View source for Potassium dichromate

{{Distinguish|Potassium chromate}}
{{chembox
| verifiedrevid  = 406377229
| Name           = Potassium dichromate
| ImageFile      = Potassium-dichromate-sample.jpg
| ImageName      = Potassium dichromate
| ImageFile1     = Potassium-dichromate-unit-cell-3D-balls.png
| ImageClass1 = bg-transparent
| ImageName1     = Unit cell of potassium dichromate
| IUPACName      = Potassium dichromate(VI)
| OtherNames     = {{ubl|potassium bichromate
|bichromate of potash
|dipotassium dichromate
|dichromic acid, dipotassium salt
|chromic acid, dipotassium salt
|[[lópezite]]<ref>{{cite web|url=http://www.epa.gov/osw/hazard/wastetypes/wasteid/inorchem/docs/pot-dich.pdf|archive-url=https://web.archive.org/web/20110628190555/http://www.epa.gov/osw/hazard/wastetypes/wasteid/inorchem/docs/pot-dich.pdf|url-status=dead|archive-date=June 28, 2011|title=POTASSIUM DICHROMATE LISTING|publisher=US EPA|date=2015-07-23}}</ref>
}}
| Section1       = {{Chembox Identifiers
| CASNo = 7778-50-9
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = T4423S18FM
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 22910
| ChEMBL = 1374101
| SMILES = [K+].[K+].[O-][Cr](=O)(=O)O[Cr]([O-])(=O)=O
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI =1S/2Cr.2K.7O/q;;2*+1;;;;;;2*-1
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = KMUONIBRACKNSN-UHFFFAOYSA-N
| PubChem = 24502
| EINECS = 231-906-6
| RTECS = HX7680000
| UNNumber = 3288
  }}
| Section2       = {{Chembox Properties
| Formula = {{chem2|K2Cr2O7}}
| MolarMass = 294.185 g/mol
| Appearance = red-orange crystalline solid
| Odor = odorless
| Density = {{val|2.676|u=g/cm3}}, solid
| Solubility = {{ubl
 |4.9 g/100 mL ({{convert|0|C|F}})
 |13 g/100 mL ({{convert|20|C|F}})
 |102 g/100 mL ({{convert|100|C|F}})
 }}
| SolubleOther = insoluble in [[ethanol|alcohol]], [[acetone]].
| MeltingPtC = 398
| BoilingPtC = 500
| BoilingPt_notes = decomposes
| RefractIndex = 1.738
  }}
| Section3       = {{Chembox Structure
| Coordination = [[Tetrahedral]] (for Cr)
| CrystalStruct = [[Triclinic]] (α-form, <241.6&nbsp;°C)
  }}
| Section4       = {{Chembox Thermochemistry
| DeltaHf = −2033 kJ/mol
| Entropy = 291.2 J/(K·mol)
| HeatCapacity = 219 J/mol<ref>{{cite book|pages=405|isbn=978-3-527-30524-7|date=2002|title=Thermochemical Data of Elements and Compounds|first1=M.|last1=Binnewies|first2=E.|last2=Milke|location=Weinheim|publisher=[[Wiley-VCH]]|edition=2}}</ref>
}}
| Section7       = {{Chembox Hazards
| MainHazards= carcinogenic,<ref>Like all compounds of hexavalent chromium, potassium dichromate is carcinogenic</ref> corrosive
| ExternalSDS = [http://www.inchem.org/documents/icsc/icsc/eics1371.htm ICSC 1371] 
| GHSPictograms = {{GHS03}}{{GHS05}}{{GHS06}}{{GHS07}}{{GHS08}}{{GHS09}}<ref name="sigma">{{Sigma-Aldrich|id=207802|name=Potassium dichromate}}</ref>
| HPhrases = {{H-phrases|H272|H301|H312|H314|H317|H330|H334|H335|H340|H350|H360|H372|H410}}<ref name="sigma" />
| PPhrases = {{P-phrases|P201|P202|P210|P220|P221|P260|P264|P270|P271|P272|P273|P280|P284|P301 + P310 + P330|P301 + P330 + P331|P303 + P361 + P353|P304 + P340 + P310|P305 + P351 + P338 + P310|P308 + P313|P333 + P313|P342 + P311|P363|P370 + P378|P391|P403 + P233|P405|P501}}<ref name="sigma" />
| NFPA-H = 4
| NFPA-F = 0
| NFPA-R = 1
| NFPA-S = OX
| FlashPt = Non-flammable
| LD50 = 25 mg/kg (oral, rat)<ref>{{cite web|url=https://pubchem.ncbi.nlm.nih.gov/#tab/sidsrcname=ChemIDplus&query=7778-50-9&input_type=text|title=ChemIDplus - 7778-50-9 - KMUONIBRACKNSN-UHFFFAOYSA-N - Potassium dichromate - Similar structures search, synonyms, formulas, resource links, and other chemical information|first=Michael|last=Chambers}}</ref>
  }}
| Section8       = {{Chembox Related
| OtherAnions = [[Potassium chromate]]<br />[[Potassium molybdate]]<br />[[Potassium tungstate]]
| OtherCations = [[Ammonium dichromate]]<br />[[Sodium dichromate]]
| OtherCompounds = [[Potassium permanganate]]
  }}
}}

