Chromic acid

File:Predominance diagram Cr.png for chromate]]

Molecular chromic acid, {{chem2|H2CrO4}}, in principle, resembles sulfuric acid, {{chem2|H2SO4}}. It would ionize accordingly:

:{{chem2|H2CrO4 ⇌ [HCrO4]− + H+}}

The pK_a for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6.[http://www.acadsoft.co.uk/scdbase/scdbase.htm IUPAC SC-Database] A comprehensive database of published data on equilibrium constants of metal complexes and ligands 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{{cite journal |title=Two Mononuclear Tetraphenylphosphonium Oxochromium Complexes: (PPh₄)[Cr^VO₃(H₂O)] and (PPh₄)[Cr^VIO₃(OH)] |date=1994 |last1=Mukherjee |first1=A. K. |last2=Mukhopadhaya |first2=A. |last3=Mukherjee |first3=M. |last4=Ray |first4=S. |journal=Acta Crystallographica Section C Crystal Structure Communications |volume=50 |issue=9 |pages=1401–1404 |bibcode=1994AcCrC..50.1401M |doi=10.1107/S0108270194003148}}

{{chem2|[HCrO4]−}} condenses to form dichromate:

:{{chem2|2 [HCrO4]− ⇌ [Cr2O7](2−) + H2O}}, logK_D = 2.05.

Furthermore, the dichromate can be protonated:

:{{chem2|[HCr2O7]− ⇌ [Cr2O7](2−) + H+}}, pK_a = 1.8{{cite journal |last=Brito |first=F. |author2=Ascanioa, J. |author3=Mateoa, S. |author4=Hernándeza, C. |author5=Araujoa, L. |author6=Gili, P. |author7=Martín-Zarzab, P. |author8=Domínguez, S. |author9=Mederos, A. |year=1997 |title=Equilibria of Chromate(VI) Species in Acid Medium and ab initio Studies of These Species |journal=Polyhedron |volume=16 |issue=21 |pages=3835–3846 |doi=10.1016/S0277-5387(97)00128-9}}

Loss of the second proton occurs in the pH range 4–8, making the ion {{chem2|[HCrO4]−}} a weak acid.{{citation needed|date=January 2024}}

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

:{{chem2|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).

File:290300-ICSD.png

Higher chromic acids with the formula {{chem2|H2Cr_{n}O_{(3n+1)}|}} are probable components of concentrated solutions of chromic acid.

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.{{cite web |title=Cleaning Glassware |author=J. M. McCormick |date=2006-06-30 |publisher=Truman State University |url=http://chemlab.truman.edu/Miscellaneous_files/Cleaning.htm |access-date=2010-12-18 |url-status=dead |archive-url=https://web.archive.org/web/20081207035502/http://chemlab.truman.edu/Miscellaneous_files/Cleaning.htm |archive-date=2008-12-07}} Furthermore, the acid leaves trace amounts of paramagnetic chromic ions ({{chem2|Cr(3+)}}) that can interfere with certain applications, such as NMR spectroscopy. This is especially the case for NMR tubes.{{cite web |publisher=Wilmad |title=NMR-010: Proper Cleaning Procedures for NMR Sample Tubes |url=http://www.wilmad-labglass.com/services/NMR_010.jsp |access-date=2008-06-27 |url-status=dead |archive-url=https://web.archive.org/web/20080513142827/http://www.wilmad-labglass.com/services/NMR_010.jsp |archive-date=2008-05-13}} 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.{{cite web |title=Watson v Buckley, Osborne, Garrett & Co Ltd and Wyrovoys Products Ltd [1940] 1 All ER 174 |url=http://www.uniset.ca/other/cs3/19401AER174.html}}

It is used as a bleach in processing black and white photographic reversal film.{{cite web |title=Fomapan R |website=Fomapan R |publisher=Foma |url=http://www.fotofachversand.at/pdf/Foma_Fomapan_R_Datenblatt.pdf |access-date=6 April 2016 |url-status=dead |archive-url=https://web.archive.org/web/20160418135416/http://www.fotofachversand.at/pdf/Foma_Fomapan_R_Datenblatt.pdf |archive-date=18 April 2016}}

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

• Chromic acid in aqueous sulfuric acid and acetone is known as the Jones reagent, 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, {{doi|10.1002/047084289X.rc164}}
• Pyridinium chlorochromate is generated from chromium trioxide and pyridinium chloride. This reagent converts primary alcohols to the corresponding aldehydes (R–CHO).
• Collins reagent is an adduct of chromium trioxide and pyridine used for diverse oxidations.
• Chromyl chloride, {{chem2|CrO2Cl2}} is a well-defined molecular compound that is generated from chromic acid.

• Oxidation of methylbenzenes to benzoic acids.{{OrgSynth |author=Kamm O. |author2=Matthews, A. O. |title=p-Nitrobenzoic Acid |collvol=1 |collvolpages=392 |year=1941 |prep=CV1P0392}}
• Oxidative scission of indene to homophthalic acid.{{OrgSynth |author=Grummitt, O. |author2=Egan, R. |author3=Buck, A. |title=Homophthalic Acid and Anhydride |collvol=3 |collvolpages=449 (1955 |prep=CV3P0449}}
• Oxidation of secondary alcohol to ketone (cyclooctanone){{OrgSynth |author=Eisenbraun, E. J. |title=Cyclooctanone |collvol=5 |collvolpages=310 (1973 |prep=CV5P0310}} and {{chem name|nortricyclanone}}.{{OrgSynth |author=Meinwald, J. |author2=Crandall, J. |author3=Hymans W. E. |title=Nortricyclanone |collvol=5 |collvolpages=866 |1973 |prep=cv5p0866}}

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.

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).{{cite journal |author1=J. M. Grill |author2=J. W. Ogle |author3=S. A. Miller |title=An Efficient and Practical System for the Catalytic Oxidation of Alcohols, Aldehydes, and α,β-Unsaturated Carboxylic Acids |journal=J. Org. Chem. |year=2006 |volume=71 |issue=25 |pages=9291–9296 |pmid=17137354 |doi=10.1021/jo0612574}} Aldehydes are relatively easily oxidized to carboxylic acids, and mild oxidizing 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.

Hexavalent chromium compounds (including chromium trioxide, chromic acids, chromates, chlorochromates) are toxic and carcinogenic. Chromium trioxide and chromic acids are strong oxidizers and may react violently if mixed with easily oxidizable organic substances.

Chromic acid burns are treated with a dilute sodium thiosulfate solution.{{cite journal

|last1=Hettiaratchy |first1=Shehan

|last2=Dziewulski |first2=Peter

|title=Pathophysiology and types of burns

|date=2004-06-12

|journal=BMJ: British Medical Journal

|volume=328

|issue=7453

|pages=1427–1429

|issn=0959-8138

|pmid=15191982

|doi=10.1136/bmj.328.7453.1427

|pmc=421790

}}

{{Reflist}}

• [http://www.hamline.edu/~rmajerle/Chem3460/Feb2105.pdf Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds]{{dead link|date=August 2017 |bot=InternetArchiveBot |fix-attempted=yes}} (PDF)

• {{ICSC|1194|11}}
• {{PGCH|0138}}
• [https://web.archive.org/web/20040701090041/http://www-cie.iarc.fr/htdocs/monographs/vol49/chromium.html IARC Monograph "Chromium and Chromium compounds"]

{{Hydrogen compounds}}

{{Chromates and dichromates}}

{{Chromium compounds}}

Category:Chromates

Category:Mineral acids

Category:Oxidizing acids

Category:Oxoacids

Category:Transition metal oxoacids