Chromic acid

Summary

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 (or VI). It is a strong and corrosive oxidising agent.

Chromic acid
Structural formulae of dichromic acid (left) and chromic acid (right)
Names
IUPAC name
Chromic acid
Systematic IUPAC name
Dihydroxidodioxidochromium
Other names
Chromic(VI) acid
Tetraoxochromic acid
Identifiers
  • 7738-94-5 checkY
3D model (JSmol)
  • Interactive image
  • Interactive image
ChEBI
  • CHEBI:33143 checkY
ChemSpider
  • 22834 checkY
ECHA InfoCard 100.028.910 Edit this at Wikidata
EC Number
  • 231-801-5
25982
  • 24425
UNII
  • SA8VOV0V7Q checkY
UN number 1755 1463
  • DTXSID8034455 Edit this at Wikidata
  • 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
H
2
CrO
4

or H
2
Cr
2
O
7

Appearance Dark red crystals
Density 1.201 g cm−3
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
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)
Lethal dose or concentration (LD, LC):
51.9 mg/kg (H2CrO4·2Na, rat, oral)[2]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 0.005 mg/m3[1]
REL (Recommended)
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).
checkY verify (what is checkY☒N ?)
Infobox references

Molecular chromic acidEdit

 
Partial predominance diagram for chromate

Molecular chromic acid, H2CrO4, has much in common with sulfuric acid, H2SO4. Only sulfuric acid can be classified as part of the 7 strong acids list. Due to the laws pertinent to the concept of "first order ionization energy", the first proton is lost most easily. It behaves extremely similar to sulfuric acid deprotonation. Since the process of polyvalent acid-base titrations have more than one proton (especially when the acid is starting substance and the base is the titrant), protons can only leave an acid one at a time. Hence the first step is as follows:

H2CrO4 ⇌ [HCrO4] + H+

The pKa for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6.[3] The value at zero ionic strength is difficult to determine because half dissociation only occurs in very acidic solution, at about pH 0, that is, with an acid concentration of about 1 mol dm−3. A further complication is that the ion [HCrO4] has a marked tendency to dimerize, with the loss of a water molecule, to form the dichromate ion, [Cr2O7]2−:

2 [HCrO4] ⇌ [Cr2O7]2− + H2O      log KD = 2.05.

Furthermore, the dichromate can be protonated:

[HCr2O7] ⇌ [Cr2O7]2− + H+      pK = 1.8[4]

The pK value for this reaction shows that it can be ignored at pH > 4.

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

but in practice the reverse reaction occurs when molecular chromic acid is dehydrated. 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 transitions.

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).

Dichromic acidEdit

Dichromic acid, H2Cr2O7 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. Dichromic acid will behave the same exact way when reacting with a primary or secondary alcohol. The caveat to this statement is that a secondary alcohol will be oxidized no further than a ketone, whereas a primary alcohol will be oxidized to a aldehyde for the first step of the mechanism and then oxidized again to a carboxylic acid, contingent on no significant steric hindrance impeding this reaction.

Dichromic acid undergoes the following reaction:

[Cr2O7]2− + 2H+ ⇌ H2Cr2O7 ⇌ H2CrO4 + CrO3

It is probably present in chromic acid cleaning mixtures along with the mixed chromosulfuric acid H2CrSO7.[citation needed]

UsesEdit

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.[5] Furthermore, the acid leaves trace amounts of paramagnetic chromic ions — Cr(III) — that can interfere with certain applications, such as NMR spectroscopy. This is especially the case for NMR tubes.[6]

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.[7]

It is used as a bleach in black and white photographic reversal processing.[8]

ReactionsEdit

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

Illustrative transformationsEdit

Use in qualitative organic analysisEdit

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 chemstry as a qualitative analytical test for the presence of primary or secondary alcohols, or aldehydes.[9]

Alternative reagentsEdit

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).[14] 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.

SafetyEdit

Hexavalent chromium compounds (including chromium trioxide, chromic acids, chromates, chlorochromates) are toxic and carcinogenic. For this reason, chromic acid oxidation is not used on an industrial scale except in the aerospace industry.

Chromium trioxide and chromic acids are strong oxidisers and may react violently if mixed with easily oxidisable organic substances. Fires or explosions may result.

Chromic acid burns are treated with a dilute sodium thiosulfate solution.[15]

See alsoEdit

  • Piranha solution – Oxidizing acid mixture containing sulfuric acid and hydrogen peroxide able to dissolve elemental carbon

NotesEdit

  1. ^ a b c NIOSH Pocket Guide to Chemical Hazards. "#0138". National Institute for Occupational Safety and Health (NIOSH).
  2. ^ "Chromic acid and chromates". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  3. ^ IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligands
  4. ^ 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.
  5. ^ J. M. McCormick (2006-06-30). "Cleaning Glassware". Truman State University. Archived from the original on 2008-12-07. Retrieved 2010-12-18.
  6. ^ "NMR-010: Proper Cleaning Procedures for NMR Sample Tubes". Wilmad. Archived from the original on 2008-05-13. Retrieved 2008-06-27.
  7. ^ "Watson v Buckley, Osborne, Garrett & Co Ltd and Wyrovoys Products Ltd [1940] 1 All ER 174".
  8. ^ "Fomapan R" (PDF). Fomapan R. Foma. Archived from the original (PDF) on 18 April 2016. Retrieved 6 April 2016.
  9. ^ a b c Freeman, F. "Chromic Acid" Encyclopedia of Reagents for Organic Synthesis (2001) John Wiley & Sons, doi:10.1002/047084289X.rc164
  10. ^ Kamm O.; Matthews, A. O. (1941). "p-Nitrobenzoic Acid". Organic Syntheses.; Collective Volume, vol. 1, p. 392
  11. ^ Grummitt, O.; Egan, R.; Buck, A. "Homophthalic Acid and Anhydride". Organic Syntheses.; Collective Volume, vol. 3, pp. 449 (1955
  12. ^ Eisenbraun, E. J. "Cyclooctanone". Organic Syntheses.; Collective Volume, vol. 5, pp. 310 (1973
  13. ^ Meinwald, J.; Crandall, J.; Hymans W. E. "Nortricyclanone". Organic Syntheses.; Collective Volume, vol. 5, p. 866
  14. ^ 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.
  15. ^ 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.

ReferencesEdit

  • Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds[permanent dead link] (PDF)

External linksEdit