Chromic acid

Last updated
Chromic acid
Dichromic acid
ChromicAcid.svg
Dichromic acid (left) and chromic acid (right)
Names
IUPAC names
Chromic acid
Dichromic acid
Systematic IUPAC name
Dihydroxidodioxidochromium
Other names
Chromic(VI) acid
Tetraoxochromic acid
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.028.910 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 231-801-5
25982
PubChem CID
UNII
UN number 1755 1463
  • InChI=1S/Cr.2H2O.2O/h;2*1H2;;/q+2;;;;/p-2 Yes check.svgY
    Key: KRVSOGSZCMJSLX-UHFFFAOYSA-L Yes check.svgY
  • InChI=1/Cr.2H2O.2O/h;2*1H2;;/q+2;;;;/p-2/rCrH2O4/c2-1(3,4)5/h2-3H
    Key: KRVSOGSZCMJSLX-OOUCQFSRAZ
  • O[Cr](O)(=O)=O
  • O=[Cr](=O)(O)O
Properties
H2CrO4 (chromic acid)
H2Cr2O7 (dichromic acid)
Molar mass 118.008 g/mol (chromic acid)
218.001 g/mol (dichromic acid)
AppearanceDark purplish-red sand-like crystalline solid or powder[ clarification needed ]
Odor Odorless
Density 1.201 g/cm3[ clarification needed ]
Melting point 197 °C (387 °F; 470 K) [ clarification needed ]
Boiling point 250 °C (482 °F; 523 K) (decomposes)[ clarification needed ]
169 g/(100 mL)[ clarification needed ]
Acidity (pKa)−0.8 to 1.6 (chromic acid)
Conjugate base Chromate and dichromate
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
highly toxic, carcinogen, corrosive
GHS labelling:
GHS-pictogram-rondflam.svg GHS-pictogram-acid.svg GHS-pictogram-skull.svg GHS-pictogram-exclam.svg GHS-pictogram-silhouette.svg GHS-pictogram-pollu.svg
Danger
H271, H300+H310+H330, H301, H314, H317, H334, H340, H341, H350, H361, H372, H410
P201, P202, P210, P220, P221, P260, P261, P262, P264, P270, P271, P272, P273, P280, P281, P283, P284, P285, P301+P310, P301+P330+P331, P302+P350, P302+P352, P303+P361+P353, P304+P340, P304+P341, P305+P351+P338, P306+P360, P308+P313, P310, P314, P320, P321, P322, P330, P333+P313, P342+P311, P361, P363, P370+P378, P371+P380+P375, P391, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704.svgHealth 4: Very short exposure could cause death or major residual injury. E.g. VX gasFlammability 0: Will not burn. E.g. waterInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazard COR: Corrosive; strong acid or base. E.g. sulfuric acid, potassium hydroxide
4
0
1
COR
Lethal dose or concentration (LD, LC):
51.9 mg/kg (H2CrO4·2Na, rat, oral) [1]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 0.005 mg/m3 [2]
REL (Recommended)
TWA 0.001 mg Cr(VI)/m3 [2]
IDLH (Immediate danger)
15 mg Cr(VI)/m3 [2]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Chromic acid is 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. [3]

Contents

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 mixture for glass. Chromic acid may also refer to the molecular species, H2CrO4 of which the trioxide is the anhydride. Chromic acid features chromium in an oxidation state of +6 (and a valence of VI or 6). It is a strong and corrosive oxidising agent and a moderate carcinogen.

Molecular chromic acid

Partial predominance diagram for chromate Predominance diagram Cr.png
Partial predominance diagram for chromate

Molecular chromic acid, H2CrO4, in principle, resembles sulfuric acid, H2SO4. It would ionize accordingly:

H2CrO4[HCrO4] + H+

The pKa for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6. [4] The structure of the mono anion has been determined by X-ray crystallography. In this tetrahedral oxyanion, three Cr-O bond lengths are 156 pm and the Cr-OH bond is 201 pm [5]

[HCrO4] condenses to form dichromate:

2 [HCrO4][Cr2O7]2− + H2O, logKD = 2.05.

Furthermore, the dichromate can be protonated:

[HCr2O7][Cr2O7]2− + H+, pKa = 1.8 [6]

Loss of the second proton occurs in the pH range 4–8, making the ion [HCrO4] a weak acid.[ citation needed ]

Molecular chromic acid could in principle be made by adding chromium trioxide to water (cf. manufacture of sulfuric acid).

