Chromates

Dichromate is a redirect to this article. For living beings see dichromate (eye) .

Chromates
Chromate anion	Dichromate anion
Potassium chromate crystals	Potassium dichromate crystals

Chromates are salts of chromic acid (H ₂ CrO ₄ ). The chromate anion CrO ₄ ²⁻ has a yellow color and a tetrahedral structure. The dichromate anion (Cr ₂ O ₇ ²⁻ ) has an orange color and is made up of two distorted, corner-sharing tetrahedra. In aqueous solutions there is a pH-dependent chemical equilibrium between chromates and dichromates , which obeys the principle of the smallest constraint :

${\displaystyle \mathrm {2\ CrO_{4}^{2-}\ +\ 2\ H_{3}O^{+}\ \rightleftharpoons \ Cr_{2}O_{7}^{2-}\ +\ 3\ H_{2}O} }$

Under very acidic conditions and concentrated solutions, the rather reddish trichromates (Cr ₃ O ₁₀ ²⁻ ), tetrachromates (Cr ₄ O ₁₃ ²⁻ ) and deep red polychromates (Cr _n O _{3n + 1} ²⁻ ) are formed in an analogous reaction .

${\displaystyle \mathrm {Cr_{2}O_{7}^{2-}\ +\ CrO_{4}^{2-}\ +\ 2\ H_{3}O^{+}\ \rightleftharpoons \ Cr_{3}O_{10}^{2-}\ +\ 3\ H_{2}O} }$

All chromates are strong oxidizing agents , especially under acidic conditions .

Use and safety information

Chromates are sometimes used as corrosion protection agents ( pigments , see also rust converters ).

Many chromates have an intense color, which is why they are (or have been) used as a coloring component in painters' paint , for example “ chrome yellow ” (PbCrO ₄ ).
Chromates are used in analytical chemistry, where they are used as precipitation reagents.

In organic chemistry, chromates are used, for example, to oxidize alcohols. The Jones reagent made from chromium (VI) oxide, concentrated sulfuric acid and acetone ( Jones oxidation ) for the production of carboxylic acids and the Collins reagent , with which the oxidation of the alcohol can be stopped at the aldehyde level, are known.

All chromates and other chromium (VI) compounds (with the exception of the water-insoluble barium chromate ) are highly poisonous, hazardous to water and carcinogenic ( carcinogenic ); Examples are ammonium dichromate , potassium dichromate , sodium chromate , lithium chromate , zinc chromate , strontium chromate and chromium (III) chromate .

See also

• Chromate allergy

Individual evidence

1. ^ ^{A b c d e} Wiberg, Egon., Wiberg, Nils: Textbook of inorganic chemistry . 102nd, heavily reworked and verb. Ed. De Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 .
2. ^ Greeves, Nick ,, Warren, Stuart G. 1938-: Organic chemistry . 2nd edition Springer, Berlin 2013, ISBN 978-3-642-34715-3 .