'''Potassium dichromate''' is the [[inorganic compound]] with the formula {{chem2|K2Cr2O7|auto=1}}.  An orange solid, it is used in diverse laboratory and industrial applications. As with all [[hexavalent chromium]] compounds, it is chronically harmful to health. It is a crystalline ionic solid with a very bright, red-orange color.  The salt is popular in laboratories because it is not [[deliquescent]], in contrast to the more industrially relevant salt [[sodium dichromate]].<ref name=Ullmann>Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. {{doi|10.1002/14356007.a07_067}}</ref>
{{TOC limit|3}}

==Production==
Potassium dichromate is usually prepared by the reaction of [[sodium dichromate]] and [[potassium chloride]].<ref name=Ullmann/>  Alternatively, it can be also obtained from [[potassium chromate]] by roasting [[chromite]] ore with [[potassium hydroxide]]:
:{{chem2|FeCr2O4  +  2 KOH  + 1.5 O2  ->  K2Cr2O7  + Fe(OH)2}}

==Structure==
The solid crystallizes as two [[Polymorphism (materials science)|polymorph]]s. These salts are soluble in water, and the dissolution process it ionizes, releasing {{chem2|Cr2O7(2-) }}.  Thus in aqueous solution, it is the [[dichromate]] ion that matters in terms of chemical reactions and environmental impact.  This anion is a corner-shared bitetrahedron, resembling [[pyrophosphate]].

==Reactions==
Potassium dichromate is an oxidising agent in [[organic chemistry]]. It is milder and more selective than [[potassium permanganate]]. It is used to [[organic redox reaction|oxidize]] [[Alcohol (chemistry)|alcohol]]s. It converts primary alcohols into [[aldehyde]]s and, under more forcing conditions, into carboxylic acids. In contrast, potassium permanganate tends to give carboxylic acids as the sole products. Secondary alcohols are converted into [[ketone]]s. For example, [[menthone]] may be prepared by oxidation of [[menthol]] when treated with acidified solution of dichromate.<ref>{{OrgSynth |title = ''l''-Menthone | author = L. T. Sandborn | collvol = 1 | collvolpages = 340 | prep = cv1p0340}}</ref> Tertiary alcohols cannot be oxidized.

