Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an

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

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 betwee ...

CrO₃. It is the acidic anhydride of

chromic 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 mixt ...

, and is sometimes marketed under the same name. This compound is a dark-purple

solid Solid is one of the State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...

under anhydrous conditions, bright orange when wet and which dissolves in water concomitant with hydrolysis. Millions of kilograms are produced annually, mainly for

electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be ...

. Chromium trioxide is a powerful oxidiser and a

carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substan ...

Production, structure, and basic reactions

Chromium trioxide is generated by treating

sodium dichromate Sodium dichromate is the inorganic compound with the formula Na2 Cr2 O7. However, the salt is usually handled as its dihydrate Na2Cr2O7·2 H2O. Virtually all chromium ore is processed via conversion to sodium dichromate and virtually all compound ...

with

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

: :H₂SO₄ + Na₂Cr₂O₇ → 2 CrO₃ + Na₂SO₄ + H₂O Approximately 100,000 tonnes are produced annually by this or similar routes. The solid consists of chains of tetrahedrally coordinated chromium atoms that share vertices. Each

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...

center therefore shares two oxygen centers with neighbors. Two oxygen atoms are not shared, giving an overall stoichiometry of 1:3. : CrO3-from-xtal-1970-chain-3D-balls

The structure of monomeric CrO₃ has been calculated using

density functional theory Density-functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground state) of many-body ...

, and is predicted to be

pyramidal A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrilater ...

(

point group In geometry, a point group is a mathematical group of symmetry operations (isometries in a Euclidean space) that have a fixed point in common. The coordinate origin of the Euclidean space is conventionally taken to be a fixed point, and every p ...

C_3v) rather than

planar Planar is an adjective meaning "relating to a plane (geometry)". Planar may also refer to: Science and technology * Planar (computer graphics), computer graphics pixel information from several bitplanes * Planar (transmission line technologies), ...

(point group D_3h). : CrO3-monomer-from-DFT-PW91-aD-2008-side-3D-balls

Chromium trioxide decomposes above 197 °C, liberating oxygen and eventually giving Cr₂O₃: :4 CrO₃ → 2 Cr₂O₃ + 3 O₂ It is used in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

as an oxidant, often as a solution in

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...

, or

acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscib ...

in the case of the

Jones oxidation The Jones oxidation is an organic reaction for the oxidation of primary and secondary alcohols to carboxylic acids and ketones, respectively. It is named after its discoverer, Sir Ewart Jones. The reaction was an early method for the oxidation o ...

. In these oxidations, the Cr(VI) converts

primary alcohol A primary alcohol is an alcohol in which the hydroxy group is bonded to a primary carbon atom. It can also be defined as a molecule containing a “–CH2OH” group. In contrast, a secondary alcohol has a formula “–CHROH” and a tertiary ...

s to the corresponding

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

s and

secondary alcohol In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...

s to

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...

s. The reactions are shown below: * Primary alcohols to carboxylic acids *:4 CrO₃ + 3 RCH₂OH + 12 H⁺ → 3 RCOOH + 4 Cr³⁺ + 9 H₂O * Secondary alcohols to ketones *:2 CrO₃ + 3 R₂CHOH + 6 H⁺ → 3 R₂C=O + 2 Cr³⁺ + 6 H₂O

Applications

Chromium trioxide is mainly used in

chrome plating Chrome plating (less commonly chromium plating) is a technique of electroplating a thin layer of chromium onto a metal object. A chrome-plated item is called ''chrome''. The chromed layer can be decorative, provide corrosion resistance, ease ...

. It is typically employed with additives that affect the plating process but do not react with the trioxide. The trioxide reacts with

cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

, and other metals to generate passivating chromate films that resist

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

. It is also used in the production of synthetic rubies. Chromic acid solution is also used in applying types of anodic coating to

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

, which are primarily used in aerospace applications. On the International Space Station, it is used to control bacteria growth in the wastewater storage tank. A chromic acid/

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...

solution is also the preferred stripping agent of anodic coatings of all types.

Safety

Chromium trioxide is highly toxic, corrosive, and carcinogenic. It is the main example of

hexavalent chromium Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium in any chemical compound that contains the element in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the ...

, an

environmental hazard An environmental hazard is a substance, state or event which has the potential to threaten the surrounding natural environment or adversely affect people's health, including pollution and natural disasters such as storms and earthquakes. It can i ...

.The environmental impact of hexavalent chromium inspired the 2000 biographical Hollywood movie ''

Erin Brockovich Erin Brockovich (née Pattee; born June 22, 1960) is an American legal clerk, consumer advocate, and environmental activist who, despite her lack of education in the law, was instrumental in building a case against Pacific Gas & Electric Compan ...

''. The related chromium(III) derivatives are not particularly dangerous; thus,

reductant 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 commonly reducing agents include the Earth meta ...

s are used to destroy chromium(VI) samples. Chromium trioxide, being a powerful oxidizer, will ignite organic materials such as

alcohols In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...

on contact.

Images

Reaction between potassium dichromate and sulfuric acid (1).jpg, A concentrated solution of potassium dichromate in water. Reaction between potassium dichromate and sulfuric acid (2).jpg, Addition of sulfuric acid to the solution. Reaction between potassium dichromate and sulfuric acid (3).jpg, Crystallization of chromium trioxide from the reaction. Reaction between chromium(VI) oxide and ethanol (1).JPG, Reaction between chromium trioxide and ethanol Reaction between chromium(VI) oxide and ethanol (2).JPG Reaction between chromium(VI) oxide and ethanol (3).JPG

References

External links

ATSDR Case Studies in Environmental Medicine: Chromium Toxicity
U.S.

Department of Health and Human Services The United States Department of Health and Human Services (HHS) is a cabinet-level executive branch department of the U.S. federal government created to protect the health of all Americans and providing essential human services. Its motto is " ...

Chromium Trioxide
at ''

The Periodic Table of Videos ''Periodic Videos'' (also known as ''The Periodic Table of Videos'') is a video project and YouTube channel on chemistry. It consists of a series of videos about chemical elements and the periodic table, with additional videos on other topics i ...

'' (University of Nottingham)
Reactions with Chromium Trioxide as Oxidizing Agent
{{oxygen compounds Acidic oxides Chromium(VI) compounds