(NH₄)₂Cr₂O₇ — Ammonium Dichromate

Ammonium dichromate \((NH_4)_2Cr_2O_7\) is an inorganic compound that appears as bright orange-red crystals. It is famous for the classic volcano demonstration in chemistry, where it decomposes with the evolution of heat, producing a green ash of chromium(III) oxide, nitrogen gas, and water vapor. This exothermic reaction visually resembles a volcanic eruption, hence its name.

Ammonium dichromate is a powerful oxidizing agent and is used in laboratories for oxidation reactions, pyrotechnics, photography, and dye production. However, due to its toxic and carcinogenic nature, its usage has decreased significantly in modern laboratories. It serves as an excellent example to study redox reactions, decomposition, and thermal instability of ammonium salts of transition metals.

The compound consists of two ammonium ions (NH4+) and one dichromate ion (Cr2O7²⁻). The dichromate ion has a structure containing two tetrahedral \(CrO_4\) units joined by a shared oxygen atom. The overall molecular structure can be represented as:

Each chromium atom is in the +6 oxidation state, making the compound a strong oxidizer. The chromium atoms are connected through oxygen bridges, and the ammonium ions stabilize the anionic framework electrostatically. The ionic nature of ammonium dichromate allows it to dissolve in water, dissociating into ions that can undergo redox reactions.

The dichromate ion is orange due to charge-transfer transitions between oxygen and chromium atoms, giving it its characteristic vivid color. This color also changes to green upon reduction to \(Cr^{3+}\), as observed during decomposition.

Ammonium dichromate can be prepared by the neutralization reaction of dichromic acid \(H_2Cr_2O_7\) with ammonium hydroxide \(NH_4OH\) or ammonia solution. The chemical equation is as follows:

Alternatively, it can be obtained by mixing ammonium carbonate and chromium trioxide (CrO3) under controlled conditions, followed by crystallization. The resulting orange crystals are collected and dried at room temperature.

The crystals are hygroscopic and should be stored in a tightly sealed container away from light and moisture to prevent decomposition.

When heated strongly, ammonium dichromate decomposes in a dramatic exothermic reaction that releases nitrogen gas and water vapor, leaving behind a fluffy green residue of chromium(III) oxide \((Cr_2O_3)\):

This reaction is often performed as a chemistry demonstration called the ammonium dichromate volcano experiment. Upon ignition, the compound decomposes with a bright orange spark, emitting steam and nitrogen gas while forming green ash resembling volcanic lava. The reaction is highly exothermic and self-sustaining once initiated.

The change in color from orange (dichromate ion) to green (chromium(III) oxide) illustrates a reduction in the oxidation state of chromium from +6 to +3. This process visually demonstrates the principle of redox chemistry and energy release in decomposition reactions.

Ammonium dichromate is an oxidizing agent that readily releases oxygen when heated or in reaction with reducing agents. Some notable chemical properties include:

• Decomposition: On heating, it decomposes into chromium(III) oxide, nitrogen, and water.
• Redox Behavior: Acts as an oxidizer, reducing itself from \(Cr^{6+}\) to \(Cr^{3+}\).
• Reaction with Acids: In acidic medium, it acts as a source of chromic acid (H2CrO4).
• Reaction with Organic Compounds: It oxidizes alcohols, aldehydes, and other organic substrates into corresponding acids or ketones.
• Reaction with Alkalis: In alkaline medium, dichromate converts to chromate ion \(CrO_4^{2-}\) which is yellow in color:

These reactions make ammonium dichromate an important reagent in analytical and industrial chemistry, despite its hazardous nature.

Ammonium dichromate has been historically used in various laboratory and industrial processes, though its use is now restricted due to toxicity. Major applications include:

• Educational Demonstrations: Used in the 'volcano experiment' to demonstrate decomposition and oxidation-reduction reactions.
• Photography: Used in chromium-based photographic printing processes such as the gum dichromate method.
• Pyrotechnics: Serves as an oxidizer in fireworks and signal flares.
• Textile Dyeing and Leather Tanning: Acts as a mordant and tanning agent due to its oxidizing ability.
• Analytical Chemistry: Used as a standard oxidizing agent in redox titrations and chemical synthesis.

Despite its versatility, ammonium dichromate’s use has declined significantly due to its carcinogenicity and environmental impact, replaced by safer alternatives such as potassium permanganate or hydrogen peroxide for oxidation purposes.

Ammonium dichromate is classified as highly toxic, corrosive, and carcinogenic. It can cause severe skin irritation, respiratory problems, and damage to the liver and kidneys upon prolonged exposure. It is also an environmental hazard, contaminating soil and water with hexavalent chromium.

Safety measures include:

• Wearing gloves, lab coats, and safety goggles when handling the compound.
• Avoiding inhalation and contact with skin or eyes.
• Performing reactions in a fume hood.
• Properly disposing of chromium-containing waste following hazardous waste regulations.

Due to these risks, the ammonium dichromate volcano experiment is rarely performed today in educational settings and, if done, is conducted under strict safety protocols.