Ammonium dichromate

Ammonium dichromate

Names
IUPAC name Ammonium dichromate
Other names Ammonium bichromate Ammonium pyrochromate
Identifiers
CAS Number	7789-09-5  Y
3D model (JSmol)	Interactive image
ChemSpider	23002  Y
ECHA InfoCard	100.029.221
PubChem CID	24600
RTECS number	HX7650000
UNII	5J18BP595G  N
UN number	1439
CompTox Dashboard (EPA)	DTXSID70872953
InChI InChI=1S/2Cr.2H3N.7O/h;;2*1H3;;;;;;;/q;;;;;;;;;2*-1/p+2 Y Key: JOSWYUNQBRPBDN-UHFFFAOYSA-P Y InChI=1/2Cr.2H3N.7O/h;;2*1H3;;;;;;;/q;;;;;;;;;2*-1/p+2/rCr2O7.2H3N/c3-1(4,5)9-2(6,7)8;;/h;2*1H3/q-2;;/p+2 Key: JOSWYUNQBRPBDN-RFRSXZKWAS
SMILES [O-][Cr](=O)(=O)O[Cr]([O-])(=O)=O.[NH4+].[NH4+]
Properties
Chemical formula	(NH4)2Cr2O7
Molar mass	252.07 g/mol
Appearance	Orange-red crystals
Odor	odorless
Density	2.115 g/cm3
Melting point	180 °C (356 °F; 453 K) decomposes
Solubility in water	18.2 g/100ml (0 °C (32 °F)) 35.6 g/100ml (20 °C (68 °F)) 40.0 g/100ml (25 °C (77 °F)) 156.0 g/100ml (100 °C (212 °F))
Solubility in ethanol	soluble
Solubility in acetone	insoluble
Structure
Crystal structure	monoclinic
HazardsSigma-Aldrich Co., Ammonium dichromate. Retrieved on 2013-07-20.
Occupational safety and health (OHS/OSH):
Main hazards	Very toxic, oxidizing, carcinogenic, mutagenic, dangerous for the environment
GHS labelling:
Pictograms
Hazard statements	H272, H301, H312, H314, H317, H330, H334, H340, H350, H360, H372, H410
Precautionary statements	P201, P220, P260, P273, P280, P284
NFPA 704 (fire diamond)	4 2 3 OX
Autoignition temperature	190 °C (374 °F; 463 K)
Threshold limit value (TLV)	0.0002 mg/m3 (TWA), 0.0005 mg/m3 (STEL), 1 mg/10m3 (C)
Lethal dose or concentration (LD, LC):
LD50 (median dose)	53 mg/kg (Rat, oral) 1860 mg/kg (Rabbit, dermal) [1]
LC50 (median concentration)	0.2 mg/l (200 mg/m3) - 4h (Rat, dust / mist)
NIOSH (US health exposure limits): [2]
PEL (Permissible)	0.005 mg/m3 (as CrO3)
REL (Recommended)	8 hours, 0.0002 mg/m3 (as Cr)
IDLH (Immediate danger)	15 mg/m3 (as Cr(VI)) [1]
Safety data sheet (SDS)	ICSC 1368
Related compounds
Other cations	Potassium dichromate Sodium dichromate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N  verify  (what is  YN ?) Infobox references

Ammonium dichromate is an inorganic compound with the formula (NH₄)₂Cr₂O₇. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state, commonly known as hexavalent chromium. It is a salt consisting of ammonium ions and dichromate ions.

Ammonium dichromate is used in demonstrations of tabletop "volcanoes". ^[3] However, this demonstration has become unpopular with school administrators due to the compound's carcinogenic nature. It has also been used in pyrotechnics and in the early days of photography.

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. ^{[4] [ page needed ]}

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. ^[5]

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. ^{[6] [ page needed ]} It is also used as a mordant for dyeing pigments, in manufacturing of alizarin, chrome alum, leather tanning and oil purification. ^{[4] [ page needed ]}

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. ^[7]

Reactions

Tabletop volcanoes and thermal decomposition

A few drops of ethanol are added to a small pile of ammonium dichromate (NH₄)₂Cr₂O₇ and ignited. Sparks are emitted and an ash-like product is formed. The phenomenon resembles the eruption of a volcano. The reaction starts at 180 °C (356 °F), becoming self-sustaining at approximately 225 °C (437 °F). ^[8]

Ammonium dichromate decomposition

The volcano demonstration involves igniting a pile of the salt, which initiates the following exothermic conversion: ^[9]

(NH₄)₂Cr₂O₇(s) → Cr₂O₃(s) + N₂(g) + 4 H₂O(g)

(ΔH = −429.1±3 kcal/mol)

Like ammonium nitrate, it is thermodynamically unstable. ^{[10] [11]} 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. ^[12]

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. ^[10] 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 produce reasonably good yields under relatively mild conditions. ^[13] 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. ^[14]

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. ^[1]

Incidents

In sealed containers, ammonium dichromate is likely to explode if heated. ^[10] 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. ^[15]

References

1. Sigma-Aldrich Co., Ammonium dichromate. Retrieved on 2013-07-20.
2. "NIOSH Pocket Guide to Chemical Hazards".
3. "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.
4. Lewis, Richard J. (2007). Hawley's condensed chemical dictionary (15th ed.). Hoboken (N.J.): Wiley-Interscience. ISBN   978-0-471-76865-4.
5. 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.
6. Patnaik, Pradyot (2003). Handbook of inorganic chemicals. New York: McGraw-Hill. ISBN   0-07-049439-8.
7. 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.
8. Planned and performed by Marina Stojanovska, Miha Bukleski and Vladimir Petruševski, Department of Chemistry, FNSM, Ss. Cyril and Methodius University, Skopje, Macedonia.
9. 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.
10. Young, Jay A. (1 November 2005). "Ammonium Dichromate". Journal of Chemical Education . 82 (11): 1617. doi:10.1021/ed082p1617.
11. 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 .
12. 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.
13. 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.
14. 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.
15. Diamond, S. (19 January 1986). "Chemical in Blast Was Being Heated". The New York Times. p. 22. Retrieved 26 June 2025.