less stable than the enol
Enol
Enols are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Alkenes with a hydroxyl group on both sides of the double bond are called enediols. Deprotonated anions of enols are called enolates...

 form. In solutions, these forms of ascorbic acid rapidly interconvert.

Antioxidant mechanism

As a mild reducing agent
Reducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...

, ascorbic acid degrades upon exposure to air, converting the oxygen to water. The redox reaction is accelerated by the presence of metal ions and light. It can be oxidized by one electron to a radical state or doubly oxidized to the stable form called dehydroascorbic acid
Dehydroascorbic acid
Dehydroascorbic acid is an oxidized form of ascorbic acid. It is actively imported into the endoplasmic reticulum of cells via glucose transporters. It is trapped therein by reduction back to ascorbate by glutathione and other thiols. Therefore, L-dehydroascorbic acid is a vitamin C compound much...

.
Ascorbate usually acts as an antioxidant. It typically reacts with oxidants of the reactive oxygen species
Reactive oxygen species
Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....

, such as the hydroxyl radical
Hydroxyl radical
The hydroxyl radical, •OH, is the neutral form of the hydroxide ion . Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides or, in...

 formed from hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

. Such radicals are damaging to animals and plants at the molecular level due to their possible interaction with nucleic acids, proteins, and lipids. Sometimes these radicals initiate chain reactions. Ascorbate can terminate these chain radical reactions by electron transfer
Electron transfer
Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...

. Ascorbic acid is special because it can transfer a single electron, owing to the stability of its own
radical ion
Radical ion
A radical ion is a free radical species that carries a charge. Radical ions are encountered in organic chemistry as reactive intermediates and in mass spectrometry as gas phase ions...

 called "semidehydroascorbate". dehydroascorbate. The net reaction is:
RO• + → ROH + .

The oxidized forms of ascorbate are relatively unreactive, and do not cause cellular damage.

However, being a good electron donor, excess ascorbate in the presence of free metal ions can not only promote but also initiate free radical reactions, thus making it a potentially dangerous pro-oxidative compound in certain metabolic contexts.

Acidity

Ascorbic acid, a reductone, behaves as a vinylogous
Vinylogous
Vinylogous is an adjective used to apply the concept of vinylogy taught in intermediate undergraduate through graduate/research organic chemistry. Vinylogy has been defined as the transmission of electronic effects through a conjugated organic bonding system...

 carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

 wherein the electrons in the double bond, hydroxyl group lone pair, and the carbonyl double bond form a conjugated system
Conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...

. Because the two major resonance structures stabilize the deprotonated conjugate base of ascorbic acid, the hydroxyl group in ascorbic acid is much more acidic than typical hydroxyl groups. In other words, ascorbic acid can be considered an enol
Enol
Enols are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Alkenes with a hydroxyl group on both sides of the double bond are called enediols. Deprotonated anions of enols are called enolates...

 in which the deprotonated form is a stabilized enolate.

Food chemistry

Ascorbic acid and its sodium, potassium, and calcium salts are commonly used as antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

 food additive
Food additive
Food additives are substances added to food to preserve flavor or enhance its taste and appearance.Some additives have been used for centuries; for example, preserving food by pickling , salting, as with bacon, preserving sweets or using sulfur dioxide as in some wines...

s. These compounds are water-soluble and thus cannot protect fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

s from oxidation: For this purpose, the fat-soluble ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

s of ascorbic acid with long-chain fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...

s (ascorbyl palmitate or ascorbyl stearate) can be used as food antioxidants. Eighty percent of the world's supply of ascorbic acid is produced in China.

The relevant European food additive E number
E number
E numbers are number codes for food additives that have been assessed for use within the European Union . They are commonly found on food labels throughout the European Union. Safety assessment and approval are the responsibility of the European Food Safety Authority...

s are
  1. E300 ascorbic acid (approved for use as a food additive in the EU, USA and Australia and New Zealand)
  2. E301 sodium ascorbate
    Sodium ascorbate
    Sodium ascorbate is a more bioavailable form of vitamin C that is an alternative to taking ascorbic acid as a supplement. The molecular formula of this chemical compound is C6H7NaO6...

