Potassium dichromate
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2008-06-21T18:46:34Z
Biscuittin
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{{Chembox new
| Name = Potassium dichromate
| ImageFile = Potassium-dichromate-sample.jpg
<!-- | ImageSize = 200px -->
| ImageName = Potassium dichromate
| ImageFile1 = Potassium-dichromate-unit-cell-3D-balls.png
| ImageName1 = Unit cell of potassium dichromate
| IUPACName = Potassium dichromate(VI)
| OtherNames = Potassium bichromate <br /> Bichromate of potash
| Section1 = {{Chembox Identifiers
| CASNo = 7778-50-9
| EINECS = 231-906-6
| RTECS = HX7680000
}}
| Section2 = {{Chembox Properties
| Formula = K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>
| MolarMass = 294.19 g/mol
| Appearance = Red-orange crystalline solid
| Density = 2.676 g/cm<sup>3</sup>, solid
| Solubility = 4.9 g/100 ml (0°C)
| MeltingPt = 398°C
| BoilingPt = 500°C ''decomp.''
}}
| Section3 = {{Chembox Structure
| Coordination = [[Tetrahedral]] (for Cr)
| CrystalStruct = [[Triclinic]] (α-form,<241.6 °C
|
}}
| Section4 = {{Chembox Thermochemistry
| DeltaHf = -2033 kJ/mol
| Entropy = 291,2 J.K<sup>−1</sup>.mol<sup>−1</sup>
}}
| Section7 = {{Chembox Hazards
| MainHazards = Highly toxic<br />Carc. Cat. 1<br />Muta. Cat. 2<br />Repr. Cat. 2<br />Oxidant<br />Dangerous for<br />the environment
| RPhrases = {{R45}}, {{R46}}, {{R60}}, {{R61}},<br />{{R8}}, {{R21}}, {{R25}}, {{R26}}, {{R34}},<br />{{R42/43}}, {{R48/23}}, {{R50/53}}
| SPhrases = {{S53}}, {{S45}}, {{S60}}, {{S61}}
| FlashPt = Non-flammable
}}
| Section8 = {{Chembox Related
| OtherAnions = [[Potassium chromate]]<br />[[Potassium molybdate]]<br />[[Potassium tungstate]]
| OtherCations = [[Ammonium dichromate]]<br />[[Sodium dichromate]]
| OtherCpds = [[Potassium permanganate]]}}
}}
'''Potassium dichromate''', K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>, is a common [[inorganic compound|inorganic]] chemical reagent, most commonly used as an [[oxidizing agent]] in various laboratory and industrial applications. As with all [[hexavalent chromium]] compounds, it is potentially harmful to health and must be handled and disposed of appropriately. It is a crystalline ionic solid with a bright, red-orange colour.
==Chemistry==
Potassium dichromate is an oxidizer. The oxidation half-equation can be seen:
:Cr<sub>2</sub>O<sub>7</sub><sup>2−</sup>(aq) + 14H<sup>+</sup> + 6e<sup>−</sup> → 2Cr<sup>3+</sup>(aq) + 7H<sub>2</sub>O (E = +1.36 V)
In organic chemistry, potassium dichromate is a mild oxidizer compared with [[potassium permanganate]]. It is used to [[organic redox reaction|oxidize]] [[alcohol]]s. It converts primary alcohols into aldehydes, or into carboxylic acids if heated under [[reflux]]. In contrast, with permanganate, carboxylic acids are the sole products. Secondary alcohols are converted into ketones — no further oxidation possible. For example, [[menthone]] may be prepared by oxidation of [[menthol]] with acidified dichromate.<ref>{{OrgSynth |title = ''l''-Menthone | author = L. T. Sandborn | collvol = 1 | collvolpages = 340 | prep = cv1p0340}}</ref> Tertiary alcohols are not oxidized by potassium dichromate.
In an aqueous solution the colour change exhibited can be used to test whether an aldehyde or ketone is present. When an aldehyde is present the chromium ions will be reduced from the +6 to the +3 [[oxidation state]], changing colour from orange to green. This is because the aldehyde can be further oxidized to the corresponding carboxylic acid. A ketone will show no such change because it cannot be oxidized further, and so the solution will remain orange.
