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Ammonium dichromate
Chemical compound

Ammonium dichromate is an inorganic compound with the chemical formula (NH4)2Cr2O7, composed of ammonium and dichromate ions. In this compound, chromium exists in the +6 oxidation state, also called hexavalent chromium. Historically, ammonium dichromate was used in early photography and pyrotechnics. It is well known for its role in tabletop "volcano" demonstrations, though this use has declined in schools due to the compound’s carcinogenic properties.

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Properties

At standard temperature and pressure, the compound exists as orange, acidic crystals soluble in water and ethanol. It is formed by the action of chromic acid on ammonium hydroxide with subsequent crystallisation.2

The (NH4)2Cr2O7 crystal (C2/c, z = 4) contains a single type of ammonium ion, at sites of symmetry C1(2,3). Each NH+4 centre is surrounded irregularly by eight oxygen atoms at N−O distances ranging from ca. 2.83 to 3.17 Å, typical of hydrogen bonds.3

Uses

It has been used in pyrotechnics and in the early days of photography as well as in lithography, as a source of pure nitrogen in the laboratory, and as a catalyst.4 It is also used as a mordant for dyeing pigments, in manufacturing of alizarin, chrome alum, leather tanning and oil purification.5

Photosensitive films containing PVA, ammonium dichromate, and a phosphor are spin-coated as aqueous slurries in the production of the phosphor raster of television screens and other devices. The ammonium dichromate acts as the photoactive site.6

Reactions

Tabletop volcanoes and thermal decomposition

The volcano demonstration involves igniting a pile of the salt, which initiates the following exothermic conversion: 7

(NH4)2Cr2O7(s) → Cr2O3(s) + N2(g) + 4 H2O(g) (ΔH = −429.1±3 kcal/mol)

Like ammonium nitrate, it is thermodynamically unstable.89 Its decomposition reaction proceeds to completion once initiated, producing voluminous dark green powdered chromium(III) oxide. Not all of the ammonium dichromate decomposes in this reaction. When the green powder is brought into water a yellow/orange solution is obtained from left over ammonium dichromate.

Observations obtained using relatively high magnification microscopy during a kinetic study of the thermal decomposition of ammonium dichromate provided evidence that salt breakdown proceeds with the intervention of an intermediate liquid phase rather than a solid phase. The characteristic darkening of (NH4)2Cr2O7 crystals as a consequence of the onset of decomposition can be ascribed to the dissociative loss of ammonia accompanied by progressive anion condensation to Cr3O2−10, Cr4O2−13, etc., ultimately yielding CrO3. The CrO3 has been identified as a possible molten intermediate participating in (NH4)2Cr2O7 decomposition.10

Oxidation reactions

Ammonium dichromate is a strong oxidising agent and reacts, often violently, with any reducing agent. The stronger the reducing agent, the more violent the reaction.11 It has also been used to promote the oxidation of alcohols and thiols. Ammonium dichromate, in the presence of Mg(HSO4)2 and wet SiO2 can act as a very efficient reagent for the oxidative coupling of thiols under solvent free conditions. The reactions produces reasonably good yields under relatively mild conditions.12 The compound is also used in the oxidation of aliphatic alcohols to their corresponding aldehydes and ketones in ZrCl4/wet SiO2 in solvent free conditions, again with relatively high yields.13

Safety

Ammonium dichromate is highly toxic. Like many hexavalent chromium compounds it is a proven carcinogen, mutagen, and reproductive toxin. It ranges from having a strong irritant effect on skin to causing severe chemical burns and skin corrosion. Inhalation of dust may be fatal.14

Incidents

In sealed containers, ammonium dichromate is likely to explode if heated.15 In 1986, two workers were killed and 14 others injured at Diamond Shamrock Chemicals in Ashtabula, Ohio, when 2,000 lb (910 kg) of ammonium dichromate exploded as it was being dried in a heater.16

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References

  1. "Ammonium Dichromate Volcano". Chemical Education Xchange. Division of Chemical Education, Inc., American Chemical Society. Archived from the original on 2024-12-08. Retrieved 2025-04-27. https://www.chemedx.org/video/ammonium-dichromate-volcano ↩

