Sodium dichromate is the
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with the formula
Na2Sodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
Cr2Chromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
O7Oxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
. Usually, however, the salt is handled as its dihydrate Na
2Cr
2O
7·2
H2OWater of crystallization is water that occurs in crystals but is not covalently bonded to a host molecule or ion. The term is archaic and predates modern structural inorganic chemistry, coming from an era when the relationships between stoichiometry and structure were poorly understood. ...
. Virtually all chromium ore is processed via conversion to sodium dichromate. In this way, many millions of kilograms of sodium dichromate are produced annually. In terms of reactivity and appearance, sodium dichromate and
potassium dichromatePotassium dichromate, K2Cr2O7, 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 potentially harmful to health and must be handled and...
are very similar.
Sodium dichromate is the
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with the formula
Na2Sodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
Cr2Chromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
O7Oxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
. Usually, however, the salt is handled as its dihydrate Na
2Cr
2O
7·2
H2OWater of crystallization is water that occurs in crystals but is not covalently bonded to a host molecule or ion. The term is archaic and predates modern structural inorganic chemistry, coming from an era when the relationships between stoichiometry and structure were poorly understood. ...
. Virtually all chromium ore is processed via conversion to sodium dichromate. In this way, many millions of kilograms of sodium dichromate are produced annually. In terms of reactivity and appearance, sodium dichromate and
potassium dichromatePotassium dichromate, K2Cr2O7, 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 potentially harmful to health and must be handled and...
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 weightEquivalent weight is a term which has been used in several contexts in chemistry. In its most general usage, it is the mass of one equivalent, that is the mass of a given substance which will:...
is also lower, which is often desirable.
In 2003, hundreds of U.S. soldiers, and many more Iraqi civilians were exposed to large amounts of sodium dichromate dust which was being released from a U.S. controlled water treatment facility in
IraqIraq , officially the Republic of Iraq , also known as Mesopotamia, is a country in Western Asia spanning most of the northwestern end of the Zagros mountain range, the eastern part of the Syrian Desert and the northern part of the Arabian Desert.Iraq shares borders with Jordan to the west, Syria...
. Many of the people who were exposed have died or had serious health complications as a result, and are sueing the
National GuardEstablished under Title 10 and Title 32 of the U.S. Code, the Army National Guard is part of the National Guard and is divided up into subordinate units stationed in each of the 50 states and territories operating under their respective governors...
for attempting to cover up the incident, and not notifying the soldiers of the potential health risk.
Production
Sodium dichromate is generated on a large scale from ores containing chromium(III) oxides. The ore is fused with base, typically
sodium carbonateSodium carbonate , , is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate; and is domestically well known for its everyday use as a water softener...
, at around 1000 °C in the presence of air (source of oxygen):
- Cr2O3 + 2 Na2CO3 + 1.5 O2 → 2 Na2CrO4 + 2 CO2
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 acidSulfuric acid, , is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion...
or
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
affords the dichromate, which is isolated at the dihydrate by crystallization. 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# 13517-17-4) as well as hexa-, tetra-, and dihydrates. Above 62 °C, these salts lose water spontaneously to give the anhydrous material.
Reactions
Dichromate and chromate salts are characteristically oxidizing agents. For the
tanningTanning is the process of making leather, which does not easily decompose, from the skins of animals, which do. Often this uses tannin, an acidic chemical compound. Coloring may occur during tanning....
of leather, sodium dichromate is first reduced with
sulfur dioxideSulfur dioxide is the chemical compound with the formula SO
2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide...
.
In the area of
organic synthesisOrganic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
, this compound oxidizes benzylic and allylic C-H bonds to
carbonylIn organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom : C=O.The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex In organic chemistry, a carbonyl group is a functional group...
derivatives. For example, 2,4,6-trinitrotoluene is oxidized to the corresponding carboxylic acid.. Similarly, 2,3-dimethylnaphthalene is oxidized by Na
2Cr
2O
7 to 2,3-naphthalenedicarboxylic acid.
Secondary alcohols are oxidized to the corresponding ketone, e.g.
mentholMenthol is an organic compound made synthetically or obtained from peppermint or other mint oils. It is a waxy, crystalline substance, clear or white in color, which is solid at room temperature and melts slightly above. The main form of menthol occurring in nature is -menthol, which is assigned...
to
menthoneMenthone is a naturally occurring organic compound with a molecular formula C10H18O. l-Menthone , shown at right, is the most abundant in nature of the four possible stereoisomers. Menthone is a monoterpene and a ketone...
; 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.
Safety
Like all
hexavalent chromiumHexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via conversion to sodium dichromate. Approximately of hexavalent chromium were produced in 1985. Other hexavalent chromium compounds are chromium...
compounds, sodium dichromate is considered hazardous. It is also a known
carcinogenA carcinogen is any substance, radionuclide or radiation that is an agent directly involved in the promotion of cancer or in the increase of its propagation. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...
.