- Potassium dichromate
Potassium dichromate Identifiers CAS number PubChem ChemSpider EC number UN number 3288 ChEMBL RTECS number HX7680000 Jmol-3D images Image 1 Properties Molecular formula K2Cr2O7 Molar mass 294.185 g/mol Appearance red-orange crystalline solid Odor odorless Density 2.676 g/cm3, solid Melting point
Solubility in water 4.9 g/100 ml (0°C)
102 g/100 mL (100 °C)
Solubility insoluble in alcohol Structure Crystal structure Triclinic (α-form, <241.6 °C) Coordination
Tetrahedral (for Cr) Thermochemistry Std enthalpy of
o298 -2033 kJ/mol Standard molar
o298 291.2 J K−1 mol−1 Hazards MSDS ICSC 1371 EU Index 024-002-00-6 EU classification Oxidant (O)
Carc. Cat. 2
Muta. Cat. 2
Repr. Cat. 2
Highly toxic (T+)
Dangerous for the environment (N)
R-phrases , , , , , , , , , , , S-phrases , , , NFPA 704 Flash point non-flammable Related compounds Other anions Potassium chromate
Other cations Ammonium dichromate
Related compounds Potassium permanganate (what is: /?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Potassium dichromate, K2Cr2O7, is a common 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 very bright, red-orange color. It is also known as potassium bichromate; bichromate of potash; dipotassium dichromate; dichromic acid, dipotassium salt; chromic acid, dipotassium salt; and lopezite.
Potassium dichromate is an oxidant (oxidizing agent). The reduction half-equation can be seen:
- Cr2O72−(aq) + 14H+ + 6e− → 2Cr3+(aq) + 7H2O (E = +1.23 V)
In organic chemistry, potassium dichromate is a mild oxidizer compared with potassium permanganate. It is used to oxidize alcohols. 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 is possible. For example, menthone may be prepared by oxidation of menthol with acidified dichromate. Tertiary alcohols are not oxidized by potassium dichromate.
In an aqueous solution the color 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 color 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.
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:
- C2H5OH + [O] → CH3COOH
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.
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 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 (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.
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 negative is attached securely onto the surface of the screen, and the whole assembly exposed to strong light - typically about half an hour in bright sunlight - hardening the exposed colloid. When the negative 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.
When dissolved in an approximately 35% nitric acid solution it is called Schwerter's solution and is used to test for the presence of various metals, notably for determination of silver purity. Pure silver will turn the solution bright red, sterling silver will turn it dark red, low grade coin silver (0.800 fine) will turn brown (largely due to the presence of copper which turns the solution brown) and even green for 0.500 silver.
Potassium dichromate occurs naturally as the rare mineral lopezite. 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.
Potassium dichromate is one of the most common causes of chromium dermatitis; chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and fore-arms, which is chronic and difficult to treat. Toxicological studies have further illustrated its highly toxic nature. With rabbits and rodents, concentrations as low as 14 mg/kg have shown a 50% fatality rate amongst test groups.  Aquatic organisms are especially vulnerable if exposed, and hence responsible disposal according local environmental regulations is advised.
As with other CrVI compounds, potassium dichromate is carcinogenic 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. Human exposure further encompasses impaired fertility, heritable genetic damage and harm to unborn children.
Potassium dichromate paper can be used to test for sulfur dioxide, as it turns distinctively from orange to green. This is typical of all redox reactions where hexavalent chromium is reduced to the less harmful trivalent chromium. Therefore, it is not a conclusive test for sulfur dioxide.
- ^ "POTASSIUM DICHROMATE LISTING". US EPA. http://www.epa.gov/osw/hazard/wastetypes/wasteid/inorchem/docs/pot-dich.pdf.
- ^ L. T. Sandborn, "l-Menthone", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv1p0340 ; Coll. Vol. 1: 340
- ^ Pekka Roto, Hannele Sainio, Timo Reunala, Pekka Laippala (January 1996). "Addition of ferrous sulfate to cement and risk of chromium dermatitis among construction workers". Contact Dermatitis 34 (1): 43–50. doi:10.1111/j.1600-0536.1996.tb02111.x. PMID 8789225. http://www.blackwell-synergy.com/doi/abs/10.1111/j.1600-0536.1996.tb02111.x
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- ^ Farokh J. Master (2003). Diseases of Skin. New Delhi: B Jain Pub Pvt Ltd. p. 223. ISBN 8170211360. http://books.google.com/?id=OBAnewDc9CYC.