CrO3 + H2O ⇌ H2CrO4

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).

Structure of tetrachromic acid
H2Cr4O13*2H2O, one component of concentrated "chromic acid". The H-atom positions are calculated, not observed. Color code: red = O, white = H, blue = Cr. 290300-ICSD.png
Structure of tetrachromic acid H2Cr4O13·2H2O, one component of concentrated "chromic acid". The H-atom positions are calculated, not observed. Color code: red = O, white = H, blue = Cr.

Higher chromic acids with the formula H2CrnO(3n+1) 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. [8] Furthermore, the acid leaves trace amounts of paramagnetic chromic ions (Cr3+) that can interfere with certain applications, such as NMR spectroscopy. This is especially the case for NMR tubes. [9] Piranha solution 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. 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 in the 1940s, under the name Melereon. [10]

It is used as a bleach in black and white photographic reversal processing. [11]

Reactions

Chromic acid is capable of oxidizing many kinds of organic compounds and many variations on this reagent have been developed:

Illustrative transformations

Use in qualitative organic analysis

In organic chemistry, dilute solutions of chromic acid can be used to oxidize primary or secondary alcohols to the corresponding aldehydes and ketones. Similarly, it can also be used to oxidize an aldehyde to its corresponding carboxylic acid. Tertiary alcohols and ketones are unaffected. Because the oxidation 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. [12]

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). [17] 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 compounds (including chromium trioxide, chromic acids, chromates, chlorochromates) are toxic and carcinogenic. Chromium trioxide and chromic acids are strong oxidisers and may react violently if mixed with easily oxidisable organic substances.

Chromic acid burns are treated with a dilute sodium thiosulfate solution. [18]

Notes

  1. "Chromic acid and chromates". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  2. 1 2 3 NIOSH Pocket Guide to Chemical Hazards. "#0138". National Institute for Occupational Safety and Health (NIOSH).
  3. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN   978-0-08-037941-8.
  4. IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligands
  5. Mukherjee, A. K.; Mukhopadhaya, A.; Mukherjee, M.; Ray, S. (1994). "Two Mononuclear Tetraphenylphosphonium Oxochromium Complexes: (PPh4)[CrVO3(H2O)] and (PPh4)[CrVIO3(OH)]". Acta Crystallographica Section C Crystal Structure Communications. 50 (9): 1401–1404. Bibcode:1994AcCrC..50.1401M. doi:10.1107/S0108270194003148.
  6. Brito, F.; Ascanioa, J.; Mateoa, S.; Hernándeza, C.; Araujoa, L.; Gili, P.; Martín-Zarzab, P.; Domínguez, S.; Mederos, A. (1997). "Equilibria of Chromate(VI) Species in Acid Medium and ab initio Studies of These Species". Polyhedron. 16 (21): 3835–3846. doi:10.1016/S0277-5387(97)00128-9.
  7. Kulikov, Vladislav; Meyer, Gerd (2013). "Dihydronium Tetrachromate(VI), (H3O)2Cr4O13". Acta Crystallographica Section E: Structure Reports Online. 69 (2): i13. Bibcode:2013AcCrE..69I..13K. doi:10.1107/S1600536813001608. PMC   3569175 . PMID   23424393.
  8. J. M. McCormick (2006-06-30). "Cleaning Glassware". Truman State University. Archived from the original on 2008-12-07. Retrieved 2010-12-18.
  9. "NMR-010: Proper Cleaning Procedures for NMR Sample Tubes". Wilmad. Archived from the original on 2008-05-13. Retrieved 2008-06-27.
  10. "Watson v Buckley, Osborne, Garrett & Co Ltd and Wyrovoys Products Ltd [1940] 1 All ER 174".
  11. "Fomapan R" (PDF). Fomapan R. Foma. Archived from the original (PDF) on 18 April 2016. Retrieved 6 April 2016.
  12. 1 2 3 Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, doi : 10.1002/047084289X.rc164
  13. Kamm O.; Matthews, A. O. (1941). "p-Nitrobenzoic Acid". Organic Syntheses .; Collective Volume, vol. 1, p. 392
  14. Grummitt, O.; Egan, R.; Buck, A. "Homophthalic Acid and Anhydride". Organic Syntheses .; Collective Volume, vol. 3, pp. 449 (1955
  15. Eisenbraun, E. J. "Cyclooctanone". Organic Syntheses .; Collective Volume, vol. 5, pp. 310 (1973
  16. Meinwald, J.; Crandall, J.; Hymans W. E. "Nortricyclanone". Organic Syntheses .; Collective Volume, vol. 5, p. 866
  17. J. M. Grill; J. W. Ogle; S. A. Miller (2006). "An Efficient and Practical System for the Catalytic Oxidation of Alcohols, Aldehydes, and α,β-Unsaturated Carboxylic Acids". J. Org. Chem. 71 (25): 9291–9296. doi:10.1021/jo0612574. PMID   17137354.
  18. Hettiaratchy, Shehan; Dziewulski, Peter (2004-06-12). "Pathophysiology and types of burns". BMJ: British Medical Journal. 328 (7453): 1427–1429. doi:10.1136/bmj.328.7453.1427. ISSN   0959-8138. PMC   421790 . PMID   15191982.