When heated strongly, it decomposes with the evolution of oxygen.{{cn|date=May 2025}}
:{{chem2|4 K2Cr2O7 -> 4 [[potassium chromate|K2CrO4]] + 2 [[chromium trioxide|Cr2O3]] + 3 O2}}

When an [[alkali]] is added to an orange-red solution containing dichromate ions, a yellow solution is obtained due to the formation of [[Chromate and dichromate|chromate]] ions ({{chem2|CrO4(2-)}}). For example, potassium chromate is produced industrially using [[potassium carbonate]]:<ref name=Ullmann/>
:{{chem2|K2Cr2O7 + K2CO3 -> 2 K2CrO4 + CO2}}

Treatment with cold [[sulfuric acid]] gives red crystals of [[chromic anhydride]] (chromium trioxide, {{chem2|CrO3}}):{{cn|date=May 2025}}
:{{chem2|K2Cr2O7 + 2 H2SO4 -> 2 CrO3 + 2 [[potassium bisulfate|KHSO4]] + H2O}}
On heating with concentrated acid, oxygen is evolved:{{cn|date=May 2025}}
:{{chem2|2 K2Cr2O7 + 8 H2SO4 -> 2 K2SO4 + 2 Cr2(SO4)3 + 8 H2O + 3 O2}}

Potassium dichromate is readily reduced by [[sulfur dioxide]]:<ref>{{cite book|author=F. Hein and S. Herzog|chapter=Chromium, Molybdenum, Tungsten, Uranium|title=Handbook of Preparative Inorganic Chemistry, 2nd Ed. |editor=G. Brauer|publisher=Academic Press|year=1963|place=NY, NY|volume=2|pages=1366}}</ref>
:{{chem2|K2Cr2O7 +  H2SO4 + 3 SO2  -> K2SO4 + Cr2(SO4)3 + H2O}}
In addition to providing a route to [[chromium(III) sulfate]], this reaction was once the basis of a test for sulfur dioxide.

==Niche or archaic uses==
Potassium dichromate has few major applications, as the sodium salt is dominant industrially.  The main use is as a precursor to [[potassium chrome alum]], used in [[leather tanning]].<ref name=Ullmann/><ref>{{Cite journal|title=Footwear dermatitis|author1=M. Saha |author2=C. R. Srinivas |author3=S. D. Shenoy |author4=C. Balachandran |journal=Contact Dermatitis|volume=28|issue=5|pages=260–264|date=May 1993|doi=10.1111/j.1600-0536.1993.tb03428.x|pmid=8365123|s2cid=23159708 }}</ref>

===Photography and printing===
{{uncited section|date=May 2025}}
In 1839, [[Mungo Ponton]] discovered that paper treated with a solution of potassium dichromate was visibly tanned by exposure to sunlight, the discoloration remaining after the potassium dichromate had been rinsed out. In 1852, [[Henry Fox Talbot]] discovered that exposure to ultraviolet light in the presence of potassium dichromate hardened organic [[colloids]] such as [[gelatin]] and [[gum arabic]], making them less soluble.

These discoveries soon led to the [[carbon print]], [[gum bichromate]], and other photographic printing processes based on differential hardening. Typically, after exposure, the unhardened portion was rinsed away with warm water, leaving a thin relief that either contained a pigment included during manufacture or was subsequently stained with a dye. Some processes depended on the hardening only, in combination with the differential absorption of certain dyes by the hardened or unhardened areas. Because some of these processes allowed the use of highly stable dyes and pigments, such as [[carbon black]], prints with an extremely high degree of archival permanence and resistance to fading from prolonged exposure to light could be produced.

Dichromated colloids were also used as [[photoresist]]s in various industrial applications, most widely in the creation of metal printing plates for use in photomechanical printing processes.

''Chromium intensification'' or ''Photochromos'' 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 (that is, 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(VI) compounds have the property of [[Tanning (leather)|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 or similar material is stretched taut onto a frame similar to the way canvas is prepared before painting. A [[colloid]] sensitized with a dichromate is applied evenly to the taut screen. Once the dichromate mixture is dry, a full-size photographic positive is attached securely onto the surface of the screen, and the whole assembly exposed to strong light – times vary from 3 minutes to a half an hour in bright sunlight – hardening the exposed colloid. When the positive is removed, the unexposed mixture on the screen can be washed off with warm water, leaving the hardened mixture intact, acting as a precise mask of the desired pattern, which can then be printed with the usual screen-printing process.