    (approved for use as a food additive in the EU, USA and Australia and New Zealand)
  3. E302 calcium ascorbate
    Calcium ascorbate
    Calcium ascorbate is a compound with the molecular formula CaC12H14O12. It is the calcium salt of ascorbic acid, one of the mineral ascorbates....

    (approved for use as a food additive in the EU, USA and Australia and New Zealand)
  4. E303 potassium ascorbate
    Potassium ascorbate
    Potassium ascorbate is a compound with formula KC6H7O6. It is the potassium salt of ascorbic acid and a mineral ascorbate. As a food additive, it has E number E303, INS number 202. Although it is not a permitted food additive in the European Union. or the USA it is approved for use in Australia...

  5. E304 fatty acid esters of ascorbic acid (i) ascorbyl palmitate
    Ascorbyl palmitate
    Ascorbyl palmitate is an ester formed from ascorbic acid and palmitic acid creating a fat-soluble form of vitamin C. In addition to its use as a source of vitamin C, it is also used as an antioxidant food additive...

     (ii) ascorbyl stearate
    Ascorbyl stearate
    Ascorbyl stearate is an ester formed from ascorbic acid and stearic acid. In addition to its use as a source of vitamin C, it is used as an antioxidant food additive in margarine . The USDA limits its use to 0.02% individually or in conjunction with other antioxidants....

    .


In plastic manufacturing, ascorbic acid can be used to assemble molecular chains more quickly and with less waste than traditional synthesis methods.

It creates volatile compounds when mixed with glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 and amino acids.

It is a cofactor in tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

 oxidation.

Niche, non-food uses

  • Ascorbic acid is easily oxidized and so is used as a reductant in photographic developer solutions (among others) and as a preservative
    Preservative
    A preservative is a naturally occurring or synthetically produced substance that is added to products such as foods, pharmaceuticals, paints, biological samples, wood, etc. to prevent decomposition by microbial growth or by undesirable chemical changes....

    .

  • In fluorescence microscopy
    Fluorescence microscope
    A fluorescence microscope is an optical microscope used to study properties of organic or inorganic substances using the phenomena of fluorescence and phosphorescence instead of, or in addition to, reflection and absorption...

     and related fluorescence-based techniques, ascorbic acid can be used as an antioxidant
    Antioxidant
    An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

     to increase fluorescent signal and chemically retard dye photobleaching
    Photobleaching
    Photobleaching is the photochemical destruction of a fluorophore. In microscopy, photobleaching may complicate the observation of fluorescent molecules, since they will eventually be destroyed by the light exposure necessary to stimulate them into fluorescing...

    .

  • It is also commonly used to remove dissolved metal stains, such as iron, from fiberglass swimming pool surfaces.

  • Heroin users are known to use ascorbic acid to dissolve heroin in water, so that it can be injected.

Biosynthesis

Ascorbic acid is found in plants, animals, and single-cell organisms where it is produced from glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

. All animals either make it, eat it, or else die from scurvy
Scurvy
Scurvy is a disease resulting from a deficiency of vitamin C, which is required for the synthesis of collagen in humans. The chemical name for vitamin C, ascorbic acid, is derived from the Latin name of scurvy, scorbutus, which also provides the adjective scorbutic...

 due to lack of it. Reptiles and older orders of birds make ascorbic acid in their kidneys. Recent orders of birds and most mammals make ascorbic acid in their liver where the enzyme L-gulonolactone oxidase
L-gulonolactone oxidase
L-gulonolactone oxidase is an enzyme that catalyzes the reaction of D-glucuronolactone with oxygen to L-xylo-hex-3-gulonolactone and hydrogen peroxide. It uses FAD as a cofactor...

 is required to convert glucose to ascorbic acid. Humans, some other primates, and guinea pigs are not able to make L-gulonolactone oxidase because of a genetic mutation and are therefore unable to make ascorbic acid. Synthesis and signalling properties are still under investigation.

