Potassium dichromate

Potassium dichromate
IUPAC name Potassium dichromate(VI)
Other names Potassium bichromate
Identifiers
CAS number	7778-50-9 Y
PubChem	516855
ChemSpider	22910 Y
EC number	231-906-6
UN number	3288
ChEMBL	CHEMBL1374101
RTECS number	HX7680000
Jmol-3D images	Image 1
SMILES [K+].[K+].[O-][Cr](=O)(=O)O[Cr]([O-])(=O)=O
InChI InChI=1S/2Cr.2K.7O/q;;2*+1;;;;;;2*-1 Y Key: KMUONIBRACKNSN-UHFFFAOYSA-N Y
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	398 °C
Boiling point	500 °C decomp.
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 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
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+) Harmful (Xn) Corrosive (C) Dangerous for the environment (N)
R-phrases	R45, R46, R60, R61, R8, R21, R25, R26, R34, R42/43, R48/23, R50/53
S-phrases	S53, S45, S60, S61
NFPA 704	0 4 1 OX
Flash point	non-flammable
Related compounds
Other anions	Potassium chromate Potassium molybdate Potassium tungstate
Other cations	Ammonium dichromate Sodium dichromate
Related compounds	Potassium permanganate
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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.

Chemistry

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.

Uses

Cleaning

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.

Construction

It is used as an ingredient in cement in which it retards the setting of the mixture and improves its density and texture. This usage commonly causes contact dermatitis in construction workers.

Ethanol determination

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.

Leather

It is used to tan leather which is used for footwear.

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.

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.

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 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.

Silver test

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.

Sulfur dioxide test

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.

Wood treatment

Potassium dichromate is used to stain certain types of wood by darkening the tannins in the wood. It produce 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 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.

Safety

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 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. Human exposure further encompasses impaired fertility, heritable genetic damage and harm to unborn children.

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"