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 Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple s ...

. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the molecular species, H₂CrO₄ of which the trioxide is the

anhydride An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...

. Chromic acid features chromium in an

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

of +6 (or VI). It is a strong and corrosive

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 ). In other words, an oxid ...

Molecular chromic acid

Molecular chromic acid, H₂CrO₄, has much in common with sulfuric acid, H₂SO₄. Only sulfuric acid can be classified as part of the 7

strong acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions ...

s list. Due to the laws pertinent to the concept of "first order ionization energy", the first proton is lost most easily. It behaves extremely similar to sulfuric acid deprotonation. Since the process of polyvalent acid-base titrations have more than one proton (especially when the acid is starting substance and the base is the titrant), protons can only leave an acid one at a time. Hence the first step is as follows: :H₂CrO₄ CrO₄sup>− + H⁺ The p''K''_a for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6. The value at zero ionic strength is difficult to determine because half dissociation only occurs in very acidic solution, at about pH 0, that is, with an acid concentration of about 1 mol dm⁻³. A further complication is that the ion CrO₄sup>− has a marked tendency to dimerize, with the loss of a water molecule, to form the dichromate ion, r₂O₇sup>2−: :2 CrO₄sup>− r₂O₇sup>2− + H₂O log ''K''_D = 2.05. Furthermore, the dichromate can be protonated: : Cr₂O₇sup>− r₂O₇sup>2− + H⁺ p''K'' = 1.8 The p''K'' value for this reaction shows that it can be ignored at pH > 4. Loss of the second proton occurs in the pH range 4–8, making the ion CrO₄sup>− a

weak acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions ...

. Molecular chromic acid could in principle be made by adding chromium trioxide to water (''cf.'' manufacture of sulfuric acid). :CrO₃ + H₂O H₂CrO₄ but in practice the reverse reaction occurs when molecular chromic acid is

dehydrated In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...

. This is what happens when concentrated sulfuric acid is added to a dichromate solution. At first the colour changes from orange (dichromate) to red (chromic acid) and then deep red crystals of chromium trioxide precipitate from the mixture, without further colour change. The colours are due to LMCT transitions. Chromium trioxide is the

of molecular chromic acid. It is a Lewis acid and can react with a Lewis base, such as

pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a ...

in a non-aqueous medium such as dichloromethane (

Collins reagent Collins reagent is the complex of chromium(VI) oxide with pyridine in dichloromethane. This metal-pyridine complex, a red solid, is used to oxidize primary alcohols to the corresponding aldehydes and secondary alcohols to the corresponding keton ...

Dichromic acid

Dichromic acid, H₂Cr₂O₇ is the fully protonated form of the dichromate ion and also can be seen as the product of adding chromium trioxide to molecular chromic acid. Dichromic acid will behave the same exact way when reacting with a primary or secondary alcohol. The caveat to this statement is that a secondary alcohol will be oxidized no further than a ketone, whereas a primary alcohol will be oxidized to a aldehyde for the first step of the mechanism and then oxidized again to a carboxylic acid, contingent on no significant steric hindrance impeding this reaction. Dichromic acid undergoes the following reaction: : r₂O₇sup>2− + 2H⁺ H₂Cr₂O₇ H₂CrO₄ + CrO₃ It is probably present in chromic acid cleaning mixtures along with the mixed chromosulfuric acid H₂CrSO₇.

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 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 ). In other words, an oxi ...

, it can be used to clean

laboratory glassware Laboratory glassware refers to a variety of equipment used in scientific work, and traditionally made of glass. Glass can be blown, bent, cut, molded, and formed into many sizes and shapes, and is therefore common in chemistry, biology, and anal ...

, particularly of otherwise insoluble organic residues. This application has declined due to environmental concerns. Furthermore, the acid leaves trace amounts of

paramagnetic Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...

chromic ions () that can interfere with certain applications, such as

NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fie ...

