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'''Ascorbic acid''' is an [[organic chemistry|organic]] acid with [[antioxidant]] properties. Its appearance is  white crystals or powder (from light yellow to brown if oxidized). It is water soluble. The L-[[enantiomer]] of ascorbic acid is commonly known as [[vitamin C]], CASNo = 50-81-7.  
| ImageFile =Ascorbic-acid-2D-skeletal.png
The name is derived from the [[alpha privative]] ''a-'' (meaning no) and ''scorbuticus'' ([[scurvy]]), the disease caused by a deficiency of vitamin C. In 1937 the [[Nobel Prize]] for chemistry was awarded to [[Walter Haworth]] for his work in determining the structure of ascorbic acid (shared with [[Paul Karrer]], who received his award for work on [[vitamin]]s), and the prize for Physiology or Medicine that year went to [[Albert Szent-Györgyi]] for his studies of the biological functions of L-ascorbic acid. At the time of its discovery in the 1920s it was called '''hexuronic acid''' by some researchers.<ref>Joseph Louis Svirbelf, Albert Szent-Gyorgyi [http://profiles.nlm.nih.gov/WG/B/B/G/W/_/wgbbgw.pdf The Chemical Nature Of Vitamin C], April 25th, 1932. Part of the [[National Library of Medicine]] collection. Accessed January 2007</ref>
| ImageFile1 =L-ascorbic-acid-3D-balls.png
: See [[Vitamin C]] for the function and effect of Ascorbic Acid on [[plant]] and [[animal]] species and its use by humans and the higher [[primate]]s.
| IUPACName = (''R'')-3,4-dihydroxy-5-((''S'')- 1,2-dihydroxyethyl)furan-2(5''H'')-one
| OtherNames =
| Section1 = {{Chembox Identifiers
|  CASNo = 50-81-7
|  EINECS = 200-066-2
|  FEMA = 2109
|  PubChem = 5785
|  SMILES =
|  InChI=1/C6H8O6/c7-1-2(8)5-3(9)4<br/>(10)6(11)12-5/h2,5,7-8,10-<br/>11H,1H2/t2-,5+/m0/s1
  }}
| Section2 = {{Chembox Properties
|  Formula = C<sub>6</sub>H<sub>8</sub>O<sub>6</sub>
|  MolarMass = 176.13 g/mol
|  Appearance = White or light yellow solid
|  Density = 1.65 g/cm³
|  MeltingPt = 190 - 192 °C (dec)
|   BoilingPt =
|  Solubility = Soluble
|  pKa = 4.17 (first), 11.6 (second)
  }}
| Section3 = {{Chembox Hazards
|  ExternalMSDS = [http://www.sciencelab.com/xMSDS-Ascorbic_acid-9922972 ScienceLab.com]
|  MainHazards =
|  FlashPt =
|  Autoignition =
|  LD50 = 11.9 g/kg (oral, rat)<ref>{{cite web |url=http://physchem.ox.ac.uk/MSDS/AS/ascorbic_acid.html |title=Safety (MSDS) data for ascorbic acid |accessdate= 2007-02-21 |date= [[2005-10-09]] | publisher= [[Oxford University]] }}</ref>
  }}
}}
:''This article deals with the molecular aspects of ascorbic acid. For information about its purpose in nutrition, see [[Vitamin C]].''
 
-->
:''This article will describe the [[biochemistry]] of Ascorbic Acid.  For the function and effect of Ascorbic Acid on [[animal]] species, and its use by humans and the higher [[primate]]s, please see [[Vitamin C]].''
 
'''Ascorbic acid''' is an [[organic chemistry|organic]] acid with [[antioxidant]] properties. Its appearance is  white to light yellow crystals or powder. It is water soluble. The L-[[enantiomer]] of ascorbic acid is commonly known as [[vitamin C]]. The name is derived from the [[alpha privative]] ''a-'' (meaning no) and ''scorbuticus'' ([[scurvy]]), the disease caused by a deficiency of vitamin C. In [[1937]] the [[Nobel Prize]] for chemistry was awarded to [[Walter Haworth]] for his work in determining the structure of ascorbic acid (shared with [[Paul Karrer]], who received his award for work on [[vitamin]]s), and the prize for Physiology or Medicine that year went to [[Albert Szent-Györgyi]] for his studies of the biological functions of L-ascorbic acid. At the time of its discovery in the 1920s it was called '''hexuronic acid''' by some researchers.<ref>Joseph Louis Svirbelf, Albert Szent-Gyorgyi [http://profiles.nlm.nih.gov/WG/B/B/G/W/_/wgbbgw.pdf The Chemical Nature Of Vitamin C], April 25th, 1932. Part of the [[National Library of Medicine]] collection. Accessed January 2007</ref>
 