===Analytical reagent===
Because it is non-hygroscopic, potassium dichromate was a common reagent in classical "wet tests" in analytical chemistry. 

====Ethanol determination====
Ethanol in a sample can be assayed using potassium dichromate, which oxidizes the ethanol to [[acetic acid]]:
:{{chem2|3 C2H5OH + 2 K2Cr2O7 + 8 H2SO4 -> 3 CH3COOH + 2  Cr2(SO4)3 + 2 K2SO4 + 11 H2O}}
This [[chemical reaction|reaction]] was once used in [[breathalyzer]] tests. When alcohol vapor makes contact with the orange dichromate-coated crystals, the color changes to Cr(III) green is directly related to the level of alcohol in the suspect's breath.

====Aldehyde test====
In an aqueous solution the color change exhibited can be used to test for distinguishing aldehydes from ketones. Aldehydes reduce dichromate from the +6 to the +3 [[oxidation state]], changing the solution color from orange to green. A ketone will show no such change because it cannot be oxidized further, and so the solution will remain orange.<ref>{{Cite web |title=oxidation of aldehydes and ketones |url=https://www.chemguide.co.uk/organicprops/carbonyls/oxidation.html |access-date=2025-10-30 |website=www.chemguide.co.uk}}</ref>

===Wood treatment===
Potassium dichromate is used to stain certain types of wood by darkening the tannins in the wood. It produces deep, rich browns that cannot be achieved with modern color dyes. It is a particularly effective treatment on [[mahogany]].<ref>{{cite book |last=Jewitt |first=Jeff |title=Hand-Applied Finishes |year=1997 |publisher=Taunton Press |location=Newtown, Connecticut |isbn=978-1-56158-154-2 |url-access=registration |url=https://archive.org/details/handappliedfinis0000jewi }}</ref>

==Natural occurrence==
[[File:Potassium dichromate (synthetic).jpg|thumb|A ~10 mm crystal of potassium dichromate in the same form as the mineral lópezite]]
Potassium dichromate occurs naturally as the rare mineral  [[lópezite]]. It has only been reported as [[vug]] fillings in the [[nitrate]] deposits of the [[Atacama Desert]] of [[Chile]] and in the [[Bushveld igneous complex]] of [[South Africa]].<ref>{{cite web|url=http://www.mindat.org/min-2433.html|title=Lópezite: Lópezite mineral information and data.}}</ref>

==Safety==
Potassium dichromate is a prevalent [[allergen]] in [[patch test]]s (4.8%). Its presence in [[cement]] can cause [[contact dermatitis]] in [[construction workers]] after extended exposure.<ref>{{cite book| doi=10.1002/14356007.a05_489.pub2 | chapter=Cement | title=Ullmann's Encyclopedia of Industrial Chemistry | date=2008 | last1=Sprung | first1=Siegbert | isbn=978-3-527-30385-4 }}</ref><ref>{{cite journal |doi=10.1111/cod.12203 |title=Severe occupational chromium allergy despite cement legislation |date=2014 |last1=Hedberg |first1=Yolanda S. |last2=Gumulka |first2=Martin |last3=Lind |first3=Marie-Louise |last4=Matura |first4=Mihály |last5=Lidén |first5=Carola |journal=Contact Dermatitis |volume=70 |issue=5 |pages=321–323 |pmid=24731090 }}</ref> In general, it is one of the most common causes of chromium [[dermatitis]].<ref>{{cite journal|doi= 10.1111/cod.12436|title= Chromium allergy and dermatitis: Prevalence and main findings|date= 2015|last1= Bregnbak|first1= David|last2= Johansen|first2= Jeanne D.|last3= Jellesen|first3= Morten S.|last4= Zachariae|first4= Claus|last5= Menné|first5= Torkil|last6= Thyssen|first6= Jacob P.|journal= Contact Dermatitis|volume= 73|issue= 5|pages= 261–280|pmid= 26104877}}</ref>  Aquatic organisms are vulnerable to poisoning by dichromate salts, but far less so than organic pollutants.<ref>{{cite journal |doi=10.1002/etc.2149 |title=Comparative Acute and Chronic Sensitivity of Fish and Amphibians: A Critical Review of Data |date=2013 |last1=Weltje |first1=Lennart |last2=Simpson |first2=Peter |last3=Gross |first3=Melanie |last4=Crane |first4=Mark |last5=Wheeler |first5=James R. |journal=Environmental Toxicology and Chemistry |volume=32 |issue=5 |pages=984–994 |pmid=23381988 |bibcode=2013EnvTC..32..984W }}</ref>

