Chromic acid

From Wikipedia, the free encyclopedia

Chromic acid
, ,
General
Systematic name	Chromium trioxide Chromium(VI) oxide
Molecular formula	H₂CrO₄
SMILES	O=[Cr](=O)(O)
Molar mass	100.0 g/mol
Appearance	dark red solid
CAS number	[1333-82-0]
Properties
Density and phase	2.70 g/cm³, solid
Solubility in water	63 g/100 ml (? °C)
Melting point	197 °C
Boiling point	250 °C decomp.
Acidity (pK_a)	0.74
Hazards
MSDS	External MSDS
EU classification	Oxidant (O) Highly toxic (T+) Carc. Cat. 1 Muta. Cat. 2 Repr. Cat. 3 Dangerous for the environment (N)
NFPA 704
R-phrases	R45, R46, R9, R24/25, R26, R35, R42/43, R48/23, R62, R50/53
S-phrases	S53, S45, S60, S61
RTECS number	GB6650000
Supplementary data page
Structure & properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Related chromates	Sodium chromate Potassium chromate
Related compounds	Vanadium pentoxide Manganese heptoxide Molybdenum trioxide Tungsten trioxide
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references

In chemistry, chromic acid is a hypothetical chromium (Cr) compound, yet to be isolated, that would have the formula H₂CrO₄. There is a related acid, also yet to be isolated called dichromic acid with the formula H₂Cr₂O₇. While these acids are not available, they are known through their divalent anions (chromate and dichromate respectively) and the compounds formed from them. Please note that the acid anhydride of chromic acid exists, chromium(VI) oxide or chromium trioxide (CrO₃), and that industrially, this compound is sometimes sold as "chromic acid". This compound is an intensely-colored dark red/orange brown, water-soluble, granular solid which is stable by itself, but is a strong oxidant which will react when mixed with many things that can be oxidized. Ethanol, for example, will ignite on contact with it.

In all of the compounds discussed here, the element chromium is in oxidation state +6 (or VI). Chromium(VI) is often referred to as hexavalent chromium. Although chromium can exist in a number of oxidation states, hexavalent chromium is commonly reduced in redox reactions to a +3 oxidation state, where the Cr³⁺ ion is a distinctively different blue green color. Hexavalent chromium compounds are toxic and carcinogenic.

Chromic acid solutions are generally acidified aqueous solutions of chromate or dichromate salts. These solutions tend to be powerful oxidants, capable of destroying all organics in glassware and so often used for the cleaning of glass. Such solutions are often red-brown, orange, or yellow colored depending on concentration.

1 Uses
- 1.1 As cleaning solution
- 1.2 Other uses
2 Reactions
3 References
4 External links

[edit] Uses

[edit] As cleaning solution

A mixture of a solution of 140 g of sodium dichromate dihydrate in 100 mL of distilled water and 2 L of sulphuric acid can be used to clean glassware. Note that the preparation of this solution is hazardous and should be done in a cold water bath in a fume hood.^{[citation needed]} In many countries this method is deprecated, since the resulting solution is highly corrosive, and high valent chromium species (some of which are rather volatile^{[citation needed]}) are carcinogenic.^{[citation needed]} Moreover, there are other cleaning methods that are just as effective, safer, and more environmentally friendly.

[edit] Other uses

It is also used to make chromates, oxidizing agents, catalysts; chromium plating intermediate; used to make ceramic glazes and colored glass; used in metal cleaning; used in inks, paints, and tanning; used in engraving and photography. Chromium trioxide has been classified as "select carcinogen" and long term exposure can cause ulceration of the respiratory system and skin.

[edit] Reactions

In organic chemistry, more dilute hexavalent chromium solutions can be used to oxidize a hydroxyl group and the carbon atom it is bonded to in a primary or secondary alcohol to a carbonyl group. Chromic acid in the presence of aqueous sulfuric acid is known as the Jones reagent, and will oxidize primary and secondary alcohols to carboxylic acids and ketones, respectively. Chromium trioxide and pyridinium hydrochloride (pyridine neutralized by HCl) can be used to form a pyridinium chlorochromate reagent under anhydrous conditions. When a primary alcohol is treated with anhydrous pyridinium chlorochromate in methylene chloride, the oxidation of the -CH₂OH group in primary alcohols will stop at the aldehyde group (-CHO).^[1] In contrast, other oxidizing solutions, such as aqueous potassium permanganate, will have a tendency to oxidize the -CH₂OH group all the way to a carboxyl group (-COOH). Dilute hexavalent chromium solutions will not oxidize a tertiary alcohol hydroxyl group. Because of the color change from a variation of orange to a blue-green color, a hexavalent chromium reagent can be formulated as a qualitative analytical test for the presence of primary or secondary alcohols. For example, chromium trioxide dissolved in aqueous sulfuric acid solution has been used as such a test reagent.^[1] Potassium dichromate, when mixed with concentrated sulfuric acid and a sample containing inorganic chloride and heated strongly, produces red fumes that condense to form a red liquid - chromyl chloride, CrO₂Cl₂. This compound hydrolyses in water to form aqueous chromic and hydrochloric acids: CrO₂Cl₂(l) + 2H₂O(l) → H₂CrO₄(aq) + 2HCl(aq) As bromides and iodides do not give respective analogues, the test is specific for chlorides.