== Chemistry ==
== Chemistry ==
===Acidity===
===Acidity===
Ascorbic acid behaves as a [[vinylogous]] [[carboxylic acid]], where the double bond ("vinyl") transmits electron pairs between the hydroxyl and the carbonyl. There are two [[resonance structure]]s for the deprotonated form, differing in the position of the double bond.
Ascorbic acid behaves as a [[vinylogous]] [[carboxylic acid]], where the double bond ("vinyl") transmits electron pairs between the [[hydroxyl]] and the [[carbonyl]]. There are two [[resonance structure]]s for the deprotonated form, differing in the position of the double bond.


Another way to look at ascorbic acid is to consider it as an [[enol]]. The deprotonated form is an [[enolate]], which are usually strongly basic. However, the adjacent double bond stabilizes the deprotonated form.
Another way to look at ascorbic acid is to consider it as an [[enol]]. The deprotonated form is an [[enolate]], which are usually strongly basic. However, the adjacent double bond stabilizes the deprotonated form.
[[Image:Ascorbic_acidity1.png|thumb|left|500px|Movement of electron pairs in deprotonation]]<br style="clear:left;">
<!--{{Image|Ascorbic_acidity1.png|left|500px|Movement of electron pairs in deprotonation}}<br style="clear:left;">-->


===Tautomerism===
===Tautomerism===
[[Image:Ascorbic diketone.png|frame|left|[[Nucleophile|Nucleophilic attack]] of ascorbic enol on proton to give 1,3-diketone]]
<!--[[Image:Ascorbic diketone.png|frame|left|[[Nucleophile|Nucleophilic attack]] of ascorbic enol on proton to give 1,3-diketone]]-->


Ascorbic acid also rapidly interconverts into two unstable [[diketone]] [[tautomer]]s by [[proton]] transfer, although it is the most stable in the enol form. The proton of the enol is lost, and reacquired by electrons from the double bond, to produce a diketone. This is an [[enol]] reaction. There are two possible forms, 1,2-diketone and 1,3-diketone.<br style="clear:both;">
Ascorbic acid also rapidly interconverts into two unstable [[diketone]] [[tautomer]]s by [[proton]] transfer, although it is the most stable in the enol form. The proton of the enol is lost, and reacquired by electrons from the double bond, to produce a diketone. This is an [[enol]] reaction. There are two possible forms, 1,2-diketone and 1,3-diketone.<br style="clear:both;">


===Determination===
===Determination of concentrations===
The concentration of a solution of ascorbic acid can be determined in many ways, the most common ways involving [[titration]] with an [[oxidizing agent]].  
The [[concentration]] of a [[solution]] of ascorbic acid can be determined in many ways, the most common ways involving [[titration]] with an [[oxidizing agent]].  


;DCPIP
;[[DCPIP]]
A commonly used oxidising agent is the dye 2,6-dichlorophenol-indophenol, or [[DCPIP]] for short. The blue dye is run into the ascorbic acid [[solution]] until a faint pink colour persists for 15 seconds.
A commonly used titration method involve oxidising agent is the dye 2,6-dichlorophenol-indophenol, or [[DCPIP]] for short. The blue dye is run into the ascorbic acid [[solution]] until a faint pink colour persists for 15 seconds.


;Iodine
;[[Iodine]]
Another method involves using [[iodine]] and a [[starch]] [[indicator]] where iodine reacts with 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.  Alternatively, ascorbic acid can be reacted with iodine in excess, followed by back titration with sodium thiosulfate while using starch as an indicator.
Another method involves using [[iodine]] and a [[starch]] [[indicator]] where iodine reacts with 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.  Alternatively, ascorbic acid can be reacted with iodine in excess, followed by back titration with sodium thiosulfate while using starch as an indicator.


;Iodate and iodine
;[[Iodate]] and [[iodine]]
The above method involving iodine requires making up and standardising the iodine solution. One way round this is to generate the iodine in the presence of the ascorbic acid by the reaction of [[iodate]] and iodide ion in [[acid]] solution.
The above method involving iodine requires making up and standardising the iodine solution. One way round this is to generate the iodine in the presence of the ascorbic acid by the reaction of [[iodate]] and iodide ion in [[acid]] solution.