As with other Cr(VI) compounds, potassium dichromate is [[carcinogen]]ic.<ref name="book100C">{{cite book
 |author      = IARC
 |author-link  = International Agency for Research on Cancer
 |title       = Volume 100C: Arsenic, Metals, Fibres, and Dusts
 |orig-year    = 17-24 March 2009
 |url         = https://publications.iarc.fr/_publications/media/download/3026/50ed50733f7d1152d91b30a803619022ef098d59.pdf
 |access-date  = 2020-01-05
 |date        = 2012
 |isbn        = 978-92-832-0135-9
 |quote       = There is ''sufficient evidence'' in humans for the carcinogenicity of chromium (VI) compounds. Chromium (VI) compounds cause cancer of the lung. Also positive associations have been observed between exposure to Chromium (VI) compounds and cancer of the nose and nasal sinuses. There is ''sufficient evidence'' in experimental animals for the carcinogenicity of chromium (VI) compounds. Chromium (VI) compounds are ''carcinogenic to humans (Group 1)''.
 |publisher   = International Agency for Research on Cancer
 |location    = Lyon
 }}</ref> The compound is also [[corrosive]] and exposure may damage eyes<ref name="MSDS">{{cite web | publisher = JT Baker| title = Potassium dichromate MSDS | url = http://hazard.com/msds/mf/baker/baker/files/p5719.htm}}</ref> Human exposure further causes impaired fertility.<ref>{{Cite web |last=PubChem |title=Potassium Dichromate |url=https://pubchem.ncbi.nlm.nih.gov/compound/24502#section=GHS-Classification |access-date=2025-10-30 |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref>

==References==
{{reflist}}

==External links==
{{commonscat}}
* [http://www.periodicvideos.com/videos/mv_potassium_dichromate.htm Potassium Dichromate] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [http://www.inchem.org/documents/icsc/icsc/eics1371.htm International Chemical Safety Card 1371]
* [https://web.archive.org/web/20060203090143/http://www.npi.gov.au/database/substance-info/profiles/25.html National Pollutant Inventory – Chromium VI and compounds fact sheet]
* [https://www.cdc.gov/niosh/npg/npgd0138.html NIOSH Pocket Guide to Chemical Hazards]
* [https://web.archive.org/web/20040701090041/http://www-cie.iarc.fr/htdocs/monographs/vol49/chromium.html IARC Monograph "Chromium and Chromium compounds"]
* [http://www.goldrecovery.us/goldrecovery/documents/reactions.pdf Gold refining article listing color change when testing metals with Schwerter's Solution]

{{Potassium compounds}}
{{Chromates and dichromates}}
{{Authority control}}

{{DEFAULTSORT:Potassium Dichromate}}
[[Category:Potassium compounds]]
[[Category:Dichromates]]
[[Category:Photographic chemicals]]
[[Category:IARC Group 1 carcinogens]]
[[Category:Light-sensitive chemicals]]
[[Category:Oxidizing agents]]