:Ammonium dichromate

At room temperature and pressure, the compound exists as orange, acidic crystals soluble in water and alcohol. It is formed by the action of chromic acid on ammonium hydroxide with subsequent crystallisation.Richard J. Lewis Hawley's Condensed Chemical Dictionary. Wiley & Sons, Inc: New York, 2007 {{ISBN|978-0-471-76865-4}}

The (NH₄)₂Cr₂O₇ crystal (C2/c, z = 4) contains a single type of ammonium ion, at sites of symmetry C₁(2,3). Each {{chem|NH|4|+}} centre is surrounded irregularly by eight oxygen atoms at N—O distances ranging from ca. 2.83 to ca. 3.17 Å, typical of hydrogen bonds.{{Cite journal |last1=Keresztury, G. |last2=Knop, O. |year=1982 |title=Infrared spectra of the ammonium ion in crystals. Part XII. Low-temperature transitions in ammonium dichromate, (NH₄)₂Cr₂O₇ |journal=Can. J. Chem. |volume=60 |issue=15 |pages=1972–1976 |doi=10.1139/v82-277}}

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.Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, {{ISBN|0-07-049439-8}} It is also used as a mordant for dyeing pigments, in manufacturing of alizarin, chrome alum, leather tanning and oil purification.

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.{{Cite journal |last1=Havard, J. M. |last2=Shim, S. Y. |last3=Fr |last4=eacute |last5=chet, J. M. |year=1999 |title=Design of Photoresists with Reduced Environmental Impact. 1. Water-Soluble Resists Based on Photo-Cross-Linking of Poly(vinyl alcohol) |journal=Chem. Mater. |volume=11 |issue=3 |pages=719–725 |doi=10.1021/cm980603y}}

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File:Ammooniumdikromaadi põlemine.JPG

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

{{Cite journal |last1=Neugebauer, C. A. |last2=Margrave, J. L. |year=1957 |title=The Heat of Formation of Ammonium Dichromate |journal=J. Phys. Chem. |volume=61 |issue=10 |pages=1429–1430 |doi=10.1021/j150556a040}}

:{{chem|(NH|4|)|2|Cr|2|O|7}}{{hsp}}(s) → {{chem|Cr|2|O|3}}{{hsp}}(s) + {{chem|N|2}}{{hsp}}(g) + 4{{hsp}}{{chem|H|2|O}}{{hsp}}(g) {{pad|5em}}(ΔH = −429.1{{hsp}}±{{hsp}}3 kcal/mol)

Like ammonium nitrate, it is thermodynamically unstable.{{Cite journal |last=Young, A.J. |year=2005 |title=CLIP, Chemical Laboratory Information Profile: Ammonium Dichromate |url=http://jchemed.chem.wisc.edu/journal/issues/2005/Nov/abs1617.html |journal=J. Chem. Educ. |volume=82 |issue=11 |pages=1617 |doi=10.1021/ed082p1617 |access-date=2009-06-14 |archive-date=2008-09-05 |archive-url=https://web.archive.org/web/20080905225615/http://jchemed.chem.wisc.edu/Journal/Issues/2005/Nov/abs1617.html |url-status=dead |url-access=subscription }}{{Cite journal |last1=G. A. P. Dalgaard |last2=A. C. Hazell |last3=R. G. Hazell |year=1974 |title=The Crystal Structure of Ammonium Dichromate, (NH₄)₂Cr₂O₇ |journal=Acta Chemica Scandinavica |volume=A28 |pages=541–545 |doi=10.3891/acta.chem.scand.28a-0541 |doi-access=free}} 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 {{chem|(NH|4|)|2|Cr|2|O|7}} crystals as a consequence of the onset of decomposition can be ascribed to the dissociative loss of ammonia accompanied by progressive anion condensation to {{chem|Cr|3|O|10|2-}}, {{chem|Cr|4|O|13|2-}}, etc., ultimately yielding {{chem|CrO|3}}. The {{chem|CrO|3}} has been identified as a possible molten intermediate participating in {{chem|(NH|4|)|2|Cr|2|O|7}} decomposition.{{Cite journal |last1=Galwey |first1=Andrew K. |last2=Pöppl |first2=László |last3=Rajam |first3=Sundara |year=1983 |title=A Melt Mechanism for the Thermal Decomposition of Ammonium Dichromate |journal=J. Chem. Soc., Faraday Trans. 1 |volume=79 |issue=9 |pages=2143–2151 |doi=10.1039/f19837902143}}

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. 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.{{Cite journal |last=Shirini, F. |display-authors=etal |year=2003 |title=Solvent free oxidation of thiols by (NH₄)₂Cr₂O₇ in the presence of Mg(HSO₄)₂ and wet SiO₂ |journal=Journal of Chemical Research |volume=2003 |pages=28–29 |doi=10.3184/030823403103172823 |s2cid=197126514|doi-access=free }} 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.{{Cite journal |last=Shirini, F. |display-authors=etal |year=2001 |title=ZrCl₄/wet SiO₂ promoted oxidation of alcohols by (NH₄)₂Cr₂O₇ in solution and solvent free condition |journal=J. Chem. Research (S) |volume=2001 |issue=11 |pages=467–477 |doi=10.3184/030823401103168541 |s2cid=197118772|doi-access=free }}{{Cite journal |last1=F. Shirini |last2=M. A. Zolfigol |last3=FOO† and M. Khaleghi |year=2003 |title=Oxidation of Alcohols Using (NH₄)₂Cr₂O₇ in the Presence of Silica Chloride/Wet SiO₂ in Solution and under Solvent Free Conditions |journal=Bull. Korean Chem. Soc. |volume=24 |issue=7 |pages=1021–1022 |doi=10.5012/bkcs.2003.24.7.1021 |doi-access=free}}

Ammonium dichromate, like all chromium (VI) compounds, is highly toxic and a proven carcinogen. It is also a strong irritant.{{citation needed|date=January 2025}}

In sealed containers, ammonium dichromate is likely to explode if heated. In 1986, two workers were killed and 14 others injured at Diamond Shamrock Chemicals in Ashtabula, Ohio, when {{cvt|2000|lb}} of ammonium dichromate exploded as it was being dried in a heater.{{Cite news |last=Diamond, S. |date=19 January 1986 |title=Chemical Explosion In Ohio |page=22 |work=The New York Times}}

{{reflist|30em}}

{{Commons category|Ammonium dichromate}}

{{Ammonium salts}}

{{Chromates and dichromates}}

Category:Ammonium compounds

Category:Dichromates

Category:Explosive chemicals

Category:Light-sensitive chemicals

Category:Oxidizing agents