- ^ "Potassium dichromate MSDS". Sigma-Aldrich. http://www.sigmaaldrich.com/catalog/ProductDetail.do?D7=0&N5=SEARCH_CONCAT_PNO%7CBRAND_KEY&N4=60190%7CFLUKA&N25=0&QS=ON&F=SPEC. Retrieved 2011-07-20.
- ^ "Potassium dichromate MSDS". JT Baker. http://hazard.com/msds/mf/baker/baker/files/p5719.htm.
- International Chemical Safety Card 1371
- National Pollutant Inventory - Chromium VI and compounds fact sheet
- NIOSH Pocket Guide to Chemical Hazards
- European Chemicals Bureau
- IARC Monograph "Chromium and Chromium compounds"
KBr · KBrO3 · KCN · KCNO · KCl · KClO3 · KClO4 · KF · KH · KHCO2 · KHCO3 · KHF2 · KHSO3 · KHSO4 · KH2AsO4 · KI · KIO3 · KIO4 · KMnO4 · KN3 · KNO2 · KNO3 · KOCN · KOH · KO2 · KPF6 · KSCN · K2CO3 · K2CrO4 · K2Cr2O7 · K2FeO4 · K2MnO4 · K2O · K2O2 · K2PtCl4 · K2PtCl6 · K2S · K2SO3 · K2SO4 · K2SO5 · K2S2O5 · K2S2O7 · K2S2O8 · K2SiO3 · K3[Fe(CN)6] · K3[Fe(C2O4)3] · K4[Fe(CN)6] · K3PO4 · Categories:
- Potassium compounds
- Photographic chemicals
- IARC Group 1 carcinogens
- Light-sensitive chemicals
- Oxidizing agents
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Look at other dictionaries:
potassium dichromate — n a soluble salt K2Cr2O7 forming large orange red crystals that is used as an oxidizing agent and in the Golgi method of staining nerve tissue called also potassium bichromate * * * K2Cr2O7, an oxidizing agent used in Zenker fixative and other… … Medical dictionary
potassium dichromate — n. a yellowish red, crystalline compound, K2Cr2O7, used as an oxidizing agent and in photography, dyeing, etc … English World dictionary
potassium dichromate — noun an orange red salt used in making dyes and in photography • Hypernyms: ↑salt * * * noun or potassium bichromate : a bitter poisonous orange red crystalline salt K2Cr2O7 used chiefly in sensitizing gelatin in photography, in textile and… … Useful english dictionary
potassium dichromate — kalio dichromatas statusas T sritis chemija formulė K₂Cr₂O₇ atitikmenys: angl. potassium dichromate rus. калия дихромат; калия хромпик ryšiai: sinonimas – dikalio heptaoksodichromatas … Chemijos terminų aiškinamasis žodynas
potassium dichromate — noun A potassium salt of chromic acid, KCrO; the orange crystals are soluble in water, and it is used as an oxidizing agent and in photography. Syn: potassium bichromate … Wiktionary
potassium dichromate — Chem. an orange red, crystalline, water soluble, poisonous powder, K2Cr2O7, used chiefly in dyeing, photography, and as a laboratory reagent. Also called potassium bichromate. [1880 85] * * * … Universalium
potassium dichromate — noun Date: 1871 a soluble salt K2Cr2O7 forming large orange red crystals used especially in dyeing, in photography, and as an oxidizing agent … New Collegiate Dictionary
potassium dichromate — potas′sium dichro′mate n. chem. an orange red, crystalline, water soluble, poisonous powder, K2Cr2O7, used chiefly in dyeing and photography and as a laboratory reagent • Etymology: 1880–85 … From formal English to slang
potassium dichromate — /pəˌtæsiəm daɪˈkroʊmeɪt/ (say puh.taseeuhm duy krohmayt) noun an orange red crystalline compound, K2Cr2O7, used in dyeing, photography, etc … Australian English dictionary
Dichromate de potassium — Général Nom IUPAC Dichromate de potassium No CAS … Wikipédia en Français