Potassium Bichromate

Potassium dichromate is the inorganic compound with the formula . 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.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.

Production

Potassium dichromate is usually prepared by the reaction of sodium dichromate and potassium chloride. Alternatively, it can be also obtained from potassium chromate by roasting chromite ore with potassium hydroxide:
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Structure

The solid crystallizes as two polymorphs. These salts are soluble in water, and the dissolution process it ionizes, releasing . 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 oxidize alcohols. It converts primary alcohols into aldehydes 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. For example, menthone may be prepared by oxidation of menthol when treated with acidified solution of dichromate. Tertiary alcohols cannot be oxidized.

When heated strongly, it decomposes with the evolution of oxygen.
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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 (). For example, potassium chromate is produced industrially using potassium carbonate:
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Treatment with cold sulfuric acid gives red crystals of chromic anhydride (chromium trioxide, ):
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On heating with concentrated acid, oxygen is evolved:
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Potassium dichromate is readily reduced by sulfur dioxide:
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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.

Photography and printing

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 photoresists 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 animal proteins 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:
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This 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.

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.

Natural occurrence

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.

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. In general, it is one of the most common causes of chromium dermatitis. Aquatic organisms are vulnerable to poisoning by dichromate salts, but far less so than organic pollutants.

As with other Cr(VI) compounds, potassium dichromate is carcinogenic. The compound is also corrosive and exposure may damage eyes Human exposure further causes impaired fertility.

References

External links

Potassium Dichromate
at ''The Periodic Table of Videos'' (University of Nottingham)

International Chemical Safety Card 1371


* ttps://www.cdc.gov/niosh/npg/npgd0138.html NIOSH Pocket Guide to Chemical Hazards
IARC Monograph "Chromium and Chromium compounds"

Gold refining article listing color change when testing metals with Schwerter's Solution

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