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Chromic acid (H2CrO4) is a strong mineral acid that is toxic and corrosive and is a fire hazard (see the hazard information table). Its anhydrous form, chromium trioxide1 (CrO3), is also called chromic acid, as is its aqueous solution with sulfuric acid, which is a common cleaning solution for laboratory glassware. Another variant is dichromic acid (H2Cr2O7), formed by the loss of 1 mole of water from 2 moles of H2CrO4.
The pKa of H2CrO4 has been estimated at–0.8 to 1.6, indicating that it is almost completely ionized in water. Because chromium is in its highest oxidation state (+6), the acid and its salts are strong oxidizers. In particular, the sodium2 and potassium3 dichromates (Na2Cr2O7 and K2Cr2O7) are widely used oxidants in organic reactions.
An early mention of chromic acid in the chemical literature is 1886 US Patent 342,578, in which William J. Chrystal of Glasgow (UK) described the manufacture of high-purity “bichromate of soda” (i.e., Na2Cr2O7) from chromium ore. The inventor’s process uses sulfuric acid, but chromic acid is not isolated.
In German Patent 182,287 (1905), inventor Max le Blanc of Leipzig described an electrolytic method for the continuous production for chromic acid solutions. The motivation of the invention was to regenerate chromic acid from acidic solutions of chromate salts.
In addition to its laboratory use as a cleaning agent and oxidizing reagent, chromic acid is used in qualitative organic analysis to detect the presence of aldehydes and primary and secondary alcohols. The reduction of chromium(VI) by these functionalities is indicated by a color change from orange to the green hue of chromium(III).
In industry, chromic acid is used in chromium plating and the production of colored glass and ceramic glazes. The global chromic acid market in 2023 was valued at more than US$700 million.
1. CAS Reg. No. 1333-82-0.
2. CAS Reg. No. 10588-01-9.
3. CAS Reg. No. 7778-50-9.
Chromic acid hazard information
| Hazard class** | GHS code and hazard statement | |
|---|---|---|
| Oxidizing solids, category 1 | H271—May cause fire or explosion; strong oxidizer |  |
| Acute toxicity, oral, category 3 | H301—Toxic if swallowed |  |
| Acute toxicity, dermal, category 3 | H311—Toxic in contact with skin | |
| Skin corrosion/irritation, category 1A | H314—Causes severe skin burns and eye damage |  |
| Sensitization, skin, category 1 | H317—May cause an allergic skin reaction |  |
| Acute toxicity, inhalation, category 2 | H330—Fatal if inhaled | |
| Sensitization, respiratory, category 1 | H334— May cause allergy or asthma symptoms or breathing difficulties if inhaled |  |
| Germ cell mutagenicity, category 1B | H340— May cause genetic defects | |
| Carcinogenicity, category 1A | H350— May cause cancer | |
| Reproductive toxicity, category 2 | H361— Suspected of damaging fertility or the unborn child | |
| Specific target organ toxicity, repeated exposure, category 1 | H372—Causes damage to the respiratory system through prolonged or repeated exposure | |
*Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. Explanation of pictograms.
MOTW updates
Vancomycin1 was the Molecule of the Week for September 05, 2016. It is an amphoteric glycopeptide antibiotic originally isolated from the soil bacterium Amycolatopsis orientalis in 1955. Vancomycin is mostly used to treat serious Gram-positive bacterial infections that do not respond to other antibiotics.
Early this month, Lynette Cegelski, Paul A. Wender, and co-workers at Stanford University (CA) reported that enhanced versions of vancomycin are broad-spectrum antibiotics effective against several otherwise resistant Gram-positive and Gram-negative pathogens. The authors prepared conjugates of vancomycin with biguanide2, a small molecule with amine and imine functionalities. The successfully treated pathogens included vancomycin-resistant Enterococcus species, mycobacteria, and all of the pathogens in the virulent ESKAPE class.
Albuterol3 was the Molecule of the Week for March 16, 2015. It is a widely used bronchodilator that dates back to 1972. Also called salbutamol, it is the active ingredient in many “rescue inhalers” for asthma attacks and other respiratory conditions.
This month, Leana S. Wen at The Washington Post reported that albuterol only alleviates the symptoms of asthma but does not resolve the underlying inflammation. According to Wen, recent research has shown that inhalers should not contain albuterol alone but should be combined with steroids that treat the inflammation. Inhaled steroids do not cause the undesirable side effects that accompany orally taken steroids.
1. CAS Reg. No. 1404-90-6.
2. CAS Reg. No. 56-03-1.
3. CAS Reg. No. 18559-94-9.
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Chromic acid fast facts
| CAS Reg. No. | 7738-94-5 |
| SciFindern name | Chromic acid (H2CrO4) |
| Empirical formula | H2CrO4 |
| Molar mass | 118.01 g/mol |
| Appearance | Dark red-purple crystals or powder |
| Melting point | 196 °C (anhydrous) |
| Water solubility | 1.4 kg/L (25 °C) |
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