Industrial preparation

Ascorbic acid is prepared industrially from glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 in a method based on the historical Reichstein process
Reichstein process
The Reichstein process in chemistry is a combined chemical and microbial method for the production of ascorbic acid from D-glucose that takes place in several steps...

. In the first of a five-step process, glucose is catalytically hydrogenated
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...

 to sorbitol
Sorbitol
Sorbitol, also known as glucitol, Sorbogem® and Sorbo®, is a sugar alcohol that the human body metabolizes slowly. It can be obtained by reduction of glucose, changing the aldehyde group to a hydroxyl group. Sorbitol is found in apples, pears, peaches, and prunes...

, which is then oxidized by the microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

 Acetobacter
Acetobacter
Acetobacter is a genus of acetic acid bacteria characterized by the ability to convert ethanol to acetic acid in the presence of oxygen. There are several species within this genus, and there are other bacteria capable of forming acetic acid under various conditions; but all of the Acetobacter are...

 suboxydans
to sorbose
Sorbose
Sorbose is a ketose belonging to the group of sugars known as monosaccharides. It has a sweetness that is equivalent to sucrose . The commercial production of vitamin C often begins with sorbose. L-Sorbose is the configuration of the naturally occurring sugar....

. Only one of the six hydroxy groups is oxidized by this enzymatic reaction. From this point, two routes are available. Treatment of the product with acetone
Acetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...

 in the presence of an acid catalyst converts four of the remaining hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 groups to acetal
Acetal
An acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.Traditional usages distinguish ketals from acetals...

s. The unprotected hydroxyl group is oxidized to the carboxylic acid by reaction with the catalytic oxidant TEMPO
TEMPO
oxyl, or oxidanyl or TEMPO is a chemical compound with the formula 32NO . This heterocycle is a red-orange, sublimable solid. As a stable radical, it has applications throughout chemistry and biochemistry. TEMPO was discovered by Lebedev and Kazarnowskii in 1960...

 (regenerated by sodium hypochlorite &emdash; bleach
Bleach
Bleach refers to a number of chemicals that remove color, whiten, or disinfect, often via oxidation. Common chemical bleaches include household chlorine bleach , lye, oxygen bleach , and bleaching powder...

ing solution). (Historically, industrial preparation via the Reichstein process used potassium permanganate
Potassium permanganate
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...

.) Acid-catalyzed hydrolysis of this product performs the dual function of removing the two acetal groups and ring-closing lactonization
Lactone
In chemistry, a lactone is a cyclic ester which can be seen as the condensation product of an alcohol group -OH and a carboxylic acid group -COOH in the same molecule...

. This step yields ascorbic acid. Each of the five steps has a yield larger than 90%.

A more biotechnological process, first developed in China in the 1960s but further developed in the 1990s, bypasses the use of acetone protecting groups. A second genetically-modified microbe species (such as mutant Erwinia
Erwinia
Erwinia is a genus of Enterobacteriaceae bacteria containing mostly plant pathogenic species which was named for the first phytobacteriologist, Erwin Smith. It is a gram negative bacterium related to E. coli, Shigella, Salmonella and Yersinia. It is primarily a rod-shaped bacteria. A well-known...

, among others) oxidises sorbose into 2-ketogluconic acid (2-KGA), which can then undergo ring-closing lactonization via dehydration. This method is used in the predominant process used by the ascorbic acid industry in China, which supplies 80% of world's ascorbic acid. American and Chinese researchers are competing to engineer a mutant which can carry out a one-pot fermentation directly from glucose to 2-KGA, bypassing both the need for a second fermentation and the need to reduce glucose to sorbitol.

Determination

The traditional way to analyze the ascorbic acid content is titration
Titration
Titration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...

 with an oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...

, and several procedures have been developed, mainly relying on iodometry
Iodometry
Iodometry, also known as iodometric titration, is a method of volumetric chemical analysis, a redox titration where the appearance or disappearance of elementary iodine indicates the end point....

. Iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

 is used in the presence of a starch indicator
Starch indicator
Starch is often used in chemistry as an indicator for redox titrations where triiodide is present. Starch forms a very dark blue-black complex with triiodide which can be made by mixing iodine with iodide . However, the complex is not formed if only iodine or only iodide is present...

. Iodine is reduced by ascorbic acid, and, when all the ascorbic acid has reacted, the iodine is then in excess, forming a blue-black complex with the starch indicator. This indicates the end-point of the titration. As an alternative, ascorbic acid can be treated with iodine in excess, followed by back titration with sodium thiosulfate using starch as an indicator. The preceding iodometric method has been revised to exploit reaction of ascorbic acid with iodate
Iodate
An iodate is a conjugate base of iodic acid. In the iodate anion, iodine is bonded to three oxygen atoms and the molecular formula is IO3−. The molecular geometry of iodate is trigonal pyramidal....

 and iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...

 in acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 solution. Electrolyzing the solution of potassium iodide produces iodine, which reacts with ascorbic acid. The end of process is determined by potentiometric titration
Potentiometric titration
Potentiometric titration is a technique similar to direct titration of a redox reaction. No indicator is used, instead the voltage across the analyte, typically an electrolyte solution is measured. To do this, two electrodes are used, a neutral electrode and a standard reference electrode. The...

 in a manner similar to Karl Fischer titration
Karl Fischer titration
Karl Fischer titration is a classic titration method in analytical chemistry that uses coulometric or volumetric titration to determine trace amounts of water in a sample. It was invented in 1935 by the German chemist Karl Fischer.-Coulometric titration:...

. The amount of ascorbic acid can be calculated by the Faraday's law.

An uncommon oxidising agent is N-bromosuccinimide
N-Bromosuccinimide
N-Bromosuccinimide or NBS is a chemical reagent which is used in radical substitution and electrophilic addition reactions in organic chemistry. NBS can be considered a convenient source of cationic bromine.-Preparation:...

, (NBS). In this titration, the NBS oxidises the ascorbic acid in the presence of potassium iodide
Potassium iodide
Potassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...

 and starch. When the NBS is in excess (i.e., the reaction is complete), the NBS liberates the iodine from the potassium iodide, which then forms the blue-black complex with starch, indicating the end-point of the titration.

Compendial status

  • British Pharmacopoeia
    British Pharmacopoeia
    The British Pharmacopoeia is an annual published collection of quality standards for UK medicinal substances. It is used by individuals and organizations involved in pharmaceutical research, development, manufacture and testing....

  • Japanese Pharmacopoeia 

See also

  • Colour retention agent
    Colour retention agent
    Colour retention agents are food additives that are added to food to prevent the colour from changing. Many of them work by absorbing or binding to oxygen before it can damage food...

  • Erythorbic acid
    Erythorbic acid
    Erythorbic acid, formerly known as isoascorbic acid and D-araboascorbic acid, is a stereoisomer of ascorbic acid . It is a vegetable-derived food additive produced from sucrose. It is denoted by E number E315, and is widely used as an antioxidant in processed foods.Clinical trials have been...

    : a diastereomer
    Diastereomer
    Diastereomers are stereoisomers that are not enantiomers.Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more of the equivalent stereocenters and are not mirror images of each other.When two diastereoisomers differ from each other at...

     of ascorbic acid.
  • Mineral ascorbates
    Mineral ascorbates
    Mineral ascorbates are salts of ascorbic acid . They are powders manufactured by reacting ascorbic acid with mineral carbonates in aqueous solutions, venting the carbon dioxide, drying the reaction product, and then milling the dried product to the desired particle size.The choice of the mineral...

    : salts of ascorbic acid
  • D-erythroascorbic acid: yeasts do not make vitamin C (L-ascorbic acid) but a similar antioxidant known as D-erythroascorbic acid
  • Acids in wine
    Acids in wine
    The acids in wine are an important component in both winemaking and the finished product of wine. They are present in both grapes and wine, having direct influences on the color, balance and taste of the wine as well as the growth and vitality of yeasts during fermentation and protecting the wine...


External links

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