AMMONIUM DICHROMATE
Formula : (NH4)2Cr2O7; MW 252.10 
Synonym : ammonium bichromate
Uses
Ammonium dichromate is used in pyrotechnics; in photoengraving and lithography; as a source of pure nitrogen in the laboratory; and as a catalyst.
Physical Properties
Bright orange-red monoclinic crystals; odorless; hygroscopic; decomposes at 180°C; density 2.115 g/cm³ at 25°C; readily dissolves in water (26.67 g/100 g at 20°C).
Preparation
(NH4)2Cr2O7 
may be prepared by the reaction of ammonia gas with chromic acid:
2NH3 + 2CrO3 + H2O → (NH4)2Cr2O7
or ammonium sulfate with sodium dichromate:
(NH4)2SO4 + Na2Cr2O7 → (NH4)2Cr2O7 + Na2SO4
Reactions
(NH4)2Cr2O7 decomposes at 180°C. On further heating to 225° C it begins to swell and dissociates exothermically, liberating nitrogen and water vapor, leaving behind a residue of chromium(III) oxide:
(NH4)2Cr2O7→N2+Cr2O3+4H2O
As an acid salt, its solution is acidic (pH 3.45 and 3.95 for a 10% and 1% solution, respectively). It undergoes acid reactions. Also, it undergoes double decomposition reactions, forming metal dichromates:
(NH4)2Cr2O7 + Pb(NO3)2 → PbCr2O7 + 2NH4NO3
As an oxidizing agent, it undergoes oxidation-reduction reactions with reducing agents at ambient and elevated temperatures.
Hazard
Ammonium dichromate is an irritant to skin. Inhalation of dusts can cause pulmonary irritation, perforation of the nasal septum and “chrome sores.” Ingestion can cause ulceration. It is also a flammable salt.
Tags: (NH4)2Cr2O7, AMMONIUM, AMMONIUM DICHROMATE, DICHROMATE
