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Ammonium Dichromate - 100g
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Ammonium dichromate is the inorganic compound with the formula (NH4)2Cr2O7. In this compound 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 sometimes known as Vesuvian Fire, because of its use in demonstrations of tabletop "volcanoes". It has been used in pyrotechnics and in the early days of photography.
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.
The (NH4)2Cr2O7 crystal (C2/c, z=4) contains a single type of ammonium ion, at sites of symmetry C1(2,3). Each NH4+ 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.
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. It is also used as a mordant for dyeing pigments, in the 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.
Tabletop volcanoes and thermal decomposition...
The volcano demonstration involves igniting a pile of the salt, which initiates the following exothermic conversion:
(NH4)2Cr2O7 (s) → Cr2O3 (s) + N2 (g) + 4 H2O (g) (ΔH=−429.1 ± 3 kcal/mol)
Like the well-known explosive ammonium nitrate, ammonium dichromate contains both an oxidizer (dichromate) and a reducer (ammonium), making it thermodynamically unstable. 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.
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