 | |
 | |
| Names | |
|---|---|
| IUPAC name Potassium dichromate(VI) | |
| Other names | |
| Identifiers | |
3D model (JSmol) | |
| ChEMBL | |
| ChemSpider |
|
| ECHA InfoCard | 100.029.005 |
| EC Number |
|
| RTECS number |
|
| UNII | |
| UN number | 3288 |
| |
| |
| Properties[2] | |
| K2Cr2O7 | |
| Molar mass | 294.182 g·mol−1 |
| Appearance | red-orange crystalline solid |
| Odor | odorless |
| Density | 2.68 g/cm3, solid |
| Melting point | 398 °C (748 °F; 671 K) |
| Boiling point | 500 °C (932 °F; 773 K) decomposes |
| 15.1 g/100g | |
Refractive index (nD) | 1.738 |
| Structure | |
| Triclinic (α-form, <241.6 °C (466.9 °F; 514.8 K)) | |
| Tetrahedral (for Cr) | |
| Thermochemistry | |
| 219 J/mol[3] | |
Std molar entropy(S⦵298) | 291.2 J/(K·mol) |
Std enthalpy of formation(ΔfH⦵298) | −2033 kJ/mol |
| Hazards | |
| GHS labelling:[6] | |
      | |
| Danger | |
| H272,H301,H312,H314,H317,H330,H334,H335,H340,H350,H360,H372,H410 | |
| 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 704 (fire diamond) | |
Threshold limit value (TLV) | 0.2 μg/m3 (TWA), 0.5 μg/m3 Skin[4] (STEL) |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 25 mg/kg (oral, rat)[5] |
LC50 (median concentration) | 0.083 mg/L (Rat, female, inhalation, dust/mist)[6] |
| NIOSH (US health exposure limits):[7] | |
PEL (Permissible) | 5 μg/m3 (TWA, asCrO3) |
REL (Recommended) | 0.2 μg/m3 (TWA, as Cr) |
IDLH (Immediate danger) | 15 mg/m3 (as Cr(VI)) |
| Related compounds | |
Otheranions | |
Othercations | |
Related compounds | Potassium permanganate |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Potassium dichromate is theinorganic compound with the formulaK2Cr2O7. An orange solid, it is used in diverse laboratory and industrial applications. As with allhexavalent 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 notdeliquescent, in contrast to the more industrially relevant saltsodium dichromate.[8]
Potassium dichromate is usually prepared by the reaction ofsodium dichromate andpotassium chloride in the presence of an acid.[8] Potassium dichromate precipitates, because it has a lower solubility than the corresponding sodium salt. Alternatively, it can be also obtained frompotassium chromate by roastingchromite ore withpotassium hydroxide:
The solid crystallizes as twopolymorphs. These salts are soluble in water, and the dissolution process it ionizes, releasingCr2O2−7. Thus in aqueous solution, it is thedichromate ion that matters in terms of chemical reactions and environmental impact. This anion is a corner-shared bitetrahedron, resemblingpyrophosphate.[9]
Potassium dichromate is an oxidising agent inorganic chemistry. It is milder and more selective thanpotassium permanganate. It is used tooxidizealcohols. It converts primary alcohols intoaldehydes 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 intoketones. For example,menthone may be prepared by oxidation ofmenthol when treated with acidified solution of dichromate.[10] Tertiary alcohols cannot be oxidized.
When heated strongly, it decomposes with the evolution of oxygen.[citation needed]
When analkali is added to an orange-red solution containing dichromate ions, a yellow solution is obtained due to the formation ofchromate ions (CrO2−4). For example, potassium chromate is produced industrially usingpotassium carbonate:[8]
Treatment with coldsulfuric acid gives red crystals ofchromic anhydride (chromium trioxide,CrO3):[citation needed]
On heating with concentrated acid, oxygen is evolved:[citation needed]
Potassium dichromate is readily reduced bysulfur dioxide:[11]
In addition to providing a route tochromium(III) sulfate, this reaction was once the basis of a test for sulfur dioxide.
Potassium trichromate (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 (K2Cr4O13) is prepared by evaporating a solution of the trichromate in nitric acid (d = 1.4-1.5) slowly on a sand bath.[12]
Potassium dichromate has few major applications, as the sodium salt is dominant industrially. The main use is as a precursor topotassium chrome alum, used inleather tanning.[8][13]
 | This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(May 2025) (Learn how and when to remove this message) |
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 organiccolloids such asgelatin andgum arabic, making them less soluble.
These discoveries soon led to thecarbon 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 ascarbon 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 asphotoresists in various industrial applications, most widely in the creation of metal printing plates for use in photomechanical printing processes.
Chromium intensification orPhotochromos uses potassium dichromate together with equal parts of concentratedhydrochloric 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 tosilver chloride. After thorough washing and exposure toactinic 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 insulfuric 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 thesilver metal tosilver 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 withsolarisation so that the end product resembles a negative and is suitable for printing in the normal way.
Cr(VI) compounds have the property oftanning animalproteins when exposed to strong light. This quality is used in photographicscreen-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. Acolloid sensitized with a dichromate is applied evenly to the taut screen. Once the dichromate mixture is dry, a full-sizephotographic 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.
Because it is non-hygroscopic, potassium dichromate was a common reagent in classical "wet tests" in analytical chemistry.
| This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(January 2026) (Learn how and when to remove this message) |
Ethanol in a sample can be assayed using potassium dichromate, which oxidizes the ethanol toacetic acid:
Thisreaction was once used inbreathalyzer 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.
In an aqueous solution the color change exhibited can be used as a test to distinguish aldehydes from ketones. Aldehydes reduce dichromate from the +6 to the +3oxidation 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.[14]
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 onmahogany.[15]
.jpg)
Potassium dichromate occurs naturally as the rare minerallópezite. It has only been reported asvug fillings in thenitrate deposits of theAtacama Desert ofChile and in theBushveld igneous complex ofSouth Africa.[16]
Potassium dichromate is a prevalentallergen inpatch tests (4.8%). Its presence incement can causecontact dermatitis inconstruction workers after extended exposure.[17][18] In general, it is one of the most common causes of chromiumdermatitis.[19] Aquatic organisms are vulnerable to poisoning by dichromate salts, but far less so than organic pollutants.[20]
As with other Cr(VI) compounds, potassium dichromate iscarcinogenic.[21] The compound is alsocorrosive and exposure may damage eyes. Human exposure further causes impaired fertility.[4]
There issufficient 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 issufficient evidence in experimental animals for the carcinogenicity of chromium (VI) compounds. Chromium (VI) compounds arecarcinogenic to humans (Group 1).
