Chromic acid is a chemical compound with the

chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...

. It is also a jargon for a solution formed by the addition of

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

to aqueous solutions of dichromate. It consists at least in part of chromium trioxide. The term "chromic acid" is usually used for a mixture made by adding concentrated

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 molecular species, of which the trioxide is the anhydride. Chromic acid features chromium in an

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 ...

of +6 (and a valence of VI or 6). It is a strong and corrosive oxidizing agent and a moderate

carcinogen A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and biologic agents such as viruse ...

Molecular chromic acid

Molecular chromic acid, , in principle, resembles

, . It would ionize accordingly: : The p''K''_a for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6. The structure of the mono anion has been determined by

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

. In this tetrahedral oxyanion, three Cr-O bond lengths are 156 pm and the Cr-OH bond is 201 pm condenses to form dichromate: :, log''K''_D = 2.05. Furthermore, the dichromate can be protonated: :, p''K''_a = 1.8 Loss of the second proton occurs in the pH range 4–8, making the ion a weak acid. Molecular chromic acid could in principle be made by adding chromium trioxide to water (''cf.'' manufacture of sulfuric acid). : In practice, the reverse reaction occurs: molecular chromic acid dehydrates. Some insights can be gleaned from observations on the reaction of dichromate solutions with sulfuric acid. The first colour change from orange to red signals the conversion of dichromate to chromic acid. Under these conditions deep red crystals of chromium trioxide precipitate from the mixture, without further colour change. Chromium trioxide is the anhydride of molecular chromic acid. It is a Lewis acid and can react with a Lewis base, such as pyridine in a non-aqueous medium such as dichloromethane ( Collins reagent). 290300-ICSD

Higher chromic acids with the formula are probable components of concentrated solutions of chromic acid.

Uses

Chromic acid is an intermediate in chromium plating, and is also used in ceramic glazes, and colored glass. Because a solution of chromic acid in sulfuric acid (also known as a sulfochromic mixture or chromosulfuric acid) is a powerful oxidizing agent, it can be used to clean laboratory glassware, particularly of otherwise insoluble organic residues. This application has declined due to environmental concerns. Furthermore, the acid leaves trace amounts of paramagnetic chromic ions () that can interfere with certain applications, such as NMR spectroscopy. This is especially the case for NMR tubes.

Piranha solution Piranha solution (), also known as piranha etch, is a mixture of sulfuric acid () and hydrogen peroxide (). The resulting mixture is used to clean organic residues off substrates, for example silicon wafers. Because the mixture is a strong o ...

can be used for the same task, without leaving metallic residues behind. Chromic acid was widely used in the musical instrument repair industry, due to its ability to "brighten" raw

brass Brass is an alloy of copper and zinc, in proportions which can be varied to achieve different colours and mechanical, electrical, acoustic and chemical properties, but copper typically has the larger proportion, generally copper and zinc. I ...

. A chromic acid dip leaves behind a bright yellow patina on the brass. Due to growing health and environmental concerns, many have discontinued use of this chemical in their repair shops. It was used in

hair dye Hair coloring, or hair dyeing, is the practice of changing the color of the hair on humans' heads. The main reasons for this are cosmetic: to cover gray or white hair, to alter hair to create a specific look, to change a color to suit preferen ...

in the 1940s, under the name ''Melereon''. It is used as a bleach in processing black and white photographic reversal film.

Reactions

Chromic acid is capable of oxidizing many kinds of

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

s and many variations on this reagent have been developed: * Chromic acid in aqueous

and

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

is known as the Jones reagent, which will oxidize primary and secondary alcohols to

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

s and

ketone 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 ( ...

s respectively, while rarely affecting unsaturated bonds.Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, * Pyridinium chlorochromate is generated from chromium trioxide and pyridinium chloride. This reagent converts primary alcohols to the corresponding aldehydes (R–CHO). * Collins reagent is an adduct of chromium trioxide and pyridine used for diverse oxidations. * Chromyl chloride, is a well-defined molecular compound that is generated from chromic acid.

Illustrative transformations

* Oxidation of methylbenzenes to benzoic acids. * Oxidative scission of indene to homophthalic acid. * Oxidation of secondary alcohol to ketone (cyclooctanone) and .

Use in qualitative organic analysis

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, dilute solutions of chromic acid can be used to oxidize primary or secondary alcohols to the corresponding aldehydes and

s. Similarly, it can also be used to oxidize an aldehyde to its corresponding

. Tertiary alcohols and ketones are unaffected. Because the

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

is signaled by a color change from orange to brownish green (indicating chromium being reduced from oxidation state +6 to +3), chromic acid is commonly used as a lab reagent in high school or undergraduate college chemistry as a qualitative analytical test for the presence of primary or secondary alcohols, or aldehydes.

Alternative reagents

In oxidations of alcohols or aldehydes into

s, chromic acid is one of several reagents, including several that are catalytic. For example, nickel(II) salts catalyze oxidations by bleach (hypochlorite). Aldehydes are relatively easily oxidized to carboxylic acids, and mild oxidizing agents are sufficient. Silver(I) compounds have been used for this purpose. Each oxidant offers advantages and disadvantages. Instead of using chemical oxidants, electrochemical oxidation is often possible.

Safety

Hexavalent chromium compounds (including chromium trioxide, chromic acids, chromates, chlorochromates) are toxic and

carcinogenic A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and Biological agent, biologic agent ...

. Chromium trioxide and chromic acids are strong oxidizers and may react violently if mixed with easily oxidizable organic substances. Chromic acid burns are treated with a dilute sodium thiosulfate solution.

Notes

References

Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds
( PDF)

External links

* *
IARC Monograph "Chromium and Chromium compounds"
{{Chromium compounds Chromates Mineral acids Oxidizing acids Oxoacids Transition metal oxoacids