;N-Bromosuccinimide
;[[N-Bromosuccinimide]]
A much less common oxidising agent is [[N-Bromosuccinimide|''N''-bromosuccinimide]], (NBS). In this titration the NBS oxidises the ascorbic acid (in the presence of [[potassium iodide]] 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.
A much less common oxidising agent is [[N-Bromosuccinimide|''N''-bromosuccinimide]], (NBS). In this titration the NBS oxidises the ascorbic acid (in the presence of [[potassium iodide]] 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.


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|}
<div style="border: none; width:150px;"><div class="thumbcaption"><small>Top: ascorbic acid<br>([[reducing agent|reduced form]] of [[Vitamin C]])<br>Bottom: [[dehydroascorbic acid]]<br>([[oxidizing agent|oxidized form]] of [[Vitamin C]])</small></div></div></div>
<div style="border: none; width:150px;"><div class="thumbcaption"><small>Top: ascorbic acid<br>([[reducing agent|reduced form]] of [[Vitamin C]])<br>Bottom: [[dehydroascorbic acid]]<br>([[oxidizing agent|oxidized form]] of [[Vitamin C]])</small></div></div></div>
===Industry===
Ascorbic acid is easily oxidized and so is used as a reductant in photographic developer solutions (among others) and as a [[preservative]].  
Ascorbic acid is easily oxidized and so is used as a reductant in photographic developer solutions (among others) and as a [[preservative]].  


Exposure to oxygen, metals, light and heat destroy ascorbic acid, so it must be stored in dark and cold and not in a metal container.   
In [[plastic]] manufacturing ascorbic acid can be used to assemble molecular chains more quickly and with less waste than traditional synthesis methods.<ref>{{cite news |first= |last= |authorlink= |author=Newswise |coauthors= |title=Vitamin C, water have benefits for plastic manufacturing |url=http://www.reliableplant.com/article.asp?pagetitle=Vitamin%20C,%20water%20have%20benefits%20for%20plastic%20manufacturing&articleid=3133 |format= |work= |publisher=Reliable Plant Magazine |id= |pages= |page= |date=2007 |accessdate=2007-06-25 |language= |quote= }}</ref>
 
The oxidized form of ascorbic acid is known as [[dehydroascorbic acid]].


===Food industry===
{{Main|vitamin C}}
The [[Optical isomerism|L-enantiomer]] of ascorbic acid is also known as [[vitamin C]].  The name "ascorbic" comes from its property of preventing and curing [[scurvy]]. [[Primate]]s, including humans, and a few other species in all divisions of the animal kingdom, notably the [[guinea pig]], have lost the ability to synthesize ascorbic acid and must obtain it in their food.  
The [[Optical isomerism|L-enantiomer]] of ascorbic acid is also known as [[vitamin C]].  The name "ascorbic" comes from its property of preventing and curing [[scurvy]]. [[Primate]]s, including humans, and a few other species in all divisions of the animal kingdom, notably the [[guinea pig]], have lost the ability to synthesize ascorbic acid and must obtain it in their food.  


====Food additive as antioxidant====
Ascorbic acid and its sodium, potassium, and calcium [[salts]] are commonly used as [[antioxidant]] [[food additive]]s. These compounds are water soluble and thus cannot protect [[fats]] from oxidation: For this purpose, the fat-[[soluble]] [[ester]]s of ascorbic acid with long-chain [[fatty acid]]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. <ref>Washington Post, [http://www.washingtonpost.com/wp-dyn/content/article/2007/05/19/AR2007051901273.html Tainted Chinese Imports Common], published May 20, 2007</ref>
Ascorbic acid and its sodium, potassium, and calcium [[salts]] are commonly used as [[antioxidant]] [[food additive]]s. These compounds are water soluble and thus cannot protect [[fats]] from oxidation: For this purpose, the fat-[[soluble]] [[ester]]s of ascorbic acid with long-chain [[fatty acid]]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. <ref>Washington Post, [http://www.washingtonpost.com/wp-dyn/content/article/2007/05/19/AR2007051901273.html Tainted Chinese Imports Common], published May 20, 2007</ref>


Line 92: Line 60:
#E304 fatty acid esters of ascorbic acid (i) [[ascorbyl palmitate]] (ii) [[ascorbyl stearate]].
#E304 fatty acid esters of ascorbic acid (i) [[ascorbyl palmitate]] (ii) [[ascorbyl stearate]].


