{{Distinguish|Potassium chromate}} {{chembox | verifiedrevid = 406377229 | Name = Potassium dichromate | ImageFile = Potassium-dichromate-sample.jpg | ImageName = Potassium dichromate | ImageFile1 = <div style="font-size: 150%">{{chem2|2 K^{+} [O3Cr\sO\sCrO3](2-) }}</div> | ImageClass1 = bg-transparent | 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}} | Properties_ref = <ref name="CRC85">{{CRC85|chapter=4|page=4-76}}</ref> | Cr=2 |K=2 | O=7 | Appearance = red-orange crystalline solid | Odor = odorless | Density = {{val|2.68|u=g/cm3}}, solid | Solubility = {{val|15.1|u=g/100g}} | MeltingPtC = 398 | BoilingPtC = 500 | BoilingPt_notes = decomposes | RefractIndex = 1.738 }} | Section3 = {{Chembox Structure | Coordination = Tetrahedral (for Cr) | CrystalStruct = Triclinic (α-form, <{{convert|241.6|C|F K}}) }} | Section4 = {{Chembox Thermochemistry | DeltaHf = {{val|−2033|u=kJ/mol}} | Entropy = {{val|291.2|u=J|up=(K·mol)}} | HeatCapacity = {{val|219|u=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 | GHS_ref = <ref name="sigma">{{Sigma-Aldrich|sigald|id=207802|name=Potassium dichromate |accessdate=13 January 2026}}</ref> | GHSSignalWord = Danger | GHSPictograms = {{GHS03}}{{GHS05}}{{GHS06}}{{GHS07}}{{GHS08}}{{GHS09}} | HPhrases = {{H-phrases|H272|H301|H312|H314|H317|H330|H334|H335|H340|H350|H360|H372|H410}} | 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}} | NFPA-H = 4 | NFPA-F = 0 | NFPA-R = 1 | NFPA-S = OX | NFPA_ref = <ref name="FisherSDS">{{cite web |title=SDS - Potassium Dichromate |url=https://www.fishersci.com/store/msds?partNumber=P186500&productDescription=POTASSIUM+DICHROMATE+TECH+500G&vendorId=VN00033897&countryCode=US&language=en |website=fishersci.com |publisher=ThermoFisher Scientific |access-date=13 January 2026 |date=18 December 2025}}</ref> | LD50 = 25 mg/kg (oral, rat)<ref>{{cite web|url=https://pubchem.ncbi.nlm.nih.gov/compound/24502|title=PubChem: Potassium dichromate}}</ref> | LC50 = {{val|0.083|u=mg/L}} (Rat, female, inhalation, dust/mist)<ref name="sigma"/> | TLV-TWA = {{val|0.2|u=μg/m3}} | TLV-STEL = {{val|0.5|u=μg/m3}} Skin<ref name="FisherSDS"/> | PEL = {{val|5|u=μg/m3}} (TWA, as {{chem2|CrO3}}) | REL = {{val|0.2|u=μg/m3}} (TWA, as Cr) | IDLH = {{val|15|u=mg/m3}} (as Cr(VI)) | NIOSH_id = 0138 }} | Section8 = {{Chembox Related | OtherAnions = {{ubl |Potassium chromate |Potassium molybdate |Potassium tungstate }} | OtherCations = {{ubl |Ammonium dichromate |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>{{Ullmann |first1=Gerd |last1=Anger |first2=Jost |last2=Halstenberg |first3=Klaus |last3=Hochgeschwender |first4=Christoph |last4=Scherhag |first5=Ulrich |last5=Korallus |first6=Herbert |last6=Knopf |first7=Peter |last7=Schmidt |first8=Manfred |last8=Ohlinger |title=Chromium Compounds |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 in the presence of an acid.<ref name=Ullmann/> Potassium dichromate precipitates, because it has a lower solubility than the corresponding sodium salt. 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== [[Image:Potassium-dichromate-unit-cell-3D-balls.png|thumb|left|Unit cell of potassium dichromate]] X-ray crystallography shows that it is a salt. Two polymorphs are known, but the structure of the dichromate anion is the same in each. It ionizes when dissolved in water, 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.<ref>{{Cite journal |last1=Silber |first1=C. |last2=Breidenstein |first2=B. |last3=Heide |first3=G. |last4=Follner |first4=H. |date=September 1999 |title=Growth Mechanism and Crystal Form of Potassium Dichromate |journal=Crystal Research and Technology |language=en |volume=34 |issue=8 |pages=969–974 |doi=10.1002/(SICI)1521-4079(199909)34:8<969::AID-CRAT969>3.0.CO;2-T |bibcode=1999CryRT..34..969S }}</ref>
==Reactions== ===Inorganic=== When heated strongly, it decomposes with the evolution of oxygen.{{cn|date=May 2025}} :{{chem2|4 K2Cr2O7 -> 4 K2CrO4 + 2 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 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 of chromic anhydride (chromium trioxide, {{chem2|CrO3}}):<ref name=Ullmann/> :{{chem2|K2Cr2O7 + 2 H2SO4 -> 2 CrO3 + 2 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 |first1=F. |last1=Hein |first2=S. |last2=Herzog |chapter=Chromium, Molybdenum, Tungsten, Uranium |title=Handbook of Preparative Inorganic Chemistry, 2nd Ed. |editor=G. Brauer |date=2 December 2012 |publisher=Academic Press |orig-date=1963|place=NY, NY |volume=2 |pages=1366 |chapter-url=https://archive.org/details/Handbook_of_Preparative_Inorganic_Chemistry_1_2_Brauer/page/n1412/mode/1up |isbn=9780323161299}}</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.