  2. Lewis, Richard J. (2007). Hawley's condensed chemical dictionary (15th ed.). Hoboken (N.J.): Wiley-Interscience. ISBN 978-0-471-76865-4. 978-0-471-76865-4 ↩

  3. Keresztury, G.; Knop, O. (1982). "Infrared spectra of the ammonium ion in crystals. Part XII. Low-temperature transitions in ammonium dichromate, (NH4)2Cr2O7". Can. J. Chem. 60 (15): 1972–1976. doi:10.1139/v82-277. /wiki/Can._J._Chem. ↩

  4. Patnaik, Pradyot (2003). Handbook of inorganic chemicals. New York: McGraw-Hill. ISBN 0-07-049439-8. 0-07-049439-8 ↩

  5. Lewis, Richard J. (2007). Hawley's condensed chemical dictionary (15th ed.). Hoboken (N.J.): Wiley-Interscience. ISBN 978-0-471-76865-4. 978-0-471-76865-4 ↩

  6. Havard, J. M.; Shim, S. Y.; Fr; eacute; chet, J. M. (1999). "Design of Photoresists with Reduced Environmental Impact. 1. Water-Soluble Resists Based on Photo-Cross-Linking of Poly(vinyl alcohol)". Chem. Mater. 11 (3): 719–725. doi:10.1021/cm980603y. /wiki/Chem._Mater. ↩

  7. Neugebauer, C. A.; Margrave, J. L. (1957). "The Heat of Formation of Ammonium Dichromate". J. Phys. Chem. 61 (10): 1429–1430. doi:10.1021/j150556a040. /wiki/J._Phys._Chem. ↩

  8. Young, Jay A. (1 November 2005). "Ammonium Dichromate". Journal of Chemical Education. 82 (11): 1617. doi:10.1021/ed082p1617. /wiki/Journal_of_Chemical_Education ↩

  9. G. A. P. Dalgaard; A. C. Hazell; R. G. Hazell (1974). "The Crystal Structure of Ammonium Dichromate, (NH4)2Cr2O7". Acta Chemica Scandinavica. A28: 541–545. doi:10.3891/acta.chem.scand.28a-0541. https://doi.org/10.3891%2Facta.chem.scand.28a-0541 ↩

  10. Galwey, Andrew K.; Pöppl, László; Rajam, Sundara (1983). "A Melt Mechanism for the Thermal Decomposition of Ammonium Dichromate". J. Chem. Soc., Faraday Trans. 1. 79 (9): 2143–2151. doi:10.1039/f19837902143. /wiki/J._Chem._Soc.,_Faraday_Trans._1 ↩

  11. Young, Jay A. (1 November 2005). "Ammonium Dichromate". Journal of Chemical Education. 82 (11): 1617. doi:10.1021/ed082p1617. /wiki/Journal_of_Chemical_Education ↩

  12. Shirini, F.; et al. (2003). "Solvent free oxidation of thiols by (NH4)2Cr2O7 in the presence of Mg(HSO4)2 and wet SiO2". Journal of Chemical Research. 2003: 28–29. doi:10.3184/030823403103172823. S2CID 197126514. https://doi.org/10.3184%2F030823403103172823 ↩

  13. Shirini, F.; et al. (2001). "ZrCl4/wet SiO2 promoted oxidation of alcohols by (NH4)2Cr2O7 in solution and solvent free condition". J. Chem. Research (S). 2001 (11): 467–477. doi:10.3184/030823401103168541. S2CID 197118772. https://doi.org/10.3184%2F030823401103168541 ↩

  14. Sigma-Aldrich Co., Ammonium dichromate. Retrieved on 2013-07-20. /wiki/Sigma-Aldrich ↩

  15. Young, Jay A. (1 November 2005). "Ammonium Dichromate". Journal of Chemical Education. 82 (11): 1617. doi:10.1021/ed082p1617. /wiki/Journal_of_Chemical_Education ↩

  16. Diamond, S. (19 January 1986). "Chemical in Blast Was Being Heated". The New York Times. p. 22. Retrieved 26 June 2025. https://www.nytimes.com/1986/01/19/us/chemical-in-blast-was-being-heated.html ↩