Ammonium dichromate is an

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

(NH₄)₂Cr₂O₇. In this compound, as in all chromates and dichromates,

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

is in a +6

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

, commonly known as

hexavalent chromium Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is any chemical compound that contains the element chromium in the +6 oxidation state (thus hexavalent). It has been identified as carcinogenic, which is of concern since approximately of ...

. It is a salt consisting of

ammonium Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) polyatomic ion, molecular ion with the chemical formula or . It is formed by the protonation, addition of a proton (a hydrogen nucleu ...

ions and dichromate ions. Ammonium dichromate is used in demonstrations of tabletop "volcanoes". However, this demonstration has become unpopular in schools due to the compound's carcinogenic nature. It has also been used in

pyrotechnic Pyrotechnics is the science and craft of creating fireworks, but also includes safety matches, oxygen candles, explosive bolts (and other fasteners), parts of automotive airbags, as well as gas-pressure blasting in mining, quarrying, and demol ...

s and in the early days of

photography Photography is the visual arts, art, application, and practice of creating images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is empl ...

Properties

At room temperature and pressure, the compound exists as orange, acidic crystals soluble in water and

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

. It is formed by the action of

chromic acid Chromic acid is a chemical compound with the chemical formula . It is also a jargon for a solution formed by the addition of sulfuric acid to aqueous solutions of dichromate. It consists at least in part of chromium trioxide. The term "chromic ...

ammonium hydroxide Ammonia solution, also known as ammonia water, ammonium hydroxide, ammoniacal liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or (inaccurately) ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). Although ...

with subsequent crystallisation.Richard J. Lewis ''Hawley's Condensed Chemical Dictionary''. Wiley & Sons, Inc: New York, 2007 The (NH₄)₂Cr₂O₇ crystal (C2/c, ''z'' = 4) contains a single type of ammonium ion, at sites of symmetry C₁(2,3). Each 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 In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

Uses

It has been used in

s and in the early days of photography as well as in

lithography Lithography () is a planographic method of printing originally based on the miscibility, immiscibility of oil and water. The printing is from a stone (lithographic limestone) or a metal plate with a smooth surface. It was invented in 1796 by ...

, as a source of pure

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

in the laboratory, and as a

catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

. It is also used as a

mordant A mordant or dye fixative is a substance used to set (i.e., bind) dyes on fabrics. It does this 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 ...

for dyeing pigments, in manufacturing of

alizarin Alizarin (also known as 1,2-dihydroxyanthraquinone, Mordant Red 11, C.I. 58000, and Turkey Red) is an organic compound with formula that has been used throughout history as a red dye, principally for dyeing textile fabrics. Historically it wa ...

chrome alum Chrome alum or Chromium(III) potassium sulfate is the potassium double sulfate of chromium. Its chemical formula is KCr(SO4)2 and it is commonly found in its dodecahydrate form as KCr(SO4)2·12(H2O). It is used in leather tanning. Production ...

, leather tanning and oil purification. Photosensitive films containing PVA, ammonium dichromate, and a

phosphor A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...

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.

Reactions

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'') (Δ''H'' = −429.1±3 kcal/mol) Like

ammonium nitrate Ammonium nitrate is a chemical compound with the formula . It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, but does not form hydrates. It is predominantly us ...

, it is thermodynamically unstable. Its decomposition reaction proceeds to completion once initiated, producing voluminous dark green powdered

chromium(III) oxide Chromium(III) oxide (or chromia) is an inorganic compound with the formula . It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite. Structure and properties has the corundum s ...

. 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 An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ''electron donor''). In ot ...

and reacts, often violently, with any

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...

. The stronger the reducing agent, the more violent the reaction. It has also been used to promote the oxidation of

s and

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

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 In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated (in which all ...

alcohols to their corresponding

aldehyde In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...

s and

ketones In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...

in ZrCl₄/wet SiO₂ in solvent free conditions, again with relatively high yields.

Safety

Ammonium dichromate, like all chromium (VI) compounds, is highly toxic and a proven carcinogen. It is also a strong irritant.

Incidents

In sealed containers, ammonium dichromate is likely to explode if heated. In 1986, two workers were killed and 14 others injured at Diamond Shamrock Chemicals in

Ashtabula, Ohio Ashtabula ( ) is the most populous city in Ashtabula County, Ohio, United States. It lies at the mouth of the Ashtabula River, on Lake Erie, northeast of Cleveland. At the 2020 census, the city had 17,975 people. Like many other cities in the ...

, when of ammonium dichromate exploded as it was being dried in a heater.

References

External links

{{Chromates and dichromates Ammonium compounds Dichromates Explosive chemicals Light-sensitive chemicals Oxidizing agents