Potassium dichromate

Source: Wikipedia, the free encyclopedia.
Not to be confused with Potassium chromate.
Potassium dichromate
Potassium dichromate
Unit cell of 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
  • lópezite[1]
Identifiers
3D model (
JSmol
)
ChEMBL
ChemSpider
ECHA InfoCard
 100.029.005 Edit this at Wikidata
EC Number
  • 231-906-6
RTECS number
  • HX7680000
UNII
UN number 3288
  • InChI=1S/2Cr.2K.7O/q;;2*+1;;;;;;2*-1 checkY
    Key: KMUONIBRACKNSN-UHFFFAOYSA-N checkY
  • [K+].[K+].[O-][Cr](=O)(=O)O[Cr]([O-])(=O)=O
Properties
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
4.9 g/100 mL (0 °C)
13 g/100 mL (20 °C)
102 g/100 mL (100 °C)
Solubility insoluble in alcohol, acetone.
1.738
Structure
Triclinic
(α-form, <241.6 °C)
Tetrahedral
(for Cr)
Thermochemistry
219 J/mol[2]
291.2 J/(K·mol)
Std enthalpy of
formation
(ΔfH⦵298)
 −2033 kJ/mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 carcinogenic,[4] corrosive
GHS labelling:
GHS03: OxidizingGHS05: CorrosiveGHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard[3]
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 4: Very short exposure could cause death or major residual injury. E.g. VX gasFlammability 0: Will not burn. E.g. waterInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazard OX: Oxidizer. E.g. potassium perchlorate
4
0
1
OX
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
25 mg/kg (oral, rat)[5]
Safety data sheet (SDS) ICSC 1371
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).
checkY verify (what is checkY☒N ?)

Potassium dichromate,

deliquescent, in contrast to the more industrially relevant salt sodium dichromate.[6]

Chemistry

Production

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

K2Cr2O7 → 2 K+ + Cr
2O2−
7
Cr
2O2−
7 + H2O ⇌ 2 CrO2−
4 + 2 H+

Reaction

Potassium dichromate is an oxidising 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.[7] 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.

4 K2Cr2O7 → 4 K2CrO4 + 2 Cr2O3 + 3 O2

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 (CrO2−4). For example, potassium chromate is produced industrially using potash:

K2Cr2O7 + K2CO3 → 2 K2CrO4 + CO2

The reaction is reversible.

Treatment with cold

chromic anhydride
(chromium trioxide, CrO3):

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

On heating with concentrated acid, oxygen is evolved:

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

Uses

Potassium dichromate has few major applications, as the sodium salt is dominant industrially. The main use is as a precursor to

leather tanning.[6][8]

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

construction workers.[9]

Photography and printing

In 1839,

colloids such as gelatin and gum arabic
, making them less soluble.

These discoveries soon led to the

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

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.

screen-printing
.

In

screen-printing
process.

Analytical reagent

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

Ethanol determination

Acidified solution of potassium dichromate

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:

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

The excess dichromate is determined by titration against

dichromate
solution against a blank.

One major application for this

Cr(VI)
orange to Cr(III) 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%

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.

SO2 + K2Cr2O7 + 3H2SO4 → K2SO4 + Cr2(SO4)3 + 3 H2O

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.[10]

Natural occurrence

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

Potassium dichromate occurs naturally as the rare

Bushveld igneous complex of South Africa.[11]

Safety

Patch test

In 2005–06, potassium dichromate was the 11th-most-prevalent allergen in patch tests (4.8%).[12]

Potassium dichromate is one of the most common causes of chromium dermatitis;[13] chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and forearms, 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.[14] Aquatic organisms are especially vulnerable if exposed, and hence responsible disposal according to the local environmental regulations is advised.

As with other

corrosive and exposure may produce severe eye damage or blindness.[16]
Human exposure further encompasses impaired fertility.

References

  1. ^ "POTASSIUM DICHROMATE LISTING" (PDF). US EPA. 2015-07-23.
  2. ISBN 978-3-527-30524-7
    .
  3. ^ Sigma-Aldrich Co., Chromium(VI) oxide. Retrieved on 2014-06-15.
  4. ^ Like all compounds of hexavalent chromium, potassium dichromate is carcinogenic
  5. ^ Chambers, Michael. "ChemIDplus - 7778-50-9 - KMUONIBRACKNSN-UHFFFAOYSA-N - Potassium dichromate - Similar structures search, synonyms, formulas, resource links, and other chemical information".
  6. ^
    doi:10.1002/14356007.a07_067
  7. ^ L. T. Sandborn. "l-Menthone". Organic Syntheses; Collected Volumes, vol. 1, p. 340.
  8. S2CID 23159708
    .
  9. S2CID 27027304
    .
  10. ISBN 978-1-56158-154-2
    .
  11. ^ "Lópezite: Lópezite mineral information and data".
  12. ^ Zug KA, Warshaw EM, Fowler JF Jr, Maibach HI, Belsito DL, Pratt MD, Sasseville D, Storrs FJ, Taylor JS, Mathias CG, Deleo VA, Rietschel RL, Marks J. Patch-test results of the North American Contact Dermatitis Group 2005–2006. Dermatitis. 2009 May–Jun;20(3):149-60.
  13. ISBN 978-81-7021-136-5
    .
  14. ^ "Potassium dichromate MSDS". Sigma-Aldrich. Retrieved 2011-07-20.
  15. ISBN 978-92-832-0135-9
    . Retrieved 2020-01-05. There is sufficient evidence in humans for the carcinogenicity of chromium (VI) compounds. Chromium (VI) compounds cause cancer of the lung. Also positive associations have been observed between exposure to Chromium (VI) compounds and cancer of the nose and nasal sinuses. There is sufficient evidence in experimental animals for the carcinogenicity of chromium (VI) compounds. Chromium (VI) compounds are carcinogenic to humans (Group 1).
  16. ^ "Potassium dichromate MSDS". JT Baker.

External links

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