Sodium dichromate
Key industrial chromium compound; toxic but widely used oxidizer.
Sodium dichromate is the inorganic compound with the formula Na2Cr2O7, usually handled as its dihydrate Na2Cr2O7·2H2O. 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. It is produced on a large scale from chromium(III) oxide ores fused with sodium carbonate at around 1000 °C in the presence of air, then acidified to yield the dichromate, which is isolated as the dihydrate by crystallization.
- chemical_formula
- Na2Cr2O7 (anhydrous); Na2Cr2O7·2H2O (dihydrate)
- molar_mass
- 261.97 g/mol (anhydrous); 298.00 g/mol (dihydrate)
- solubility_in_water
- 49 g/L at 0 °C (sodium salt); around twenty times more soluble than potassium dichromate
- appearance
- Yellow-orange crystals (dihydrate); color change to green upon reduction to Cr(III)
- hazards
- Carcinogenic, corrosive, may produce severe eye damage or blindness; impairs fertility, causes heritable genetic damage and harm to unborn children
- common_uses
- Tanning of leather, hexavalent chrome plating, organic oxidation reagent
Lore & Background
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, which solubilizes the chromium and allows it to be extracted into hot water. Acidification of the resulting aqueous extract with sulfuric acid or carbon dioxide affords the dichromate, which is isolated as the dihydrate by crystallization. Many millions of kilograms are produced annually. Since chromium(VI) is toxic, especially as dust, factories are subject to stringent regulations; effluent is treated with reducing agents to return any chromium(VI) to chromium(III).
Reader's Guide
Sodium dichromate is a central compound in chromium chemistry, as virtually all chromium-based materials are prepared from it. Its high solubility in water and polar solvents (around twenty times that of potassium dichromate) makes it often desirable for industrial and laboratory use. In organic chemistry, it serves as an oxidizing agent, converting primary alcohols to aldehydes or carboxylic acids and secondary alcohols to ketones, with a characteristic color change from yellow-orange to green. However, use of chromate-based reagents has declined owing to environmental concerns, as hexavalent chromium compounds are carcinogenic and corrosive. The compound's legacy is thus one of industrial indispensability tempered by significant health and environmental risks.
Did You Know?
- Sodium dichromate is around twenty times more soluble in water than potassium dichromate (49 g/L at 0 °C).
- It is produced by fusing chromium(III) oxide ore with sodium carbonate at around 1000 °C in the presence of air.
- The color change from yellow-orange to green upon reduction can be used as a test to distinguish aldehydes from ketones.
- Above 62 °C, the hydrates lose water spontaneously to give the anhydrous material.
More in Sodium Compounds 1-24
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