|Jmol-3D images||Image 1|
|Molar mass||261.97 g/mol (anhydrous)
298.00 g/mol (dihydrate)
|Melting point||356.7 °C
dehydrates at 100 °C
|Boiling point||decomposes 400 °C|
|Solubility in water||73 g/100 mL at 25 °C|
|Solubility in other solvents||soluble in methanol|
|Refractive index (nD)||1.661 (dihydrate)|
|EU Index||024-004-00-7 (anhydrous)
|EU classification||Oxidant (O)
Carc. Cat. 2
Muta. Cat. 2
Repr. Cat. 2
Very toxic (T+)
Dangerous for the environment (N)
|R-phrases||R45, R46, R60, R61, R8, R21, R25, R26, R34, R42/43, R48/23, R50/53,|
|S-phrases||S53, S45, S60, S61|
|Other anions||Sodium chromate
|Other cations||Potassium dichromate
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Sodium dichromate is the chemical compound with the formula Na2Cr2O7. Usually, however, the salt is handled as its dihydrate Na2Cr2O7·2H2O. 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 dichromate 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 is also lower, which is often desirable.
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):
- 2 Cr2O3 + 4 Na2CO3 + 3 O2 → 4 Na2CrO4 + 4 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 acid or carbon dioxide 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.
In the area of organic synthesis, this compound oxidizes benzylic and allylic C-H bonds to carbonyl derivatives. For example, 2,4,6-trinitrotoluene is oxidized to the corresponding carboxylic acid. Similarly, 2,3-dimethylnaphthalene is oxidized by Na2Cr2O7 to 2,3-naphthalenedicarboxylic acid.
- 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.
- 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.
- Freeman, F. "Sodium Dichromate" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.
- Clarke, H. T.; Hartman, W. W. (1941), "2,4,6-Trinitrobenzoic Acid", Org. Synth.; Coll. Vol. 1: 543
- Friedman, L. (1973), "2,3-Naphthalenedicarboxylic Acid", Org. Synth.; Coll. Vol. 5: 810
- L. T. Sandborn (1929), "l-Menthone", Org. Synth. 9: 59; Coll. Vol. 1: 340
- W. F. Bruce (1941), "Cholestanone", Org. Synth.; Coll. Vol. 2: 139
- Template:The Facts on File Dictionary of Chemistry
- ILO 1369 - Sodium Dichromate