(NH4)2Cr2O7
Ammonium dichromate

Chemical properties

Ammonium dichromate forms odourless, orange crystals. When heated to 180°C, it decomposes, developing nitrogen. However, it can also explode due to strong friction. As a strong oxidant, it reacts explosively with metal powders, sulphur or phosphorus.

The “volcano test” was formerly demonstrated in schools. A small pile of ammonium dichromate was ignited at the tip with a burner. The reaction continues on its own, the orange-coloured substance is transformed into a grey-green substance under lively spraying. The volume increases many times over, so that a “volcano” with a crater gradually forms.

Production

It is produced by the reaction of sodium dichromate and ammonium chloride. The concentrated solutions of the two salts are added together and heated to boiling point.

Na2Cr2O7 + 2 NH4Cl → (NH4)2Cr2O7 + 2 NaCl

The sodium chloride precipitating in the hot solution is immediately filtered off. After cooling, the ammonium dichromate precipitates from the solution.

Use

Ammonium dichromate is used in various printing processes in lithography and photography. In lithography, a moist mixture of gelatine and ammonium dichromate (or potassium dichromate) is applied in two layers to a flat mirror glass plate and dried. This plate is light sensitive. Depending on the light intensity, the gelatine dissolves better or worse in water due to the chromium content. The hardening of the gelatine varies, which leads to better or worse adhesion of the printing ink. Today, lithography is no longer of importance as a printing process, occasionally it is still used for art prints.

Ammonium dichromate is used for the production of wood preservatives and catalysts in organic syntheses. Other substances can be synthesized, for example

  • Chromium(III) oxide pigments are widely used for the production of green colors,
  • Chromium(IV) oxide used to be used as a carrier material for magnetic tapes,
  • Potassium chrome alum was formerly used as a tanning agent for leather.

Some applications are only possible to a very limited extent today due to their toxicity. The use as mordant in textile dyeing is just as problematic as the use in fireworks or in smokeless powders.

Security

Toxicity

Like potassium dichromate, ammonium dichromate is an extremely toxic substance. Inhaling the carcinogenic dusts is particularly dangerous for humans. It is absorbed through absorption in the lungs or through skin contact. As with other chromium salts, the chromium accumulates in the kidneys, liver and brain. Conjunctivitis and gastritis can then occur.

Ammonium dichromate is toxicologically classified by the EU Commission as

  • Carcinogenic Category: 1B (H350: May cause cancer.)
  • Mutagenic Category: 1B (H340: May cause genetic defects.)
  • Toxic to reproduction Category: 1B (H360FD: May impair fertility. May damage the unborn child).

It is also classified as lethal if inhaled (H330) and toxic if swallowed (H301). Contact with the skin is classified as harmful (H312). Another hazard of ammonium dichromate is the possibility of sensitization by inhalation and skin contact (H334/317).

Ecotoxicologically it is considered very toxic to aquatic organisms and may cause long-term adverse effects in the aquatic environment (H410). In the administrative regulation on substances hazardous to water (VwVwS as of July 2005), ammonium dichromate is classified in the highest water hazard class 3 with the identification number 290. Due to the bioconcentration factor of 200-2000 given in the literature, accumulation in organisms is possible.

Explosion hazard

Ammonium dichromate decomposes exothermically from 100 °C; spontaneous combustion occurs between 130 and 180 °C, the reaction is explosive even in the absence of oxygen from 240 °C. In the case of an initial ignition using picric acid, it detonates only incompletely, even under containment.

The sensitivity to mechanical stress is extremely low. For example, ammonium dichromate does not react at all when rubbed in an unglazed mortar; sensitivity to impact is roughly equivalent to that of ammonium perchlorate (15 cm under a 10 kg drop hammer; ammonium nitrate, which is not classified as explosive, detonates under the 10 kg drop hammer from a height of 20 cm). Even under optimum conditions, the detonation propagates only over very short distances.

Ammonium dichromate is not an explosive, but is used occasionally in pyrotechnic compositions and as a catalyst in ammonium nitrate-based propellants.