CHROMIC ACID

CAS No: 7738-94-5
EC No: 231-801-5

Synonyms:
CHROMIC ACID; Tetraoxochromic acid; Chromic(VI) acid; 7738-94-5; Chromic acid (H2CrO4); Caswell No. 221; Acide chromique; kromik asit; dikromik asit; kromik asid; kromikasit; chromic acid; dichromic acid; 1333-82-0; 7738-94-5; EC No 231-801-5 ; CHROMIC ACID; Chromic(VI) acid; Chromic acid (H2CrO4); tetraoxochromic acid; tetraoksokromik asit; Caswell No. 221; dihydroxy-dioxo-chromium; Acide chromique [French]; Acide chromique; dioxochromiumdiol; chromic acid anhydride; dihydroxy-dioxochromium; dihydroxidodioxidochromium; Acide chromique; dioxochromiumdiol; chromic acid anhydride; dihydroxy-dioxochromium; dihydroxidodioxidochromium; dihydrogen(tetraaoxidochromate); Chromic Acid; Kromic Acid; Khromic Acid; Chromic Acit; Kromic Acit; Khromic Acit; Kromik asit; Acide chromique; L'acide chromique; Kıromik asit

Chromic acid

Chromic acid
Structural formulae of dichromic acid (left) and chromic acid (right)
Chromic Acid 3D BallStick.png
Names
IUPAC name
Chromic acid
Systematic IUPAC name
Dihydroxidodioxidochromium
Other names
Chromic(VI) acid
Tetraoxochromic acid
Identifiers
CAS Number    
7738-94-5 check
3D model (JSmol)    
Interactive image
Interactive image
ChEBI    
CHEBI:33143 check
ChemSpider    
22834 check
ECHA InfoCard    100.028.910 Edit this at Wikidata
EC Number    
231-801-5
Gmelin Reference    25982
PubChem CID    
24425
UNII    
SA8VOV0V7Q check
UN number    1755 1463
CompTox Dashboard (EPA)    
DTXSID8034455 Edit this at Wikidata
InChI[show]
SMILES[show]
Properties
Chemical formula    H
2CrO
4
or H
2Cr
2O
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)
Solubility in water    169 g/100 mL
Acidity (pKa)    -0.8 to 1.6
Conjugate base    Chromate and dichromate
Hazards
Main hazards    highly toxic, carcinogen, corrosive
GHS pictograms    GHS03: OxidizingGHS05: CorrosiveGHS06: ToxicGHS07: HarmfulGHS08: Health hazardGHS09: Environmental hazard
GHS Signal word    Danger
GHS hazard statements    H271, H300, H301, H310, H314, H317, H318, H330, H334, H340, H341, H350, H361, H372
GHS precautionary statements    P201, P202, P210, P220, P221, P260, P261, P262, P264, P270, P271, P272, P273, P280, P281, P283, P284, P285, P301+310, P301+330+331, P302+350, P302+352, P303+361+353, P304+340, P304+341
NFPA 704 (fire diamond)    
NFPA 704 four-colored diamond
041COR
Lethal dose or concentration (LD, LC):
LD50 (median dose)    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).
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Infobox references
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.

Molecular chromic acid

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 acid
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 an aldehyde or ketone. The caveat to this statement, however, is that a secondary ketone will be oxidized no further than a ketone and dichromic acid will oxidize the aldehyde only. The aldehyde will be oxidized to a ketone for the first step of the mechanism and oxidized again to a carboxylic acid, contingent on no significant steric hindrance impeding this reaction. The same thing would happened for PCC regarding the oxidation of a secondary ketone, a more mild oxidizing agent. Dichromic acid undergo 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]

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.[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]

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 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.[9]
Pyridinium chlorochromate is generated from chromium trioxide and pyridinium chloride. This reagent converts primary alcohols to the corresponding aldehydes (R–CHO).[9]
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.[10]
Oxidative scission of indene to homophthalic acid.[11]
Oxidation of secondary alcohol to ketone (cyclooctanone)[12] and nortricyclanone.[13]
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. Tertiary alcohol groups are unaffected. Because of the oxidation is signaled by a color change from orange to a blue-green, chromic acid is used as a qualitative analytical test for the presence of primary or secondary alcohols.[9]

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).[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.

Safety
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.