====Vitaminisation of food====
====Other food uses====
It can be added to water that has been treated with iodine to make it potable, neutralizing the unpleasant iodine taste.
It can be added to water that has been treated with iodine to make it potable, neutralizing the unpleasant iodine taste.


In plastic manufacturing ascorbic acid can be used to assemble molecular chains more quickly and with less waste than traditional synthesis methods.<ref>{{cite news |first= |last= |authorlink= |author=Newswise |coauthors= |title=Vitamin C, water have benefits for plastic manufacturing |url=http://www.reliableplant.com/article.asp?pagetitle=Vitamin%20C,%20water%20have%20benefits%20for%20plastic%20manufacturing&articleid=3133 |format= |work= |publisher=Reliable Plant Magazine |id= |pages= |page= |date=2007 |accessdate=2007-06-25  |language= |quote= }}</ref>
===Pharmacology===
In pharmacology ascorbic acid used as vitamin (pure substance and mixtures with other vitamins and minerals) or in complex preparations, as ''Ferroplex'' (the mixture of ascorbic acid and FeSO<sub>4</sub>).
 
===Analytical chemistry===
Exposure to oxygen, metals, light and heat destroy ascorbic acid, so it must be stored in dark and cold and not in a metal container.
 
The oxidized form of ascorbic acid is known as [[dehydroascorbic acid]].
 
==Redox reactions==
===Antioxidant mechanism===
Ascorbate acts as an [[antioxidant]] by being itself available for energetically favourable oxidation. Many oxidants (typically, [[reactive oxygen species]]) such as the [[hydroxyl]] radical (formed from [[hydrogen peroxide]]), contain an unpaired [[electron]] and thus are highly reactive and damaging to humans and plants at the molecular level. This is due to their interaction with [[nucleic acid]], [[protein]]s and [[lipids]]. Reactive oxygen species oxidize (take electrons from) ascorbate first to monodehydroascorbate and then dehydroascorbate. The reactive oxygen species are reduced to water while the oxidized forms of ascorbate are relatively stable and unreactive, and do not cause cellular damage.
 
But "no effect of supplementation with [[vitamin E]], ascorbic acid, or [[coenzyme Q10]] on oxidative [[DNA]] damage estimated by 8-oxo-7,8-dihydro-2'-deoxyguanosine excretion in smokers".<ref>http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=9022536&cmd=showdetailview&indexed=google</ref>


==Antioxidant mechanism==
===Prooxidant mechanism===
Ascorbate acts as an antioxidant by being itself available for energetically favourable oxidation. Many oxidants (typically, [[reactive oxygen species]]) such as the hydroxyl radical (formed from [[hydrogen peroxide]]), contain an unpaired electron and thus are highly reactive and damaging to humans and plants at the molecular level. This is due to their interaction with [[nucleic acid]], proteins and lipids. Reactive oxygen species oxidize (take electrons from) ascorbate first to monodehydroascorbate and then dehydroascorbate. The reactive oxygen species are reduced to water while the oxidized forms of ascorbate are relatively stable and unreactive, and do not cause cellular damage.


