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Chromic acid
From Wikipedia, the free encyclopedia
| Chromic acid | |
|---|---|
 ,  |
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| General | |
| Systematic name | ? |
| Molecular formula | H2CrO4 |
| SMILES | ? |
| Molar mass | ? |
| Appearance | ? |
| CAS number | ? |
| Properties | |
| Density and phase | ? |
| Solubility in water | ? |
| Melting point | ? |
| Boiling point | ? |
| Acidity (pKa) | ? |
| Hazards | |
| MSDS | External MSDS |
| EU classification | ? |
| NFPA 704 | ? |
| R-phrases | ? |
| S-phrases | ? |
| RTECS number | ? |
| Supplementary data page | |
| Structure & properties | n, εr, etc. |
| Thermodynamic data | Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | Sodium chromate Potassium chromate |
| Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references |
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In chemistry, chromic acid is a chromium (Cr) compound, yet to be isolated, with the formula H2CrO4. A related acid, also yet to be isolated, is dichromic acid with the formula H2Cr2O7. Although these acids are not available in a pure state, their conjugate bases, chromate and dichromate form well characterized salts. The acid anhydride of chromic acid is chromium trioxide, also called chromium(VI) oxide; industrially, this compound is sometimes sold as "chromic acid".
In all chromic acid and its direct derivatives, 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 the [[Cr(H2O)6]]3+ ion, which has a distinctive blue green colour. Hexavalent chromium compounds are toxic and carcinogenic.
Chromic acid solutions are generated by acidifying aqueous solutions of chromate or dichromate salts. These solutions tend to be powerful oxidants, capable of oxidizing organics in glassware and often used for the cleaning of glass. Such solutions are often red-brown, orange, or yellow-coloured, depending on the concentration of the chromium(VI).
Contents |
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.
Chromic acid is 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.
In organic chemistry, dilute solutions of hexavalent chromium can be used to oxidize primary or secondary alcohols to the corresponding aldehydes and ketones. Chromic acid in the presence of 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. Chromium trioxide and pyridinium hydrochloride (pyridine neutralized by HCl) form the pyridinium chlorochromate reagent. 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 added to concentrated sulfuric acid in the presence of chloride ions 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.
- Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds (PDF)
- Index of Organic Synthesis procedures utilizing Chromic acid