Ammonium dichromate
Encyclopedia
Ammonium dichromate is the inorganic compound
with the formula
(NH4)2Cr2O7. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state
, commonly known as hexavalent chromium
. It is a salt consisting of ammonium
ions and dichromate ions.
Ammonium dichromate is sometimes known as Vesuvian Fire, because of its use in demonstrations of tabletop "volcanoes". It has been used in pyrotechnics and in the early days of photography
.
on ammonium hydroxide
with subsequent crystallisation.
The (NH4)2Cr2O7 crystal (C2/c, z=4) contains a single type of ammonium ion, at sites of symmetry C1(2,3). Each NH4+ centre is surrounded irregularly by eight oxygen atoms at N---O distances ranging from ca. 2.83 to ca. 3.17 Å, typical of hydrogen bond
s.
as well as in lithography
, as a source of pure nitrogen
in the laboratory, and as a catalyst. It is also used as a mordant
for dyeing pigments, in the manufacturing of alizarin
, chrome alum
, leather tanning and oil purification.
Photosensitive films containing PVA
, ammonium dichromate, and a phosphor
are spin-coated as aqueous slurries in the production of the phosphor raster of television screens and other devices. The ammonium dichromate acts as the photoactive site.
Like the well-known explosive ammonium nitrate, ammonium dichromate contains both an oxidizer (dichromate) and a reducer (ammonium
), making it thermodynamically unstable. Its decomposition reaction proceeds to completion once initiated, producing voluminous dark green powdered chromium(III) oxide
. Not all of the ammonium dichromate decomposes in this reaction. When the green powder is brought into water a yellow/orange solution is obtained from left over ammonium dichromate.
Observations obtained using relatively high magnification microscopy during a kinetic study of the thermal decomposition of ammonium dichromate provided evidence that salt breakdown proceeds with the intervention of an intermediate liquid phase rather than a solid phase. The characteristic darkening of crystals as a consequence of the onset of decomposition can be ascribed to the dissociative loss of ammonia accompanied by progressive anion condensation to , , etc., ultimately yielding . The has been identified as a possible molten intermediate participating in decomposition.
s and thiol
s. Ammonium dichromate, in the presence of Mg(HSO4)2 and wet SiO2 can act as a very efficient reagent for the oxidative coupling of thiols under solvent free conditions. The reactions produces reasonably good yields under relatively mild conditions. The compound is also used in the oxidation of aliphatic alcohols to their corresponding aldehyde
s and ketones in ZrCl4/wet SiO2 in solvent free conditions, again with relatively high yields.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
(NH4)2Cr2O7. In this compound, as in all chromates and dichromates, chromium is in a +6 oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
, commonly known as hexavalent chromium
Hexavalent chromium
Hexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately of hexavalent chromium were produced in 1985...
. It is a salt consisting of ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
ions and dichromate ions.
Ammonium dichromate is sometimes known as Vesuvian Fire, because of its use in demonstrations of tabletop "volcanoes". It has been used in pyrotechnics and in the early days of photography
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
.
Properties
At room temperature and pressure, the compound exists as orange, acidic crystals soluble in water and alcohol. It is formed by the action of chromic acidChromic acid
The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the...
on ammonium hydroxide
Ammonium hydroxide
Ammonia solution, also known as ammonium hydroxide, ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or simply ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3...
with subsequent crystallisation.
The (NH4)2Cr2O7 crystal (C2/c, z=4) contains a single type of ammonium ion, at sites of symmetry C1(2,3). Each NH4+ centre is surrounded irregularly by eight oxygen atoms at N---O distances ranging from ca. 2.83 to ca. 3.17 Å, typical of hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s.
Uses
It has been used in pyrotechnics and in the early days of photographyPhotography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
as well as in lithography
Lithography
Lithography is a method for printing using a stone or a metal plate with a completely smooth surface...
, as a source of pure nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
in the laboratory, and as a catalyst. It is also used as a mordant
Mordant
A mordant is a substance used to set dyes on fabrics or tissue sections by forming a coordination complex with the dye which then attaches to the fabric or tissue. It may be used for dyeing fabrics, or for intensifying stains in cell or tissue preparations. The term mordant comes from the Latin...
for dyeing pigments, in the manufacturing of alizarin
Alizarin
Alizarin or 1,2-dihydroxyanthraquinone is an organic compound with formula that has been used throughout history as a prominent dye, originally derived from the roots of plants of the madder genus.Alizarin was used as a red dye for the English parliamentary "new model" army...
, chrome alum
Alum
Alum is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate with the formula KAl2.12H2O. The wider class of compounds known as alums have the related empirical formula, AB2.12H2O.-Chemical properties:Alums are...
, leather tanning and oil purification.
Photosensitive films containing PVA
Polyvinyl alcohol
Polyvinyl alcohol is a water-soluble synthetic polymer .-Properties:...
, ammonium dichromate, and a phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
are spin-coated as aqueous slurries in the production of the phosphor raster of television screens and other devices. The ammonium dichromate acts as the photoactive site.
Tabletop volcanoes and thermal decomposition
The volcano demonstration involves igniting a pile of the salt, which initiates the following exothermic conversion:- (s) → (s) + (g) + 4 (g) (ΔH=−429.1 ± 3 kcal/mol)
Like the well-known explosive ammonium nitrate, ammonium dichromate contains both an oxidizer (dichromate) and a reducer (ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
), making it thermodynamically unstable. Its decomposition reaction proceeds to completion once initiated, producing voluminous dark green powdered chromium(III) oxide
Chromium(III) oxide
Chromium oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.-Structure and properties:...
. Not all of the ammonium dichromate decomposes in this reaction. When the green powder is brought into water a yellow/orange solution is obtained from left over ammonium dichromate.
Observations obtained using relatively high magnification microscopy during a kinetic study of the thermal decomposition of ammonium dichromate provided evidence that salt breakdown proceeds with the intervention of an intermediate liquid phase rather than a solid phase. The characteristic darkening of crystals as a consequence of the onset of decomposition can be ascribed to the dissociative loss of ammonia accompanied by progressive anion condensation to , , etc., ultimately yielding . The has been identified as a possible molten intermediate participating in decomposition.
Oxidation reactions
Ammonium dichromate is a strong oxidising agent and reacts, often violently, with any reducing agent. The stronger the reducing agent, the more violent the reaction. It has also been used to promote the oxidation of alcoholAlcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
s and thiol
Thiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...
s. Ammonium dichromate, in the presence of Mg(HSO4)2 and wet SiO2 can act as a very efficient reagent for the oxidative coupling of thiols under solvent free conditions. The reactions produces reasonably good yields under relatively mild conditions. The compound is also used in the oxidation of aliphatic alcohols to their corresponding aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
s and ketones in ZrCl4/wet SiO2 in solvent free conditions, again with relatively high yields.