Potassium dichromate

Potassium dichromate, K₂Cr₂O₇, is a common inorganic chemical reagent, most commonly used as an oxidising 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 vivid red-orange colour.

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Chemistry

Potassium dichromate is an oxidizer. The oxidation half-equation can be seen:

Cr₂O₇²⁻(aq) + 14H⁺ + 6e⁻ → 2Cr³⁺(aq) + 7H₂O +1.36 V

In organic chemistry, potassium dichromate is a milder oxidizer compared with potassium permanganate. It is used to oxidise 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 possible. For example, menthone may be prepared by oxidation of menthol with acidified dichromate.^[1]

Tertiary alcohols are not able to be oxidized to give a carbonyl group.

In an aqueous solution the colour 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 colour from orange to green. This is because the aldehyde can be further oxidised to the corresponding carboxylic acid. A ketone will show no such change because it cannot be oxidised further, and so the solution will remain orange.

Uses

Like other chromium(VI) compounds (chromium trioxide, sodium dichromate), potassium dichromate may be used to prepare "chromic acid", which can be used for cleaning glassware and etching materials.

Photography

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.

Chromium intensification 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.

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 of similar material that is required to be printed is then taped securely onto the surface of the screen and the whole thing exposed to strong light for a period - typically about half an hour in bright sunlight. When the design is removed, the gelatine on the screen is washed off with hot water. All the gelatine exposed to sun-light will have been hardened by the dichromate and will be retained on the screen leaving a precise mask of the required design which can be printed in the usual way.

Ethanol determination

The concentration of ethanol in a sample can be determined by back titration with acidified potassium dichromate. Adding 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.

Homeopathy

Due its irritant properties,^[2] potassium dichromate is used as a purported anti-pain remedy in homeopathy, where it is also called kalium bichromicum.^[3] However, in these remedies the substance is used in minute quantities. It is claimed that at these levels the compound instead acts to suppress symptoms of irritation and inflammation, though a recent meta-analysis of homeopathic treatments showed them to be unlikely to do any better than a placebo.^[4] One notable homeopathic product containing potassium dichromate is the headache treatment HeadOn, which contains the compound diluted to one part per million.

Wood treatment

Potassium dichromate is used to finish certain types of wood. It brings out the color and grain to a very deep, rich appearance. This is particularly effective on mahogany.^[5]

Safety

Potassium dichromate is one of the most common culprits in causing 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. It is also toxic, with an doses of approximately 100 mg/kg being fatal in rabbits and rodents.^[6]

As with other Cr^VI 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.^[2]

References

1. ^ L. T. Sandborn. "l-Menthone". Org. Synth.; Coll. Vol. 1: 340. 
2. ^ ^{a b} MSDS-Potassium_dichromate
3. ^ KALIUM BICHROMICUM http://homeoint.org
4. ^ Shang A, Huwiler-Müntener K, Nartey L, et al (2005). "Are the clinical effects of homoeopathy placebo effects? Comparative study of placebo-controlled trials of homoeopathy and allopathy". Lancet 366 (9487): 726–732. doi:10.1016/S0140-6736(05)67177-2. PMID 16125589.
5. ^ "(copper gilded changing screen)". Woodworks. DIY Network.
6. ^ Safety data for potassium dichromate HSci project