==Uses==
===Cleaning===
Like other [[chromium(VI)]] compounds ([[chromium trioxide]], [[sodium dichromate]]), potassium dichromate may be used to prepare "[[chromic acid]]", which can be used for cleaning glassware and etching materials.
===Construction===
It is used as an ingredient in [[cement]] in which it retards the setting of the mixture and improves its density and texture. This usage commonly causes [[contact dermatitis]] in [[construction workers]].<ref>{{citation|title=Addition of ferrous sulfate to cement and risk of chromium dermatitis among construction workers|author=Pekka Roto, Hannele Sainio, Timo Reunala, Pekka Laippala|journal=Contact Dermatitis|Volume=34|Issue=1|Pages=43-50|date=January 1996|url=http://www.blackwell-synergy.com/doi/abs/10.1111/j.1600-0536.1996.tb02111.x}}</ref>
===Ethanol determination===
The concentration of ethanol in a sample can be determined by [[back titration]] with acidified potassium dichromate. Reacting the sample with an excess of potassium dichromate, all ethanol is oxidized to [[acetic acid]]:
: C<sub>2</sub>H<sub>5</sub>OH + [O] → CH<sub>3</sub>COOH
The excess dichromate is determined by titration against [[sodium thiosulfate]]. Subtracting the amount of excess dichromate from the initial amount, gives the amount of ethanol present. Accuracy can be improved by calibrating the [[dichromate]] solution against a blank.
One major application for this [[reaction]] is in old police [[breathalyzer]] tests. When alcohol vapor makes contact with the yellow dichromate-coated crystals, the [[color]] changes from yellow to green. The degree of the color change is directly related to the level of alcohol in the suspect's breath.
===Homeopathic uses===
Potassium dichromate is used in the [[homeopathy|homeopathic]] preparation of [[List of homeopathic preparations|Kali (or Kalium) bichromicum]], although little or none remains in the final dilution.<ref>{{cite book|title=The Guiding Symptoms of Our Materia Medica|author=Constantine Hering|url=http://books.google.co.uk/books?id=-j08jXFbLdIC&pg=RA1-PA312&dq=Kali+bichromicum&sig=vuco16in0xSOSyouForAoCr69zQ#PRA1-PA312,M1|page=312|isbn=8170214378|publisher=B.Jain}}</ref> One notable homeopathic product that contains it as the active ingredient is [[HeadOn]],<ref>{{citation|publisher=Daily Herald|title=Intriguing infomercials beg the question: Does it work?|author=Jamie Sotonoff|date=21 Dec 2006|url=http://goliath.ecnext.com/coms2/summary_0199-6100693_ITM}}</ref> in which the potassium dichromate is diluted to one [[parts-per notation|part per million]].
===Leather===
It is used to [[tanning|tan leather]] which is used for [[footwear]].<ref>{{citation|title=Footwear dermatitis|author=M. Saha, C. R. Srinivas, S. D. Shenoy, C. Balachandran|journal=Contact Dermatitis|Volume=28|Issue=5|Pages=260-264|date=May 1993|url=http://www.blackwell-synergy.com/doi/abs/10.1111/j.1600-0536.1993.tb03428.x}}</ref>
===Photography===
Potassium dichromate has important uses in [[photography]] and in photographic [[screen printing]], where it is used as an oxidizing agent together with a strong mineral acid.
[[Gum bichromate]] printing was one of the very first stable photographic printing processes, dating back to about 1850. A solution of [[gum arabic]] and potassium dichromate, once applied to paper and dried, will harden when exposed to ultraviolet light.
''Chromium intensification'' uses potassium dichromate together with equal parts of concentrated [[hydrochloric acid]] diluted down to approximately 10% v/v to treat weak and thin negatives of black and white photograph roll. This solution reconverts the elemental silver particles in the film to [[silver chloride]]. After thorough washing and exposure to [[actinic]] light, the film can be redeveloped to its end-point yielding a stronger negative which is able to produce a more satisfactory print.