Related Research Articles

<span class="mw-page-title-main">Carboxylic acid</span> Organic compound containing a –C(=O)OH group

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 often written as R−COOH or R−CO2H, sometimes as R−C(O)OH with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic acids occur widely. Important examples include the amino acids and fatty acids. Deprotonation of a carboxylic acid gives a carboxylate anion.

<span class="mw-page-title-main">Aldehyde</span> Organic compound containing the functional group R−CH=O

In organic chemistry, an aldehyde is an organic compound containing a functional group with the structure R−CH=O. The functional group itself can be referred to as an aldehyde but can also be classified as a formyl group. Aldehydes are a common motif in many chemicals important in technology and biology.

<span class="mw-page-title-main">Oxidizing agent</span> Chemical compound used to oxidize another substance in a chemical reaction

An oxidizing agent is a substance in a redox chemical reaction that gains or "accepts"/"receives" an electron from a reducing agent. In other words, an oxidizer is any substance that oxidizes another substance. The oxidation state, which describes the degree of loss of electrons, of the oxidizer decreases while that of the reductant increases; this is expressed by saying that oxidizers "undergo reduction" and "are reduced" while reducers "undergo oxidation" and "are oxidized". Common oxidizing agents are oxygen, hydrogen peroxide, and the halogens.

<span class="mw-page-title-main">Chromate and dichromate</span> Chromium(VI) anions

Chromate salts contain the chromate anion, CrO2−
4
. Dichromate salts contain the dichromate anion, Cr
2
O2−
7
. They are oxyanions of chromium in the +6 oxidation state and are moderately strong oxidizing agents. In an aqueous solution, chromate and dichromate ions can be interconvertible.

<span class="mw-page-title-main">Potassium dichromate</span> Chemical compound

Potassium dichromate, K2Cr2O7, is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health. It is a crystalline ionic solid with a very bright, red-orange color. The salt is popular in laboratories because it is not deliquescent, in contrast to the more industrially relevant salt sodium dichromate.

<span class="mw-page-title-main">Pyridinium chlorochromate</span> Chemical compound

Pyridinium chlorochromate (PCC) is a yellow-orange salt with the formula [C5H5NH]+[CrO3Cl]. It is a reagent in organic synthesis used primarily for oxidation of alcohols to form carbonyls. A variety of related compounds are known with similar reactivity. PCC offers the advantage of the selective oxidation of alcohols to aldehydes or ketones, whereas many other reagents are less selective.

<span class="mw-page-title-main">Permanganate</span> Chemical compound

A permanganate is a chemical compound with the manganate(VII) ion, MnO
4
, the conjugate base of permanganic acid. Because the manganese atom has a +7 oxidation state, the permanganate(VII) ion is a strong oxidising agent. The ion is a transition metal ion with a tetrahedral structure. Permanganate solutions are purple in colour and are stable in neutral or slightly alkaline media. The exact chemical reaction depends on the carbon-containing reactants present and the oxidant used. For example, trichloroethane (C2H3Cl3) is oxidised by permanganate ions to form carbon dioxide (CO2), manganese dioxide (MnO2), hydrogen ions (H+), and chloride ions (Cl).