Potassium trichromate ({{chem2|K2Cr3O10}}) is prepared by evaporating a solution of potassium dichromate in nitric acid (d = 1.19) over concentrated sulfuric acid at low temperature. The tetrachromate ({{chem2|K2Cr4O13}}) is prepared by evaporating a solution of the trichromate in nitric acid (d = 1.4-1.5) slowly on a sand bath.<ref name="EExploV3">{{cite tech report |title=Encyclopedia of Explosives and Related Items |volume=3, Chlorides through Detonating Relays |chapter=C - Trichromates - Potassium trichromate & Tetrachromates - Potassium tetrachromate |pages=C288-9 |first1=Basil T. |last1=Fedoroff |first2=Oliver E. |last2=Sheffield |publisher=U.S. Army Armament Research Development And Engineering Center - TACOM, ARDEC - Warheads, Energetics And Combat Support Armaments Center |date=1 January 1966 |url=https://apps.dtic.mil/sti/pdfs/AD0653029.pdf#page=74 |location=Picatinny Arsenal, NJ |id=AD0653029, PATR 2700}}</ref>
Potassium dichromate reacts with hydrogen chloride to give chromyl chloride:<ref>{{cite book |last=Sisler |first=Harry H. |title=Chromyl Chloride [Chromium(VI) Dioxychloride] |year=2006 |orig-year=1st pub. 1946 |series=Inorganic Syntheses |volume=2 |page=205 |editor-last=Fernelius |editor-first=W. Conard |url=https://onlinelibrary.wiley.com/doi/10.1002/9780470132333.ch63 |access-date=2026-05-15 |edition=1 |publisher=Wiley |language=en |doi=10.1002/9780470132333.ch63 |isbn=978-0-470-13161-9}}</ref> :{{chem2|K2Cr2O7 + 6 HCl → 2 CrO2Cl2 + 2 KCl + 3 H2O}}
===Analytical reagent=== Because it is non-hygroscopic, potassium dichromate was a common reagent in classical "wet tests" in analytical chemistry.
===Organic chemistry=== {{see|Oxidation with chromium(VI) complexes}} Potassium dichromate is an oxidising agent in organic chemistry. It is a member of a large collection of chromium-based oxidants.<ref>{{cite book|author=Cainelli, G.; Cardillo, G.|title=Chromium Oxidations in Organic Chemistry |publisher=Open Court Publishers | location=La Salle, IL|year=1981|page= 118–216.}}</ref> Oxidations are often conducted in the presence of sulfuric acid, which favors the formation of chromium trioxide, which is the active oxidation reagent.
It is milder and more selective than potassium permanganate. It is often used interchangeably with sodium dichromate, although the use of chromates has declined owing to environmental concerns (see Hexavalent chromium). Especially in the presence of acids such as sulfuric acid, it is well known to oxidize alcohols. It converts primary alcohols into aldehydes<ref>{{cite journal |title=Propionaldehyde |journal=Organic Syntheses |date=1932 |volume=12 |page=64 |doi=10.15227/orgsyn.012.0064|author=Charles D. Hurd and R. N. Meinert}}</ref> 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 ketones.<ref>{{cite journal |title=Methylenation of Carbonyl Compounds: (+)-3-Methylene-''cis''-p-Menthane |journal=Organic Syntheses |date=1987 |volume=65 |page=81 |doi=10.15227/orgsyn.065.0081|author=Luciano Lombardo}}</ref> Tertiary alcohols are not oxidized.
Oxidations by dichromate are accompanied by a color change from yellow-orange for Cr(VI) to green for Cr(III). The color change, a colorimetric test, exhibited can be used as a test to distinguish aldehydes from ketones. Aldehydes are oxidized further, whereas ketones resist oxidation by dichromate salts.<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>
In some cases, potassium dichromate can cleave C=C bonds.<ref>{{cite journal |title=Homophthalic Acid and Anhydride |journal=Organic Syntheses |date=1949 |volume=29 |page=49 |doi=10.15227/orgsyn.029.0049|author1=Oliver Grummitt |author2=Richard Egan |author3=Allen Buck}}</ref>
Potassium dichromate is a classical reagent for the preparation of naphthoquinones.<ref>{{cite journal |author=Louis F. Fieser|title=1,4-Naphthoquinone |journal=Organic Syntheses |date=1925 |volume=5 |page=79 |doi=10.15227/orgsyn.005.0079}}</ref>
===Leather=== 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>
===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== 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>{{Mindat |name=Lópezite |id=2433 |accessdate=13 January 2026}}</ref>
==Safety== Potassium dichromate is a prevalent allergen in patch tests (4.8%). Its presence in cement can cause contact dermatitis in construction workers after extended exposure.<ref>{{Ullmann |doi=10.1002/14356007.a05_489.pub2 |title=Cement |date=2008 |last1=Sprung |first1=Siegbert}}</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 carcinogenic.<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 |archive-date = 2020-03-17 |archive-url = https://web.archive.org/web/20200317095517/https://publications.iarc.fr/_publications/media/download/3026/50ed50733f7d1152d91b30a803619022ef098d59.pdf |url-status = dead }}</ref> The compound is also corrosive and exposure may damage eyes. Human exposure further causes impaired fertility.<ref name="FisherSDS"/>
==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) * [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://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] {{Webarchive|url=https://web.archive.org/web/20180403160333/http://www.goldrecovery.us/goldrecovery/documents/reactions.pdf |date=2018-04-03 }}
{{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