Infomation Page – Ammonium Dichromate – Properties and Uses

Ammonium dichromate is an inorganic chemical compound containing chromium in the +6 oxidation state. It is best known for its bright orange crystalline appearance and for its role in classic laboratory demonstrations, as well as its industrial and analytical applications.

Physical Properties

  • Appearance: Bright orange to reddish-orange crystalline solid
  • Molar Mass: ~252.06 g/mol
  • Density: ~2.15 g/cm³
  • Solubility: Soluble in water; solubility increases with temperature
  • Melting Point: Decomposes at ~180–190 °C (does not melt cleanly)
  • Odour: Odourless

Chemical Properties

  • Oxidizing Agent: Strong oxidizer due to chromium(VI)
  • Thermal Decomposition: On heating, decomposes exothermically to form chromium(III) oxide (Cr₂O₃), nitrogen gas, and water vapor
  • Reactivity: Reacts vigorously with reducing agents and organic materials
  • Stability: Stable under normal conditions, but sensitive to heat and contamination

Key Reaction Equation (Thermal Decomposition)

(NH4)2Cr2O7(s)→ΔCr2O3(s)+N2(g)+4H2O(g)(NH_4)_2Cr_2O_7 (s) \xrightarrow{\Delta} Cr_2O_3 (s) + N_2 (g) + 4H_2O (g)

This reaction is highly exothermic and produces the green chromium(III) oxide “ash” along with nitrogen gas and steam. The rapid gas evolution gives the reaction its volcano-like appearance.

Oxidation–Reduction Explanation

  • Chromium: Reduced from oxidation state +6 (in dichromate) to +3 (in chromium(III) oxide)
  • Nitrogen: Oxidized from −3 (in ammonium ion) to 0 (in nitrogen gas)

This confirms that the reaction is a redox reaction involving internal oxidation and reduction.

Simple Reaction Diagram (Conceptual)

          Heat (Δ)
 (NH4)2Cr2O7  ─────────▶  Cr2O3  +  N2  +  H2O
  (orange)                  (green)   (gas)  (steam)

Uses

1. Laboratory Demonstrations

  • Commonly used in the “ammonium dichromate volcano” experiment to demonstrate exothermic decomposition and oxidation–reduction reactions

2. Analytical Chemistry

  • Used as a reagent in certain oxidation reactions
  • Occasionally employed in chemical analysis involving chromium compounds

3. Pyrotechnics and Special Effects

  • Historically used in controlled ignition compositions and visual effects

4. Photography and Imaging (Historical)

  • Chromium(VI) compounds, including ammonium dichromate, were once used in early photographic and photoengraving processes

Safety and Health Hazards

  • Toxicity: Highly toxic if ingested or inhaled
  • Carcinogenicity: Chromium(VI) compounds are known carcinogens
  • Skin and Eye Damage: Causes severe irritation and burns
  • Environmental Hazard: Harmful to aquatic life; persistent environmental pollutant

Handling Precautions:

  • Use only in well-ventilated laboratories
  • Wear gloves, eye protection, and lab coats
  • Avoid dust formation and direct contact

Storage and Disposal

  • Store in tightly sealed containers away from heat and combustible materials
  • Dispose of as hazardous waste according to local regulations; never release into drains or the environment

Summary

Ammonium dichromate is a visually striking but hazardous chemical with significant oxidizing power. While it has educational and analytical value, its toxicity and environmental risks have led to restricted use and replacement by safer alternatives in many applications.