<span class="mw-page-title-main">Chromium trioxide</span> Chemical compound

Chromium trioxide 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 solid under anhydrous conditions and bright orange when wet. The substance dissolves in water accompanied by hydrolysis. Millions of kilograms are produced annually, mainly for electroplating. Chromium trioxide is a powerful oxidiser, a mutagen, and a carcinogen.

<span class="mw-page-title-main">Sodium dichromate</span> Inorganic compound

Sodium dichromate is the inorganic compound with the formula Na2Cr2O7. However, the salt is usually handled as its dihydrate Na2Cr2O7·2H2O. Virtually all chromium ore is processed via conversion to sodium dichromate and virtually all compounds and materials based on chromium are prepared from this salt. In terms of reactivity and appearance, sodium dichromate and potassium dichromate are very similar. The sodium salt is, however, around twenty times more soluble in water than the potassium salt (49 g/L at 0 °C) and its equivalent weight is also lower, which is often desirable.

<span class="mw-page-title-main">Chromyl chloride</span> Chemical compound

Chromyl chloride is an 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 compounds.

<span class="mw-page-title-main">Chromium compounds</span> Chemical compounds containing chromium

Chromium compounds are compounds containing the element chromium (Cr). Chromium is a member of group 6 of the transition metals. The +3 and +6 states occur most commonly within chromium compounds, followed by +2; charges of +1, +4 and +5 for chromium are rare, but do nevertheless occasionally exist.

The Étard reaction is a chemical reaction that involves the direct oxidation of an aromatic or heterocyclic bound methyl group to an aldehyde using chromyl chloride. For example, toluene can be oxidized to benzaldehyde.

<span class="mw-page-title-main">Sarett oxidation</span> Organic reaction

The Sarett oxidation is an organic reaction that oxidizes primary and secondary alcohols to aldehydes and ketones, respectively, using chromium trioxide and pyridine. Unlike the similar Jones oxidation, the Sarett oxidation will not further oxidize primary alcohols to their carboxylic acid form, neither will it affect carbon-carbon double bonds. Use of the original Sarett oxidation has become largely antiquated however, in favor of other modified oxidation techniques. The unadulterated reaction is still occasionally used in teaching settings and in small scale laboratory research.

<span class="mw-page-title-main">Collins reagent</span> Chemical compound

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 ketones. This complex is a hygroscopic orange solid.

<span class="mw-page-title-main">Cornforth reagent</span> Chemical compound

The pyridinium dichromate(PDC) or Cornforth reagent is a pyridinium salt of dichromate with the chemical formula [C5H5NH]2[Cr2O7]. This compound is named after the Australian-British chemist Sir John Warcup Cornforth (b. 1917) who introduced it in 1962. The Cornforth reagent is a strong oxidizing agent which can convert primary and secondary alcohols to aldehydes and ketones respectively. In its chemical structure and functions it is closely related to other compounds made from hexavalent chromium oxide, such as pyridinium chlorochromate and Collins reagent. Because of their toxicity, these reagents are rarely used nowadays.

<span class="mw-page-title-main">Sodium chromate</span> Chemical compound

Sodium chromate is the inorganic compound with the formula Na2CrO4. It exists as a yellow hygroscopic solid, which can form tetra-, hexa-, and decahydrates. It is an intermediate in the extraction of chromium from its ores.

Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters where the carbon carries a higher oxidation state. The reaction mainly applies to primary and secondary alcohols. Secondary alcohols form ketones, while primary alcohols form aldehydes or carboxylic acids.

<span class="mw-page-title-main">Chromium(VI) oxide peroxide</span> Chemical compound

Chromium(VI) oxide peroxide or chromium oxide peroxide or more accurately chromium(VI) oxide diperoxide is an inorganic compound with the chemical formula CrO5 or more accurately CrO(O2)2. It is an unstable dark blue compound. This compound contains one oxo ligand and two peroxo ligands, making a total of five oxygen atoms per chromium atom.

Oxidation with chromium(VI) complexes involves the conversion of alcohols to carbonyl compounds or more highly oxidized products through the action of molecular chromium(VI) oxides and salts. The principal reagents are Collins reagent, PDC, and PCC. These reagents represent improvements over inorganic chromium(VI) reagents such as Jones reagent.

<span class="mw-page-title-main">Jones oxidation</span> Oxidation of alcohol

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 of alcohols. Its use has subsided because milder, more selective reagents have been developed, e.g. Collins reagent.

References