Chromic acid
Dichromic acid
Structural formulae of dichromic acid (left) and chromic acid (right)
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
EC Number
  • 231-801-5
25982
UNII
UN number 1755 1463
  • InChI=1S/Cr.2H2O.2O/h;2*1H2;;/q+2;;;;/p-2 checkY
    Key: KRVSOGSZCMJSLX-UHFFFAOYSA-L checkY
  • 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)
Appearance Dark purplish-red sand-like crystalline solid or powder
Odor Odorless
Density 1.201 g/cm3
Melting point 197 °C (387 °F; 470 K)
Boiling point 250 °C (482 °F; 523 K) (decomposes)
169 g/(100 mL)
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:
GHS03: OxidizingGHS05: CorrosiveGHS06: ToxicGHS07: Exclamation markGHS08: Health hazardGHS09: Environmental hazard
Danger
H271, H300, H301, H310, H314, H317, H330, 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 four-colored diamond
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).
checkY verify (what is checkY☒N ?)
Infobox references

Chromic acid is jargon for a solution of formed by the addition of sulfuric acid to aqueous solutions of dichromate. It consists at least in part of chromium trioxide.[3]

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

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.

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.[7]

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.

References

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