Sodium dichromate is the
inorganic compound with the formula
Na2 Cr2 O7. However, the salt is usually handled as its
dihydrate
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
Na
2Cr
2O
7·2
H2O. Virtually all
chromium ore is processed via conversion to sodium dichromate and virtually all compounds and materials based on chromium are prepared from this salt.
[Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. ] In terms of
reactivity and appearance, sodium dichromate and
potassium dichromate
Potassium dichromate, , is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health ...
are very similar. The sodium salt is, however, around twenty times more soluble in water than the potassium salt (49 g/L at 0 °C) and its
equivalent weight
In chemistry, equivalent weight (also known as gram equivalent) is the mass of one equivalent, that is the mass of a given substance which will combine with or displace a fixed quantity of another substance. The equivalent weight of an element is ...
is also lower, which is often desirable.
[Freeman, F. "Sodium Dichromate" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. .]
Preparation
Sodium dichromate is generated on a large scale from ores containing chromium(III) oxides. The ore is fused with a base, typically
sodium carbonate, at around 1000 °C in the presence of air (source of oxygen):
2Cr2O3 + 4Na2CO3 + 3O2 -> 4Na2CrO4 + 4CO2
This step solubilizes the chromium and allows it to be extracted into hot water. At this stage, other components of the ore such as aluminium and iron compounds, are poorly soluble. Acidification of the resulting aqueous extract with
sulfuric acid or
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
affords the dichromate:
2Na2CrO4 + 2CO2 + H2O -> Na2Cr2O7 + 2NaHCO3
2Na2CrO4 + H2SO4 -> Na2Cr2O7 + Na2SO4 + H2O
The dichromate is isolated as the dihydrate by crystallization. In this way, many millions of kilograms of sodium dichromate are produced annually.
Since chromium(VI) is toxic, especially as the dust, such factories are subject to stringent regulations. For example, effluent from such refineries is treated with reducing agents to return any chromium(VI) to chromium(III), which is less threatening to the environment.
[ A variety of hydrates of this salt are known, ranging from the decahydrate below 19.5 °C (CAS# ) as well as hexa-, tetra-, and dihydrates. Above 62 °C, these salts lose water spontaneously to give the anhydrous material.
It is crystallised around 30 to 35 degrees C
]
Reactions
Dichromate and chromate salts are oxidizing agents. For the tanning
Tanning may refer to:
*Tanning (leather), treating animal skins to produce leather
*Sun tanning, using the sun to darken pale skin
**Indoor tanning, the use of artificial light in place of the sun
**Sunless tanning, application of a stain or dye t ...
of leather, sodium dichromate is first reduced with sulfur dioxide.
In the area of organic synthesis,[ this compound oxidizes benzylic and allylic C-H bonds to ]carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
derivatives. For example, 2,4,6-trinitrotoluene
Trinitrotoluene (), more commonly known as TNT, more specifically 2,4,6-trinitrotoluene, and by its preferred IUPAC name 2-methyl-1,3,5-trinitrobenzene, is a chemical compound with the formula C6H2(NO2)3CH3. TNT is occasionally used as a reagen ...
is oxidized to the corresponding carboxylic acid. Similarly, 2,3-dimethylnaphthalene is oxidized by Na2Cr2O7 to 2,3-naphthalenedicarboxylic acid.
Secondary alcohols are oxidized to the corresponding ketone, e.g. menthol to menthone
Menthone is a monoterpene with a minty flavor that occurs naturally in a number of essential oils. ''l''-Menthone (or (2''S'',5''R'')-''trans''-2-isopropyl-5-methylcyclohexanone), shown at right, is the most abundant in nature of the four possible ...
; dihydrocholesterol to cholestanone:
:3 R2CHOH + Cr2O72− + 2 H+ → 3 R2C=O + Cr2O3 + 4 H2O
Relative to the potassium salt, the main advantage of sodium dichromate is its greater solubility in water and polar solvents like acetic acid.
Sodium dichromate can be used in fluorene to fluorenone
Fluorenone is an aromatic organic compound with the chemical formula C13H8O. It is bright fluorescent yellow in color and is a solid at room temperature.
It can be synthesised from fluorene with the addition of glacial acetic acid and sodium hy ...
conversion.
Safety
Like all hexavalent chromium
Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium in any chemical compound that contains the element in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the ...
compounds, sodium dichromate is carcinogen
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subs ...
ic. The compound is also corrosive
A corrosive substance is one that will damage or destroy other substances with which it comes into contact by means of a chemical reaction.
Etymology
The word ''corrosive'' is derived from the Latin verb ''corrodere'', which means ''to gnaw'', ...
and exposure may produce severe eye damage or blindness. Human exposure further encompasses impaired fertility, heritable genetic damage and harm to unborn children.
References
{{Chromates and dichromates
Sodium compounds
Dichromates
Oxidizing agents