==Ascorbic acid synthesis in non-primates==
==Ascorbic acid synthesis in non-primates==
Ascorbic acid is found in plants, animals, and single cell organisms.<ref>
Ascorbic acid is found in plants, animals, and single cell organisms.<ref>
[http://www.seanet.com/~alexs/ascorbate/197x/stone-i-orthomol_psych-1972-v1-n2-3-p82.htm ''The Natural History of Ascorbic Acid in the Evolution of Mammals and Primates'', Irwin Stone, 1972]</ref> All living animals either make it, eat it, or die from scurvy 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 livers where the enzyme [[L-gulonolactone oxidase]] is required to convert glucose to ascorbic acid.<ref>Stone</ref> Humans, guinea pigs, and some other primates are not able to make L-gulonolactone oxidase because of a genetic defect and are therefore unable to make ascorbic acid in their livers.  This genetic mutation occurred about 63 million years ago <ref>Stone</ref> This would have had lethal consequences for the mutated primate were it not for the fact that it occurred to an arboreal animal living in a tropical environment where plenty of foodstuffs containing ascorbic acid were available throughout the year.  Although ascorbic acid is a vital food nutrient for humans and is therefore termed a vitamin, it is a natural liver metabolite in most other animals.
[http://www.seanet.com/~alexs/ascorbate/197x/stone-i-orthomol_psych-1972-v1-n2-3-p82.htm ''The Natural History of Ascorbic Acid in the Evolution of Mammals and Primates'', Irwin Stone, 1972]</ref> All living animals either make it, eat it, or die from scurvy 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 livers where the enzyme [[L-gulonolactone oxidase]] is required to convert glucose to ascorbic acid.<ref>Stone</ref> Humans, guinea pigs, and some other primates are not able to make L-gulonolactone oxidase because of a genetic defect and are therefore unable to make ascorbic acid in their livers.  This genetic mutation occurred about 63 million years ago <ref>Stone</ref> This would have had lethal consequences for the mutated primate were it not for the fact that it occurred to an arboreal animal living in a tropical environment where plenty of foodstuffs containing ascorbic acid were available throughout the year.  Although ascorbic acid is a vital food nutrient for humans and is therefore termed a vitamin, it is a natural liver metabolite in most other animals.
-->
 
==See also==
== Methods of manufacture (chemical synthesis) ==
*[[Vitamin C]]: a discussion of the medical properties of ascorbic acid as well as its historic and social role
Vitamin C is produced from [[glucose]] by two main routes. The Reichstein process developed in the 1930s uses a single pre-fermentation followed by a purely chemical route. The more modern two-step fermentation process was originally developed in China in the 1960s, uses additional fermentation to replace part of the later chemical stages.  Both processes yield approximately 60% vitamin C from the glucose feed.<ref>[http://www.competition-commission.org.uk/rep_pub/reports/2001/fulltext/456a4.2.pdf#search=%22Two-Step%20fermentation%20process%20vitamin%20c%201960s%22 UK Competition Commission Report on Vitamin C - 2001 ] APPENDIX 4.2.</ref>
*[[Erythorbic acid]]: a [[diastereomer]] of ascorbic acid.  
 
*[[Mineral ascorbates]]: salts of ascorbic acid
Research is underway at the [[Scottish Crop Research Institute]] to create yeast micro organisms to synthesise ascorbic acid in a single fermentation step, a technology which is expected to reduce manufacturing costs considerably.<ref>
*[[D-erythroascorbic acid]]: yeasts do not make vitamin C (L-ascorbic acid), but a similar antioxidant known as [http://www.scri.sari.ac.uk/SCRI/Web/Site/home/ResearchAreas/Theme2~GenestoProducts/QHN/External/vitaminC.as D-erythroascorbic acid]
[http://www.scri.sari.ac.uk/SCRI/Web/Site/home/ResearchAreas/Theme2~GenestoProducts/QHN/External/vitaminC.asp Scottish Crop Research Institute] -Development of a Yeast-Based Single-Step Process for the Manufacture of L-Ascorbic Acid (vitamin C)</ref>
 
World production of synthesised vitamin C is currently estimated at approximately 110,000 tonnes annually.
Main producers today are [[BASF]]/[[Takeda Chemical Industries|Takeda]], [[DSM]], [[Merck KGaA|Merck]] and the China Pharmaceutical Group Ltd. of the [[People's Republic of China]]. China is slowly becoming the major world supplier as its prices undercut those of the US and European manufacturers.<ref>[http://www.nutraingredients.com/news/ng.asp?n=63349-dsm-vitamin-c nutraingredients.com] "DSM makes last stand against Chinese vitamin C" 20/10/2005 accessed June 2006 .</ref>
 
==Attribution==
{{WPAttribution}}


==References==
==References==
* Clayden, Greeves, Warren, Wothers. ''Organic Chemistry.'' Oxford University Press (2001), ISBN 0-19-850346-6.
<references>
* {{cite journal | author = Derek Denby | title = Vitamin C | journal = [http://www.york.ac.uk/depts/chem/chemrev/ Chemistry Review] | publisher = [[University of York]] | volume = 5 | issue = 5 | date = May 1996}}
*''Vitamin C: Its Chemistry and Biochemistry'' Michael B. Davies, John Austin, David A. Partridge. Royal Society of Chemistry. ISBN 0-85186-333-7
*''Food: The Chemistry of Its Components; Third Edition'' T.P. Coultate. Royal Society of Chemistry. ISBN 0-85404-513-9
<references/>
 