. This is especially the case for

NMR tubes An NMR tube is a thin glass walled tube used to contain samples in nuclear magnetic resonance spectroscopy. Typically NMR tubes come in 5 mm diameters but 10 mm and 3 mm samples are known. It is important that the tubes are uniforml ...

. 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 (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other wit ...

. 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 hair color. The main reasons for this are cosmetic: to cover gray or white hair, to change to a color regarded as more fashionable or desirable, or to restore the original hair color ...

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

Reactions

Chromic acid is capable of oxidizing many kinds of

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...

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

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 ...

is known as the

Jones reagent Jones may refer to: People *Jones (surname), a common Welsh and English surname *List of people with surname Jones * Jones (singer), a British singer-songwriter Arts and entertainment * Jones (''Animal Farm''), a human character in George Orwell ...

, which will oxidize primary and secondary alcohols to carboxylic acids and ketones respectively, while rarely affecting unsaturated bonds.Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, *

Pyridinium chlorochromate Pyridinium chlorochromate (PCC) is a yellow-orange salt with the formula 5H5NHsup>+ rO3Clsup>−. It is a reagent in organic synthesis used primarily for oxidation of alcohols to form carbonyls. A variety of related compounds are known with simil ...

is generated from chromium trioxide and

pyridinium chloride Pyridinium chloride is an organic chemical compound with a formula of C5H5NHCl. Preparation Pyridinium chloride can be produced by passing hydrogen chloride in pyridine dissolved in diethyl ether. The chemical formula is as follows: :C5H5N + ...

. This reagent converts primary alcohols to the corresponding

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

s (R–CHO). *

is an adduct of chromium trioxide and

used for diverse oxidations. *

Chromyl chloride Chromyl chloride is the inorganic compound with the formula CrO2Cl2. It is a reddish brown compound that is a volatile liquid at room temperature, which is unusual for transition metal complexes. Preparation Chromyl chloride can be prepared by th ...

, CrO₂Cl₂ is a well-defined molecular compound that is generated from chromic acid.

Illustrative transformations

* Oxidation of methylbenzenes to

benzoic acid Benzoic acid is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, ...

s. * Oxidative scission of

indene Indene is a flammable polycyclic hydrocarbon with chemical formula . It is composed of a benzene ring fused with a cyclopentene ring. This aromatic liquid is colorless although samples often are pale yellow. The principal industrial use of i ...

homophthalic acid Homophthalic acid is a dicarboxylic acid with the formula C6H4(CO2H)CH2CO2H. It is a colorless solid. The compounds can be prepared by the Willgerodt reaction from 2-acetylbenzoic acid. One of the uses is in the preparation of the NSAID ...

. * Oxidation of secondary alcohol to ketone (cyclooctanone) and nortricyclanone.

Use in qualitative organic analysis

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

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

s and ketones. Similarly, it can also be used to oxidize an aldehyde to its corresponding carboxylic acid.

Tertiary 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), whic ...

s and ketones are unaffected. Because the

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

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 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, or aldehydes.

Alternative reagents

In oxidations of alcohols or

s into carboxylic acids, 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 oxidised to carboxylic acids, and mild oxidising 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 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 ...

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

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subs ...

and

carcinogenic 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 subs ...

. For this reason, chromic acid oxidation is not used on an industrial scale except in the

aerospace Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial and military applications. Aerospace engineering consists of aeronautics and astr ...

industry. Chromium trioxide and chromic acids are strong oxidisers and may react violently if mixed with easily oxidisable organic substances. Fires or explosions may result. Chromic acid burns are treated with a dilute

sodium thiosulfate Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula . Typically it is available as the white or colorless pentahydrate, . The solid is an efflorescent (loses water readily) crystalline substance that dissolves well in ...

solution.

Notes

References

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

External links

* *
IARC Monograph "Chromium and Chromium compounds"
{{DEFAULTSORT:Chromic Acid Hydrogen compounds Chromates Oxidizing agents Oxidizing acids Mineral acids Transition metal oxoacids