Potassium dichromate

Not to be confused with Potassium chromate.

Potassium dichromate

Names
IUPAC name Potassium dichromate(VI)
Other names potassium bichromate bichromate of potash dipotassium dichromate dichromic acid, dipotassium salt chromic acid, dipotassium salt lopezite
Identifiers
CAS Number	7778-50-9 Y
ChEMBL	ChEMBL1374101
ChemSpider	22910 Y
EC Number	231-906-6
Jmol 3D model	Interactive image
PubChem	24502
RTECS number	HX7680000
UN number	3288
InChI InChI=1S/2Cr.2K.7O/q;;2*+1;;;;;;2*-1 Y Key: KMUONIBRACKNSN-UHFFFAOYSA-N Y
SMILES [K+].[K+].[O-][Cr](=O)(=O)O[Cr]([O-])(=O)=O
Properties
Chemical 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 (748 °F; 671 K)
Boiling point	500 °C (932 °F; 773 K) decomposes
Solubility in water	4.9 g/100 mL (0 °C) 102 g/100 mL (100 °C)
Solubility	insoluble in alcohol
Refractive index (nD)	1.738
Structure
Crystal structure	Triclinic (α-form, <241.6 °C)
Coordination geometry	Tetrahedral (for Cr)
Thermochemistry
Std molar entropy (So298)	291.2 J K−1 mol−1
Std enthalpy of formation (ΔfHo298)	-2033 kJ/mol
Vapor pressure	{{{value}}}
Related compounds
Other anions	Potassium chromate Potassium molybdate Potassium tungstate
Other cations	Ammonium dichromate Sodium dichromate
Related compounds	Potassium permanganate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y verify (what is YN ?)
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 acutely and chronically harmful to health. It is a crystalline ionic solid with a very bright, red-orange color. The salt is popular in the laboratory because it is not deliquescent, in contrast to the more industrially relevant salt sodium dichromate.

Chemistry

Production

Potassium dichromate is usually prepared by the reaction of potassium chloride on sodium dichromate. Alternatively, it can be obtained from potassium chromate by roasting chrome ore with potassium hydroxide. It is soluble in water and in the dissolution process it ionizes:

K₂Cr₂O₇ → 2 K⁺ + Cr₂O₇²⁻

Cr₂O₇²⁻ + H₂O ⇌ 2 CrO₄²⁻ + 2 H⁺

Reactions

Potassium dichromate is an oxidizing agent in organic chemistry, and is milder 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 with acidified dichromate. Tertiary alcohols cannot be oxidized.

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 color from orange to green. This color change arises because the aldehyde can be 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.

When heated strongly, it decomposes with the evolution of oxygen.

4K₂Cr₂O₇ → 4K₂CrO₄ + 2Cr₂O₃ + 3O₂

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 potash:

K₂Cr₂O₇ + K₂CO₃ → 2 K₂CrO₄ + CO₂

The reaction is reversible.

Treatment with cold sulphuric acid gives red crystals of chromic anhydride (CrO₃):

K₂Cr₂O₇ + 2H₂SO₄ → 2CrO₃ + 2 KHSO₄ + H₂O

On heating with concentrated acid, oxygen is evolved:

2 K₂Cr₂O₇ + 8H₂SO₄ → 2 K₂SO₄ + 2 Cr₂(SO₄)₃ + 8 H₂O + 3O₂

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.

Cleaning

Like other chromium(VI) compounds (chromium trioxide, sodium dichromate), potassium dichromate has been used to prepare "chromic acid" for cleaning glassware and etching materials. Because of safety concerns associated with hexavalent chromium, this practice has been largely discontinued.

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.

Photography

Potassium dichromate has uses in photography and in photographic screen printing, where it is used as an oxidizing agent together with a strong mineral acid.

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 (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.

As an analytical reagent

Because it is non-hygroscopic, potassium dichromate is a common reagent in classical "wet tests" in analytical chemistry.

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:

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

Full reaction of converting ethanol to acetic acid:

3C₂H₅OH + 2K₂Cr₂O₇ + 8H₂SO₄ → 3CH₃COOH + 2Cr₂(SO4)₃ + 2K₂SO₄ + 11H₂O

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 orange dichromate-coated crystals, the color changes from orange to green. The degree of the color change is directly related to the level of alcohol in the suspect's breath.

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. Brass turns dark brown, copper turns brown, lead and tin both turn yellow while gold and palladium do not change.

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 trivalent chromium. Therefore, it is not a conclusive test for sulfur dioxide. The final product formed is Cr₂(SO₄)₃.

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

A ~10 mm crystal of potassium dichromate in the same form as the mineral lopezite

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

Patch test

In 2005–06, potassium dichromate was the 11th-most-prevalent allergen in patch tests (4.8%).

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

• Potassium Dichromate at The Periodic Table of Videos (University of Nottingham)
• International Chemical Safety Card 1371
• National Pollutant Inventory – Chromium VI and compounds fact sheet
• 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