Chromic acid
Dichromic acid (left) and chromic acid (right)
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Names | |
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IUPAC names
Chromic acid
Dichromic acid | |
Systematic IUPAC name
Dihydroxidodioxidochromium | |
Other names
Chromic(VI) acid
Tetraoxochromic acid | |
Identifiers | |
3D model (
JSmol ) |
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ChEBI | |
ChemSpider | |
ECHA InfoCard
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100.028.910 |
EC Number |
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25982 | |
PubChem CID
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UNII | |
UN number | 1755 1463 |
CompTox Dashboard (EPA)
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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[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
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Chromate and dichromate |
Hazards | |
Occupational safety and health (OHS/OSH): | |
Main hazards
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highly toxic, carcinogen, corrosive |
GHS labelling: | |
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) | |
Lethal dose or concentration (LD, LC): | |
LD50 (median dose)
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51.9 mg/kg (H2CrO4·2Na, rat, oral)[2] |
NIOSH (US health exposure limits): | |
PEL (Permissible)
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TWA 0.005 mg/m3[1] |
REL (Recommended)
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TWA 0.001 mg Cr(VI)/m3[1] |
IDLH (Immediate danger) |
15 mg Cr(VI)/m3[1] |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Chromic acid is jargon for a solution formed by the addition of
The term chromic acid is usually used for a mixture made by adding concentrated
Molecular chromic acid
Molecular chromic acid, H2CrO4, in principle, resembles sulfuric acid, H2SO4. It would ionize accordingly:
- H2CrO4 ⇌ [HCrO4]− + H+
The
[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
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
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
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
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:
- Chromic acid in aqueous alcohols to carboxylic acids and ketones respectively, while rarely affecting unsaturated bonds.[12]
- Pyridinium chlorochromate is generated from chromium trioxide and pyridinium chloride. This reagent converts primary alcohols to the corresponding aldehydes (R–CHO).[12]
- Collins reagent is an adduct of chromium trioxide and pyridine used for diverse oxidations.
- Chromyl chloride, CrO2Cl2 is a well-defined molecular compound that is generated from chromic acid.
Illustrative transformations
- Oxidation of methylbenzenes to benzoic acids.[13]
- Oxidative scission of indene to homophthalic acid.[14]
- Oxidation of secondary alcohol to ketone (cyclooctanone)[15] and nortricyclanone.[16]
Use in qualitative organic analysis
In
Alternative reagents
In oxidations of
Safety
Chromic acid burns are treated with a dilute sodium thiosulfate solution.[18]
Notes
- ^ a b c NIOSH Pocket Guide to Chemical Hazards. "#0138". National Institute for Occupational Safety and Health (NIOSH).
- ^ "Chromic acid and chromates". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
- ISBN 978-0-08-037941-8.
- ^ IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligands
- .
- .
- PMID 23424393.
- ^ J. M. McCormick (2006-06-30). "Cleaning Glassware". Truman State University. Archived from the original on 2008-12-07. Retrieved 2010-12-18.
- ^ "NMR-010: Proper Cleaning Procedures for NMR Sample Tubes". Wilmad. Archived from the original on 2008-05-13. Retrieved 2008-06-27.
- ^ "Watson v Buckley, Osborne, Garrett & Co Ltd and Wyrovoys Products Ltd [1940] 1 All ER 174".
- ^ "Fomapan R" (PDF). Fomapan R. Foma. Archived from the original (PDF) on 18 April 2016. Retrieved 6 April 2016.
- ^
- ^ Kamm O.; Matthews, A. O. (1941). "p-Nitrobenzoic Acid". Organic Syntheses; Collected Volumes, vol. 1, p. 392.
- ^ Grummitt, O.; Egan, R.; Buck, A. "Homophthalic Acid and Anhydride". Organic Syntheses; Collected Volumes, vol. 3, pp. 449 (1955.
- ^ Eisenbraun, E. J. "Cyclooctanone". Organic Syntheses; Collected Volumes, vol. 5, pp. 310 (1973.
- ^ Meinwald, J.; Crandall, J.; Hymans W. E. "Nortricyclanone". Organic Syntheses; Collected Volumes, vol. 5, p. 866.
- PMID 17137354.
- PMID 15191982.
References
- Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds[permanent dead link] (PDF)
External links
- International Chemical Safety Card 1194
- NIOSH Pocket Guide to Chemical Hazards. "#0138". National Institute for Occupational Safety and Health (NIOSH).
- IARC Monograph "Chromium and Chromium compounds"