Ammonium dichromate

Properties

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.

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. 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:-

Like ammonium nitrate, it is 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.

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 (NH
4)
2Cr
2O
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 Cr
3O2−
10, Cr
4O2−
13, etc., ultimately yielding CrO
3. The CrO
3 has been identified as a possible molten intermediate participating in (NH
4)
2Cr
2O
7 decomposition.

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. 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. 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.

Safety

Ammonium dichromate, like all chromium(VI) compounds, is highly toxic and a proven carcinogen. It is also a strong irritant.

Incidents

In sealed containers, ammonium dichromate is likely to explode if heated. On January 19, 1986, The New York Times reported that two workers had been killed and 14 others injured at Diamond Shamrock Chemicals, Ashtabula, Ohio, when 2,000 lbs of ammonium dichromate exploded as it was being dried in a heater.