A potassium dichromate solution in [[sulfuric acid]] can be used to produce a reversal negative (i.e,. a positive transparency from a negative film). This is effected by developing a black and white film but allowing the development to proceed more or less to the end point. The development is then stopped by copious washing and the film then treated in the acid dichromate solution. This converts the [[silver]] metal to [[silver sulfate]], a compound that is insensitive to light. After thorough washing and exposure to actinic light, the film is developed again allowing the previously unexposed silver halide to be reduced to silver metal.
The results obtained can be unpredictable, but sometimes excellent results are obtained producing images that would otherwise be unobtainable. This process can be coupled with [[solarisation]] so that the end product resembles a negative and is suitable for printing in the normal way.
Cr<sup>VI</sup> compounds have the property of [[tanning]] animal [[protein]]s when exposed to strong light. This quality is used in photographic [[screen printing]]. In screen printing a fine screen of bolting silk of similar material that is required to be printed is then taped securely onto the surface of the screen and the whole thing exposed to strong light for a period - typically about half an hour in bright sunlight. When the design is removed, the gelatine on the screen is washed off with hot water. All the gelatine exposed to sun-light will have been hardened by the dichromate and will be retained on the screen leaving a precise mask of the required design which can be printed in the usual way.
===Wood treatment===
Potassium dichromate is used to finish certain types of wood. It brings out the color and grain to a very deep, rich appearance. This is particularly effective on [[mahogany]].<ref>{{cite web | title = Woodworking: Contemporary Headboard | publisher = DIY Network | url = http://www.diynetwork.com/diy/ww_beds_furniture/article/0,,DIY_14438_2274602,00.html | accessdate = 2008-01-28}}</ref>
==Safety==
Potassium dichromate is one of the most common causes of chromium [[dermatitis]]<ref>{{cite book|title= Diseases of Skin|pages=223|author=Farokh J. Master|year=2003|url=http://books.google.co.uk/books?id=OBAnewDc9CYC|isbn=8170211360}}</ref>; chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and fore-arms, which is chronic and difficult to treat. It is also toxic, with doses of approximately 100 mg/kg being fatal in rabbits and rodents.<ref>[http://physchem.ox.ac.uk/MSDS/PO/potassium_dichromate.html Safety data for potassium dichromate] HSci project</ref>
As with other Cr<sup>VI</sup> compounds, potassium dichromate is [[carcinogen]]ic and should be handled with gloves and appropriate health and safety protection. The compound is also [[corrosive]] and exposure may produce severe eye damage or blindness.<ref name="MSDS">{{cite web | publisher = ScienceLab.com | title = Potassium dichromate MSDS | url = http://www.sciencelab.com/xMSDS-Potassium_dichromate-9927404 | accessdate = 2008-01-28}}</ref>
==Sulfur dioxide==
Potassium dichromate(VI) paper can be used to test for [[sulfur dioxide]], as it turns distinctively from orange to green.
==References==
{{reflist}}
==External links==
*[http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc13/icsc1371.htm International Chemical Safety Card 1371]
* [http://www.npi.gov.au/database/substance-info/profiles/25.html National Pollutant Inventory - Chromium VI and compounds fact sheet]
*[http://www.cdc.gov/niosh/npg/npgd0138.html NIOSH Pocket Guide to Chemical Hazards]
*[http://ecb.jrc.it/ European Chemicals Bureau]
*[http://www-cie.iarc.fr/htdocs/monographs/vol49/chromium.html IARC Monograph "Chromium and Chromium compounds"]
[[Category:Potassium compounds]]
[[Category:Chromates]]
[[Category:Photographic chemicals]]
[[Category:IARC Group 1 carcinogens]]
[[de:Kaliumdichromat]]
[[es:Dicromato de potasio]]
[[fr:Dichromate de potassium]]
[[it:Bicromato di potassio]]
[[nl:Kaliumdichromaat]]
[[ja:二クロム酸カリウム]]
[[pl:Dichromian(VI) potasu]]
[[ru:Дихромат калия]]
[[zh:重铬酸钾]]