Ammonium dichromate

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Ammonium dichromate
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UN number 1439
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Molar mass 252.07 g/mol
Appearance Orange-red crystals
Odor odorless
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Solubility in ethanol soluble
Solubility in acetone insoluble
Template:Longitem monoclinic
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Ammonium dichromate is an inorganic compound with the formula Template:Chem2. 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".[2] However, this demonstration has become unpopular in schools 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.[3]Script error: No such module "Unsubst".

The Template:Chem2 crystal (C2/c, z = 4) contains a single type of ammonium ion, at sites of symmetry C1(2,3). Each Template:Chem2 centre is surrounded irregularly by eight oxygen atoms at Template:Chem2 distances ranging from ca. Script error: No such module "val"., typical of hydrogen bonds.[4]

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.[5]Script error: No such module "Unsubst". It is also used as a mordant for dyeing pigments, in manufacturing of alizarin, chrome alum, leather tanning and oil purification.[3]Script error: No such module "Unsubst".

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

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Ammonium dichromate decomposition

The volcano demonstration involves igniting a pile of the salt, which initiates the following exothermic conversion: [8]

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Like ammonium nitrate, it is thermodynamically unstable.[9][10] 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 Template:Chem2 crystals as a consequence of the onset of decomposition can be ascribed to the dissociative loss of ammonia accompanied by progressive anion condensation to Template:Chem2, Template:Chem2, etc., ultimately yielding Template:Chem2. The Template:Chem2 has been identified as a possible molten intermediate participating in Template:Chem2 decomposition.[11]

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.[9] It has also been used to promote the oxidation of alcohols and thiols. Ammonium dichromate, in the presence of Template:Chem2 and wet Template:Chem2 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 Template:Chem2/wet Template:Chem2 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.[1]

Incidents

In sealed containers, ammonium dichromate is likely to explode if heated.[9] In 1986, two workers were killed and 14 others injured at Diamond Shamrock Chemicals in Ashtabula, Ohio, when Template:Cvt of ammonium dichromate exploded as it was being dried in a heater.[14]

References

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  1. ↑ a b c Sigma-Aldrich Co., Ammonium dichromate. Retrieved on 2013-07-20.
  2. ↑ Script error: No such module "citation/CS1".
  3. ↑ a b Script error: No such module "citation/CS1".
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  6. ↑ Script error: No such module "Citation/CS1".
  7. ↑ Planned and performed by Marina Stojanovska, Miha Bukleski and Vladimir Petruševski, Department of Chemistry, FNSM, Ss. Cyril and Methodius University, Skopje, Macedonia.
  8. ↑ Script error: No such module "Citation/CS1".
  9. ↑ a b c Script error: No such module "Citation/CS1".
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External links

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Template:Ammonium salts Template:Chromates and dichromates

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