Chromic acid

Chromic acid

Names
IUPAC name Chromic acid
Systematic IUPAC name Dihydroxidodioxidochromium
Other names Chromic(VI) acid Tetraoxochromic acid
Identifiers
CAS Number	7738-94-5 Y
3D model (Jmol)	Interactive image Interactive image
ChEBI	CHEBI:33143 Y
ChemSpider	22834 Y
ECHA InfoCard	100.028.910
EC Number	231-801-5
Gmelin Reference	25982
PubChem	24425
UNII	SA8VOV0V7Q Y
InChI InChI=1S/Cr.2H2O.2O/h;2*1H2;;/q+2;;;;/p-2 Y Key: KRVSOGSZCMJSLX-UHFFFAOYSA-L Y InChI=1/Cr.2H2O.2O/h;2*1H2;;/q+2;;;;/p-2/rCrH2O4/c2-1(3,4)5/h2-3H Key: KRVSOGSZCMJSLX-OOUCQFSRAZ
SMILES O[Cr](O)(=O)=O O=[Cr](=O)(O)O
Properties
Chemical formula	CrH2O4
Molar mass	118.01 g·mol−1
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
Hazards
Lethal dose or concentration (LD, LC):
LD50 (median dose)	51.9 mg/kg (H2CrO4·2Na, rat, oral)
US health exposure limits (NIOSH):
PEL (Permissible)	TWA 0.005 mg/m3
REL (Recommended)	TWA 0.001 mg Cr(VI)/m3
IDLH (Immediate danger)	15 mg Cr(VI)/m3
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
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, H₂CrO₄, has much in common with sulfuric acid, H₂SO₄. Both are classified as strong acids, though only the first proton is lost easily.

The pK_a for the equilibrium is not well characterized. Reported values vary between about −0.8 to 1.6. 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⁻:

Furthermore, the dichromate can be protonated:

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).

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.