The term
chromic acid is usually used for a mixture made by adding concentrated
sulfuric acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
to a dichromate, which may contain a variety of compounds, including solid
chromium trioxideChromium trioxide is the 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-red/orange brown solid, which dissolves in water concomitant with hydrolysis...
. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the molecular species, H
2CrO
4 of which the trioxide is the
anhydrideAn acidic oxide is an oxide that either*reacts with water to form an acid; or*reacts with a base to form a salt.Examples include:...
. Chromic acid features chromium in an
oxidation stateIn 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...
of +6 (or VI). It is a strong and corrosive oxidising agent.
Molecular chromic acid
Molecular chromic acid, H
2CrO
4, has much in common with
sulfuric acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, H
2SO
4. Both are classified as
strong acidA strong acid is an acid that ionizes completely in an aqueous solution by losing one proton, according to the equationFor sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first protonMore precisely, the acid must be stronger in aqueous solution than...
s, though only the first proton is lost easily.
- H2CrO4 , [HCrO4]− + H+
The
pKaAn acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
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 zero, that is, with an acid concentration of about 1 mol dm
−3. A further complication is that the ion [HCrO
4]
− has a marked tendency to dimerize, with the loss of a water molecule, to form the dichromate ion, [Cr
2O
7]
2−
- 2 [HCrO4]− [Cr2O7]2− + H2O, log KD = 2.05.
Furthermore, The dichromate can be protonated.
- [HCr2O7]− [Cr2O7]2− + H+, pK = 1.8
The pK value for this reaction shows that is can be ignored at pH > 4.
Loss of the second proton occurs in the pH range 4–8, making the ion [HCrO
4]
− a
weak acidA weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...
.
Molecular chromic acid could in principle be made by adding chromium trioxide to water (c.f.
manufacture of sulfuric acidThe contact process is the current method of producing sulphuric acid in the high concentrations needed for industrial processes. Platinum was formerly employed as a catalyst for the reaction, but as it is susceptible to poisoning by arsenic impurities in the sulfur feedstock, vanadium oxide is...
).
- CrO3 + H2O H2CrO4
but in practice the reverse reaction occurs when molecular chromic acid is
dehydratedIn chemistry and the biological sciences, a dehydration reaction is usually defined as a chemical reaction that involves the loss of water from the reacting molecule. Dehydration reactions are a subset of elimination reactions...
. 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 charge transfer transitions.
Chromium trioxide is the
anydrideAn acidic oxide is an oxide that either*reacts with water to form an acid; or*reacts with a base to form a salt.Examples include:...
of molecular chromic acid. It is a
Lewis acid]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
and can react with a Lewis base, such as
pyridinePyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
in a non-aqueous medium such as dichloromethane (
Collins reagentCollins reagent is the complex of chromium oxide with pyridine in dichloromethane. It is used to selectively oxidize primary alcohols to the aldehyde, and will tolerate many other functional groups within the molecule....
).
Dichromic acid
Dichromic acid, H
2Cr
2O
7, (structure illustrated top right) 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.
- [Cr2O7]2− + 2H+ H2Cr2O7 H2CrO4 + CrO3
It is probably present in chromic acid cleaning mixtures along with the mixed chromic-sulfuric acid H
2CrSO
7
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 agentAn oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
, it can be used to
cleanFor the article about the tube cleaning of steam locomotive boilers see: Tube cleaning .Tube cleaning describes the activity of, or device for, the cleaning and maintenance of fouled tubes....
laboratory glasswareLaboratory glassware refers to a variety of equipment, traditionally made of glass, used for scientific experiments and other work in science, especially in chemistry and biology laboratories...
, particularly of otherwise insoluble organic residues. This application has declined due to environmental concerns. Furthermore the acid leaves trace amounts of paramagnetic chromic ions — Cr(III) — that can interfere with certain applications, such as
NMR spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
. This is especially the case for NMR tubes.
Chromic acid was widely used in the instrument repair industry, due to its ability to "brighten" raw brass. 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.
Reactions
Chromic acid is capable of oxidizing many kinds of
organic compoundAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s and many variations on this reagent have been developed:
- Chromic acid in aqueous sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
and acetone is known as the Jones reagent, which will oxidize primary and secondary alcohols to carboxylic acidCarboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s and ketoneIn organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
s respectively, while rarely affecting unsaturated bonds.
- Pyridinium chlorochromate
Pyridinium chlorochromate is a reddish orange solid reagent used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. Pyridinium chlorochromate, or PCC, will not fully oxidize a primary alcohol to the carboxylic acid as does the Jones reagent. A disadvantage to using PCC is...
is generated from chromium trioxide and pyridinium chloride. This reagent converts primary alcohols to the corresponding aldehydeAn 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 (R-CHO).
- Collins reagent
Collins reagent is the complex of chromium oxide with pyridine in dichloromethane. It is used to selectively oxidize primary alcohols to the aldehyde, and will tolerate many other functional groups within the molecule....
is an adduct of chromium trioxide and pyridine used for diverse oxidations.
- Chromyl chloride
Chromyl chloride is a chemical compound with the formula CrO2Cl2. This compound is an opaque dark blood-red liquid at room temperature and pressure. It is tetrahedral, somewhat like SO2Cl2. CrO2Cl2 is similar to the most commonly encountered chromium derivative chromate, [CrO4]2−; both are...
, CrO2Cl2 is a well-defined molecular compound that is generated from chromic acid.
Illustrative transformations
- Oxidation of methylbenzenes to benzoic acid
Benzoic acid , C7H6O2 , is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis...
s.
- Oxidative scission of indene
Indene is a flammable polycyclic hydrocarbon with chemical formula C9H8. It is composed of a benzene ring fused with a cyclopentene ring. This aromatic liquid is colorless although samples often are pale yellow...
to homophthalic acid.
- Oxidation of secondary alcohol to ketone (cyclooctanone) and nortricyclanone.
Use in qualitative organic analysis
In
organic chemistryOrganic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, dilute
solutionIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
s of hexavalent chromium can be used to oxidize primary or secondary
alcoholIn 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 to the corresponding
aldehydeAn 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
ketoneIn organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
s. Tertiary alcohol groups are unaffected. Because of the oxidation is signaled by a color change from orange to a blue-green, chromic acid is used as a qualitative analytical test for the presence of primary or secondary alcohols.
Alternative reagents
In oxidations of alcohols or aldehydes 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.
Safety
Hexavalent chromiumHexavalent 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...
compounds are toxic and carcinogenic. For this reason, chromic acid oxidation is not used on an industrial scale.
External links