Ammonium dichromate is an inorganic salt with the formula (NH4)2Cr2O7. It forms orange–red crystalline solids and contains the dichromate anion in which chromium is in the hexavalent state. As a powerful oxidizing agent it reacts readily with reducing substances and decomposes vigorously when heated under some conditions.

Chemical behaviour and decomposition

On heating, ammonium dichromate undergoes a redox decomposition that is exothermic and yields nitrogen gas, water vapor and a green chromium(III) oxide residue. The dramatic release of gas and glowing solids has been used as a classroom "volcano" demonstration; the remaining green powder is chromium(III) oxide. Because chromium in the dichromate ion is hexavalent (Cr(VI)), the compound is strongly oxidizing and can ignite or cause violent reactions if it contacts organic matter or is confined.

Preparation and physical properties

Ammonium dichromate may be prepared by combining ammonium salts with dichromate-containing acid solutions or by neutralizing chromic acid with ammonium hydroxide. It is water-soluble and solutions are acidic because of the dichromate/chromate equilibrium. The solid is typically granular or crystalline and ranges in color from bright orange to reddish-orange depending on crystal habit and purity.

Uses and historical notes

Historically the compound has been employed as a laboratory oxidizing reagent, in pyrotechnic formulations, and as a pedagogical demonstration of rapid decomposition. Safer modern practices have reduced its use in public demonstrations because of health and environmental concerns. In synthetic chemistry it has seen more limited use compared with less hazardous oxidants.

Safety, toxicity, and environmental impact

Ammonium dichromate contains hexavalent chromium, a toxic and carcinogenic form of chromium. Exposure can cause severe health effects if inhaled, swallowed, or absorbed through the skin. It is corrosive to tissue and harmful to aquatic life. Typical safety controls include using enclosed systems, local exhaust ventilation, appropriate personal protective equipment (gloves, eye protection, respirators as needed), and strict waste segregation. Disposal must follow regulatory guidance because of the environmental persistence and toxicity of Cr(VI).

Precautions and distinguishing facts

  • Do not use ammonium dichromate for public demonstrations; many instructors now prefer safer alternatives.
  • Store away from combustible materials and reducing agents; avoid confinement that could promote pressure build-up.
  • Reduction of dichromate produces chromium(III), which is much less toxic than Cr(VI) but still requires appropriate handling.

Further resources