potassium dichromate

Potassium dichromate
IUPAC name	Potassium dichromate(VI)
Other names	Potassium bichromate
Identifiers
CAS number	7778-50-9
EINECS number	231-906-6
RTECS number	HX7680000
Properties
Molecular formula	K2Cr2O7
Molar mass	294.19 g/mol
Appearance	Red-orange crystalline solid
Density	2.676 g/cm3, solid
Melting point	398°C
Boiling point	500°C decomp.
Solubility in water	4.9 g/100 ml (0°C)
Structure
Crystal structure	Triclinic (α-form,<241.6 °C
Coordination geometry	Tetrahedral (for Cr)
Thermochemistry
Std enthalpy of formation ΔfHo298	-2033 kJ/mol
Standard molar entropy So298	291,2 J.K−1.mol−1
Hazards
Main hazards	Highly toxic Carc. Cat. 1 Muta. Cat. 2 Repr. Cat. 2 Oxidant Dangerous for the environment
R-phrases	R45, R46, R60, R61, R8, R21, R25, R26, R34, R42/43, R48/23, R50/53
S-phrases	S53, S45, S60, S61
Flash point	Non-flammable
Related Compounds
Other anions	Potassium chromate Potassium molybdate Potassium tungstate
Other cations	Ammonium dichromate Sodium dichromate
Related compounds	Potassium permanganate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

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.

Uses in organic chemistry

Potassium dichromate is used to oxidise alcohols. It converts primary alcohols into aldehydes, or into carboxylic acids if heated under reflux. Secondary alcohols are converted into ketones, with no further oxidation possible. Tertiary alcohols are not oxidized, due to lack of an additional hydrogen to be eliminated to form a carbonyl group.

In an aqueous solution the colour change exhibited can be use 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.

Ethanol determination

Potassium dichromate (usually acidified with sulfuric acid), or any other dichromate for that matter, can be used to determine the amount of ethanol in a solution using back titration. First a known amount of dichromate is added, enough to react with all the ethanol and leave some spare in the solution. A blank solution with no ethanol is titrated using sodium thiosulfate with iodide ions (from e.g. potassium iodide) in the solution and a small amount of starch used as an indicator. The dichromate reacts with the iodide ions forming iodine, which will then react with thiosulfate ions forming iodide ions again. In this way, when all the dichromate has been used up, the thiosulfate will convert iodine to iodide ions and the blue black colour from the starch will disappear, giving a clear endpoint. From here, calculations using balanced formulae easily yield the amount of ethanol in the original solution.

Other Applications

K₂Cr₂O₇ is used as an oxidizing agent in many chemical applications, and is often used for cleaning laboratory glassware of organic contaminants, usually in a solution with concentrated sulfuric acid. This solution must not be used to clean the glass tubes used in NMR spectroscopy, as residual contamination of the glass by the paramagnetic Chromium disrupts the NMR procedure.

Potassium dichromate also 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.

Hazards

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. As with other Cr^VI compounds, potassium dichromate is carcinogenic and should be handled with gloves and appropriate health and safety protection. Potassium dichromate is listed as one of the ingredients in the migraine over-the-counter homeopathic medication called HeadOn, though the product contains only one part per million of dichromate.

References

External links

• 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"

Donate to Wikimedia