Chromate and dichromate
Dichromate is a redirect to this article. For living things, see dichromate (eye).
Chromates are salts of chromic acid (H2CrO4). The chromate anion CrO42- has a yellow color and a tetrahedral structure. The dichromate anion (Cr2O72-) has an orange color and is composed of two distorted, corner-linked tetrahedra. There is a pH-dependent chemical equilibrium between chromates and dichromates in aqueous solutions, which obeys the principle of least constraint:
Under very acidic conditions and concentrated solutions, the more reddish trichromates (Cr3O102-), tetrachromates (Cr4O132-) and deep red polychromates (CrnO3n+12-) are formed in an analogous reaction.
All chromates are - especially under acidic conditions - strong oxidizing agents.
Use and safety instructions
Chromates are sometimes used as corrosion inhibitors (pigments, see also rust converter).
Many chromates have an intense coloration, which is why they are (or were) used as a color-imparting component in paints, for example "chrome yellow" (PbCrO4).
Chromates find application in analytical chemistry, where they are used as precipitation reagents.
In organic chemistry, chromates are used, for example, for the oxidation of alcohols. The Jones reagent consisting of chromium(VI) oxide, concentrated sulfuric acid and acetone (Jones oxidation) for the production of carboxylic acids and the Collins reagent, which can be used to stop the oxidation of the alcohol at the aldehyde stage, are well known.
All chromates and other chromium(VI) compounds (with the exception of the water-insoluble barium chromate) are highly toxic, hazardous to water and carcinogenic; examples include ammonium dichromate, potassium dichromate, sodium chromate, lithium chromate, zinc chromate, strontium chromate and chromium(III) chromate.
See also
- Chromate allergy