Molecular Weight of Chromic acid:    118.01 g/mol    Computed by PubChem 2.1 (PubChem release 2019.06.18)
Hydrogen Bond Donor Count of Chromic acid:    2    Computed by Cactvs 3.4.6.11 (PubChem release 2019.06.18)
Hydrogen Bond Acceptor Count of Chromic acid:    4    Computed by Cactvs 3.4.6.11 (PubChem release 2019.06.18)
Rotatable Bond Count of Chromic acid:    0    Computed by Cactvs 3.4.6.11 (PubChem release 2019.06.18)
Exact Mass of Chromic acid:    117.935814 g/mol    Computed by PubChem 2.1 (PubChem release 2019.06.18)
Monoisotopic Mass of Chromic acid:    117.935814 g/mol    Computed by PubChem 2.1 (PubChem release 2019.06.18)
Topological Polar Surface Area of Chromic acid:    74.6 Ų    Computed by Cactvs 3.4.6.11 (PubChem release 2019.06.18)
Heavy Atom Count of Chromic acid:    5    Computed by PubChem
Formal Charge of Chromic acid:    0    Computed by PubChem
Complexity of Chromic acid:    81.3    Computed by Cactvs 3.4.6.11 (PubChem release 2019.06.18)
Isotope Atom Count of Chromic acid:    0    Computed by PubChem
Defined Atom Stereocenter Count of Chromic acid:    0    Computed by PubChem
Undefined Atom Stereocenter Count of Chromic acid:    0    Computed by PubChem
Defined Bond Stereocenter Count of Chromic acid:    0    Computed by PubChem
Undefined Bond Stereocenter Count of Chromic acid:    0    Computed by PubChem
Covalently-Bonded Unit Count of Chromic acid:    1    Computed by PubChem
Compound of Chromic acid Is Canonicalized    Yes

Chromic Acid - H2CrO4
What is Chromic acid?
Chromic Acid is a naturally occurring oxide with a formula H2CrO4.

Chromic Acid is also called Tetraoxochromic acid or Chromic(VI) acid. It is usually a mixture made by adding concentrated sulphuric acid (H2SO4) to a dichromate which consists of a variety of compounds and solid chromium trioxide.

Molecular chromic acid – H2CrO4 is similar to sulphuric acid (H2SO4) as both are strong acids, however, only the first proton is lost easily.
Dichromic acid – H2Cr2O7 is the fully protonated form of the dichromate (Cr2O7–) ion. Also, it is seen as the product of adding chromium trioxide (CrO3) to molecular chromic acid.
Properties of Chromic acid – H2CrO4
Chromic acid    H2CrO4
Molecular Weight of Chromic acid    118.008 g/mol
Density of Chromic acid    1.201 g/cm3
Melting Point of Chromic acid    197 °C
Boiling Point of Chromic acid    250 °C
Structure of Chromic acid (H2CrO4)
 

Structure of Chromic acid
Structure of Chromic acid – H2CrO4

Uses of Chromic acid (H2CrO4)
Chromic acid acts as an intermediate in chromium plating,
It is used in ceramic glazes, coloured glass.
Chromosulfuric acid or Sulfochromic mixture is a strong oxidizing agent that is used to clean laboratory glassware.
It has the ability to brighten raw brass and therefore it is used in the instrument repair industry.
In the year 1940, it was used in hair dye.
The completely protonated form of the dichromate ion is dichromic acid, H2Cr2O7 and can also be seen as the result of adding chromium trioxide to molecular chromic acid. When reacting with an aldehyde or ketone, Dichromic acid can behave the same exact way. However, the caveat to this argument is that no more than a ketone will oxidise a secondary ketone and dichromic acid will only oxidise the aldehyde. For the first step of the mechanism, the aldehyde would be oxidised to a ketone and oxidised to a carboxylic acid again, subject to no major steric hindrance impeding this reaction.

Chromic acid is capable of oxidising many forms of organic compounds, and many variants have been created for this reagent. Chromic acid is referred to as the Jones reagent in aqueous sulfuric acid and acetone, which oxidises primary and secondary alcohols into carboxylic acids and ketones, respectively, though rarely affecting unsaturated bonds.
Chromium trioxide and pyridinium chloride produce pyridinium chlorochromate. This reagent converts to the corresponding aldehydes (R-CHO) primary alcohols.

Health hazards
Hexavalent chromium compounds such as chromium trioxide, chromates, chromic acids, and chlorochromate are toxic and carcinogenic. Therefore, chromic acid oxidation is used only in the aerospace industry and not on any other industrial scale.

Chromic acids are strong oxidizers and can react violently if mixed with some easily oxidizable organic substances which can cause explosions or fires. In case of any burns caused by this acid, it is treated with a dilute solution of sodium thiosulfate.

Frequently Asked Questions
What are the uses of chromic acid?
This compound is widely used as an intermediate in chromium plating. Chromic acid is also used in coloured glass and ceramic glazes. In the 1940s, this compound was an integral part of several hair dyes.

How can chromic acid be prepared?
First, sodium dichromate, or potassium dichromate, must be  combined with a little water to produce a paste. While applying sulphuric acid to the paste and continuously mixing, chromic acid is formed.

Which neutralizing agents are ideal for neutralizing chromic acid?
Chromic acid must be diluted in a good amount of water first. Then, it can be neutralized with a reductant after the dilution process. Some excellent neutralizing agents are sodium / potassium metabisulfite, sodium thiosulfate, or sodium sulfite.