==External links==
*[http://ptcl.chem.ox.ac.uk/MSDS/AS/ascorbic_acid.html Safety data] from the [[University of Oxford]].
*[http://www.inchem.org/documents/pims/pharm/ascorbic.htm International Programme on Chemical Safety]


<!--
</references>[[Category:Suggestion Bot Tag]]
{{vitamin}}
[[Category:Antioxidants]]
[[Category:Dietary antioxidants]]
[[Category:Coenzymes]]
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[[Category:Organic acids]]

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Ascorbic acid is an organic acid with antioxidant properties. Its appearance is white crystals or powder (from light yellow to brown if oxidized). It is water soluble. The L-enantiomer of ascorbic acid is commonly known as vitamin C, CASNo = 50-81-7. The name is derived from the alpha privative a- (meaning no) and scorbuticus (scurvy), the disease caused by a deficiency of vitamin C. In 1937 the Nobel Prize for chemistry was awarded to Walter Haworth for his work in determining the structure of ascorbic acid (shared with Paul Karrer, who received his award for work on vitamins), and the prize for Physiology or Medicine that year went to Albert Szent-Györgyi for his studies of the biological functions of L-ascorbic acid. At the time of its discovery in the 1920s it was called hexuronic acid by some researchers.[1]

See Vitamin C for the function and effect of Ascorbic Acid on plant and animal species and its use by humans and the higher primates.

Chemistry

Acidity

Ascorbic acid behaves as a vinylogous carboxylic acid, where the double bond ("vinyl") transmits electron pairs between the hydroxyl and the carbonyl. There are two resonance structures for the deprotonated form, differing in the position of the double bond.

Another way to look at ascorbic acid is to consider it as an enol. The deprotonated form is an enolate, which are usually strongly basic. However, the adjacent double bond stabilizes the deprotonated form.

Tautomerism

Ascorbic acid also rapidly interconverts into two unstable diketone tautomers by proton transfer, although it is the most stable in the enol form. The proton of the enol is lost, and reacquired by electrons from the double bond, to produce a diketone. This is an enol reaction. There are two possible forms, 1,2-diketone and 1,3-diketone.

Determination of concentrations

The concentration of a solution of ascorbic acid can be determined in many ways, the most common ways involving titration with an oxidizing agent.

DCPIP

A commonly used titration method involve oxidising agent is the dye 2,6-dichlorophenol-indophenol, or DCPIP for short. The blue dye is run into the ascorbic acid solution until a faint pink colour persists for 15 seconds.

Iodine

Another method involves using iodine and a starch indicator where iodine reacts with 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. Alternatively, ascorbic acid can be reacted with iodine in excess, followed by back titration with sodium thiosulfate while using starch as an indicator.

Iodate and iodine

The above method involving iodine requires making up and standardising the iodine solution. One way round this is to generate the iodine in the presence of the ascorbic acid by the reaction of iodate and iodide ion in acid solution.

N-Bromosuccinimide

A much less common oxidising agent is N-bromosuccinimide, (NBS). In this titration the NBS oxidises the ascorbic acid (in the presence of potassium iodide 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.

Uses

Ascorbic acid structure.png
Dehydroascorbic acid.png

Industry

Ascorbic acid is easily oxidized and so is used as a reductant in photographic developer solutions (among others) and as a preservative.

In plastic manufacturing ascorbic acid can be used to assemble molecular chains more quickly and with less waste than traditional synthesis methods.[2]

Food industry

For more information, see: vitamin C.

The L-enantiomer of ascorbic acid is also known as vitamin C. The name "ascorbic" comes from its property of preventing and curing scurvy. Primates, including humans, and a few other species in all divisions of the animal kingdom, notably the guinea pig, have lost the ability to synthesize ascorbic acid and must obtain it in their food.

Food additive as antioxidant

Ascorbic acid and its sodium, potassium, and calcium salts are commonly used as antioxidant food additives. These compounds are water soluble and thus cannot protect fats from oxidation: For this purpose, the fat-soluble esters of ascorbic acid with long-chain fatty acids (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. [3]

The relevant European food additive E numbers are:

  1. E300 ascorbic acid,
  2. E301 sodium ascorbate,
  3. E302 calcium ascorbate,
  4. E303 potassium ascorbate,
  5. E304 fatty acid esters of ascorbic acid (i) ascorbyl palmitate (ii) ascorbyl stearate.

