Ammonium dichromate

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.

The crystal (C2/c, ) contains a single type of ammonium ion, at sites of symmetry C(2,3). Each centre is surrounded irregularly by eight oxygen atoms at distances ranging from ca. , typical of hydrogen bonds.

Uses

Pyrotechnics

It has been used in pyrotechnics as part of a conversion coating method for magnesium powder. It may also be used when a dichromate compatible with ammonium perchlorate () is needed; potassium dichromate () is commonly used, but reacts with ammonium perchlorate over time to form potassium perchlorate and ammonium dichromate. This mixture occupies 5% more volume than the original + mixture and leads to eventual cracking of stars and consolidated compositions.

It was used in a composition in the early 20th century in a combination firework called a serpent's nest to produce the green grass (or nest) from which a separately made pharaoh's serpent or black snake composition emerged.

Holography

In holography, it has been used since the 1960s to sensitize dichromated gelatins or other materials on which to expose a hologram. This method has the advantage of being low cost, using relatively common materials, and only requiring isopropyl alcohol for development, but the gelatins created with these methods are sensitive to humidty. Potassium dichromate is also used for this purpose.

Other

It was used in the early days of photography as well as in lithography, as a source of pure nitrogen in the laboratory, and as a catalyst. 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.

Reactions

Tabletop volcanoes and thermal decomposition

The volcano demonstration involves igniting a pile of the salt, which initiates the following exothermic conversion: : :(ΔH = )

Like ammonium nitrate, it is thermodynamically unstable. Its decomposition reaction produces voluminous dark green powdered chromium(III) oxide. Not all of the ammonium dichromate decomposes in this reaction. When the green powder is added to 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 crystals as a consequence of the onset of decomposition can be ascribed to the dissociative loss of ammonia accompanied by progressive anion condensation to , , etc., ultimately yielding . The has been identified as a possible molten intermediate participating in decomposition.

Oxidation reactions

Ammonium dichromate is a strong oxidising agent and reacts, often violently, with many reducing agents. In general, 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 and wet 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. The compound is also used in the oxidation of aliphatic alcohols to their corresponding aldehydes and ketones in /wet in solvent free conditions, again with relatively high yields.

Safety

Ammonium dichromate is highly acutely toxic. It ranges from having a strong irritant effect on skin to causing severe chemical burns and skin corrosion. Inhalation of dust may be fatal. Ingestion results in death from multi-organ failure if not treated quickly. Ascorbic acid may be a useful early treatment.

Like many hexavalent chromium compounds it is a proven carcinogen, mutagen, and reproductive toxin.

Incidents

In sealed containers, ammonium dichromate is likely to explode if heated.

References

References

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