Chromic acid
| Structural formula | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
||||||||||||||||
| General | ||||||||||||||||
| Surname | Chromic acid | |||||||||||||||
| other names |
Dihydrogen chromate ( IUPAC ) |
|||||||||||||||
| Molecular formula | H 2 CrO 4 | |||||||||||||||
| Brief description |
dark red to yellow odorless aqueous solution |
|||||||||||||||
| External identifiers / databases | ||||||||||||||||
|
||||||||||||||||
| properties | ||||||||||||||||
| Molar mass | 116.95 g mol −1 | |||||||||||||||
| Physical state |
only stable as a solution in water |
|||||||||||||||
| density |
1.5 g cm −3 (50% CrO 3 ) |
|||||||||||||||
| boiling point |
thermal decomposition |
|||||||||||||||
| solubility |
CrO 3 readily soluble in water: 1854 g l −1 (at 20 ° C) |
|||||||||||||||
| safety instructions | ||||||||||||||||
|
||||||||||||||||
| Authorization procedure under REACH |
of particular concern : carcinogenic ( CMR ); subject to approval |
|||||||||||||||
| MAK |
|
|||||||||||||||
| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . | ||||||||||||||||
Chromic acid with the structure H 2 CrO 4 only exists in dilute aqueous solutions. Pure, anhydrous chromic acid is unknown. It is a strong biprotonic acid and forms the chromates as salts . Hydrogen chromates, on the other hand, are unstable and only known in aqueous solution.
If chromic acid is offered as a solid, it is always the anhydride of chromic acid, chromium trioxide . Like anhydride, chromic acid is corrosive and poisonous.
Aqueous solution
Chromic acid is a strong acid ( pKa value : −0.61) and is only present in equilibrium as H 2 CrO 4 to a very small extent . HCrO 4 - however, is a weak acid (pK s -value: 6.45). Therefore, chromic acid in dilute aqueous solution is mainly present as hydrogen chromate HCrO 4 - .
If the aqueous solutions are mixed with a base , CrO 4 2− ions are predominantly present at a pH value> 7 .
At pH values between 2 and 6, HCrO 4 - are present and dichromates (Cr 2 O 7 2− ) are formed in an equilibrium reaction . During this reaction, the color changes from yellow to orange. If the pH value of the solution is <1, polychromates of the general formula
Cr n O 3n + 1 2− are formed with further intensification of color .
presentation
Diluted chromic acid can be obtained by hydrolysis of chromium (VI) oxide dichloride (chromyl chloride). Hydrochloric acid and chlorine are produced as by-products.
use
Chromic acid is used in chemistry as an oxidizing agent. It can be used to oxidize primary alcohols to aldehydes or carboxylic acids , and secondary alcohols to ketones .
In electroplating technology , chromic acid is used to passivate electroplated layers - primarily with cadmium - and to pretreat aluminum surfaces by anodic oxidation. It is also the main component of the galvanic chrome electrolytes that are used for chrome plating. Furthermore, chromic acid or chromium trioxide are used to produce potassium dichromate , ammonium dichromate and chromium dioxide .
Chromic acid solutions are also often used to quickly determine whether silver is present in the metal or alloy. The object of the sample is here before the test for reasons of assumed silver plating z. B. filed and dabbed this point with the acid. The acid and silver result in a silver chromate precipitate in blood-red tones. Silver chromate does not form with base metals. The darker the blood-red precipitate, the higher the silver fineness.
Chromic acid should not be confused with chromic sulfuric acid .
toxicology
Chromic acid is extremely toxic (the lethal dose is 1 to 2 g) and has long been known to be carcinogenic . If they come into contact with the skin, they cause irritation to the eyes, skin and mucous membranes. Chronic contact with chromic acid can lead to permanent damage if treatment is neglected.
Chromic acid can cause toxic pulmonary edema and, despite subsequent treatment, death. After a temporary freedom from discomfort (up to 48 hours), the full picture with hemoptysis , cyanosis , aspiration or cardiac arrest can occur.
Since 1958 the WHO (World Health Organization) has recommended a maximum concentration of 0.05 mg / liter for chromium (VI) compounds in drinking water.
See also
literature
- AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , pp. 1567-1569.
Individual evidence
- ↑ a b c d e f Entry on chromic acid in the GESTIS substance database of the IFA , accessed on July 23, 2016(JavaScript required) .
- ↑ Not explicitly listed in Regulation (EC) No. 1272/2008 (CLP) , but with the indicated labeling it falls under the group entry Chromium (VI) compounds, with the exception of barium chromate and of compounds specified elsewhere in this Annex in the Classification and Labeling inventory of the European Chemicals Agency (ECHA), accessed on August 14, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
- ↑ chromic acid; Safety data sheet on emsdiasum.com, accessed on August 16, 2016.
- ↑ Entry in the SVHC list of the European Chemicals Agency , accessed on July 16, 2014.
- ↑ Entry in the register of substances subject to authorization of the European Chemicals Agency , accessed on July 16, 2014.
- ↑ Swiss Accident Insurance Fund (Suva): Limit values - current MAK and BAT values (search for 7738-94-5 or chromic acid ), accessed on October 27, 2015.
- ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1568.
- ^ S. Moeschlin: Clinic and Therapy of Poisonings. 7th edition, Thieme-Verlag, Stuttgart 1986, ISBN 978-3-13-378407-8 .
- ↑ Max Daunderer : Textbook Clinical Toxicology. 42nd amendment, May 1989.