Vitaminisation of food

Other food uses

It can be added to water that has been treated with iodine to make it potable, neutralizing the unpleasant iodine taste.

Pharmacology

In pharmacology ascorbic acid used as vitamin (pure substance and mixtures with other vitamins and minerals) or in complex preparations, as Ferroplex (the mixture of ascorbic acid and FeSO4).

Analytical chemistry

Exposure to oxygen, metals, light and heat destroy ascorbic acid, so it must be stored in dark and cold and not in a metal container.

The oxidized form of ascorbic acid is known as dehydroascorbic acid.

Redox reactions

Antioxidant mechanism

Ascorbate acts as an antioxidant by being itself available for energetically favourable oxidation. Many oxidants (typically, reactive oxygen species) such as the hydroxyl radical (formed from hydrogen peroxide), contain an unpaired electron and thus are highly reactive and damaging to humans and plants at the molecular level. This is due to their interaction with nucleic acid, proteins and lipids. Reactive oxygen species oxidize (take electrons from) ascorbate first to monodehydroascorbate and then dehydroascorbate. The reactive oxygen species are reduced to water while the oxidized forms of ascorbate are relatively stable and unreactive, and do not cause cellular damage.

But "no effect of supplementation with vitamin E, ascorbic acid, or coenzyme Q10 on oxidative DNA damage estimated by 8-oxo-7,8-dihydro-2'-deoxyguanosine excretion in smokers".[4]

Prooxidant mechanism

Ascorbic acid synthesis in non-primates

Ascorbic acid is found in plants, animals, and single cell organisms.[5] All living animals either make it, eat it, or die from scurvy 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 livers where the enzyme L-gulonolactone oxidase is required to convert glucose to ascorbic acid.[6] Humans, guinea pigs, and some other primates are not able to make L-gulonolactone oxidase because of a genetic defect and are therefore unable to make ascorbic acid in their livers. This genetic mutation occurred about 63 million years ago [7] This would have had lethal consequences for the mutated primate were it not for the fact that it occurred to an arboreal animal living in a tropical environment where plenty of foodstuffs containing ascorbic acid were available throughout the year. Although ascorbic acid is a vital food nutrient for humans and is therefore termed a vitamin, it is a natural liver metabolite in most other animals.

Methods of manufacture (chemical synthesis)

Vitamin C is produced from glucose by two main routes. The Reichstein process developed in the 1930s uses a single pre-fermentation followed by a purely chemical route. The more modern two-step fermentation process was originally developed in China in the 1960s, uses additional fermentation to replace part of the later chemical stages. Both processes yield approximately 60% vitamin C from the glucose feed.[8]

Research is underway at the Scottish Crop Research Institute to create yeast micro organisms to synthesise ascorbic acid in a single fermentation step, a technology which is expected to reduce manufacturing costs considerably.[9]

World production of synthesised vitamin C is currently estimated at approximately 110,000 tonnes annually. Main producers today are BASF/Takeda, DSM, Merck and the China Pharmaceutical Group Ltd. of the People's Republic of China. China is slowly becoming the major world supplier as its prices undercut those of the US and European manufacturers.[10]

Attribution

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References

  1. ↑ Joseph Louis Svirbelf, Albert Szent-Gyorgyi The Chemical Nature Of Vitamin C, April 25th, 1932. Part of the National Library of Medicine collection. Accessed January 2007
  2. ↑ Newswise. Vitamin C, water have benefits for plastic manufacturing, Reliable Plant Magazine, 2007. Retrieved on 2007-06-25.
  3. ↑ Washington Post, Tainted Chinese Imports Common, published May 20, 2007
  4. ↑ http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=9022536&cmd=showdetailview&indexed=google
  5. ↑ The Natural History of Ascorbic Acid in the Evolution of Mammals and Primates, Irwin Stone, 1972
  6. ↑ Stone
  7. ↑ Stone
  8. ↑ UK Competition Commission Report on Vitamin C - 2001 APPENDIX 4.2.
  9. ↑ Scottish Crop Research Institute -Development of a Yeast-Based Single-Step Process for the Manufacture of L-Ascorbic Acid (vitamin C)
  10. ↑ nutraingredients.com "DSM makes last stand against Chinese vitamin C" 20/10/2005 accessed June 2006 .
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