sodium dichromate

Sodium dichromate
IUPAC name	Sodium dichromate
Other names	Chromic acid, (H2Cr2O7), disodium salt
Identifiers
CAS number	10588-01-9 Y, 7789-12-0 (dihydrate)
PubChem	25408
EC number	234-190-3
UN number	3288
RTECS number	HX7750000 HX7750000 (dihydrate)
Properties
Molecular formula	Na2Cr2O7
Molar mass	261.97 g/mol (anhydrous) 298.00 g/mol (dihydrate)
Density	2.52 g/cm3
Melting point	356.7 °C dehydrates at 100 °C
Boiling point	decomposes 400 °C
Solubility in water	730 g/L at 25 °C
Hazards
MSDS	ICSC 1369
EU Index	024-004-00-7 (anhydrous) 024-004-01-4 (dihydrate)
EU classification	Oxidant (O) Carc. Cat. 2 Muta. Cat. 2 Repr. Cat. 2 Very toxic (T+) Harmful (Xn) Corrosive (C) 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
NFPA 704	0 3 0 OX
LD50	50 mg/kg
Related compounds
Other anions	Sodium chromate Sodium molybdate Sodium tungstate
Other cations	Potassium dichromate Ammonium dichromate
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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.^[1] 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.^[2]

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 Iraq. Many of the people who were exposed have died or had serious health complications as a result, and are sueing the National Guard for attempting to cover up the incident, and not notifying the soldiers of the potential health risk.^[3]

Production

Sodium dichromate is generated on a large scale from ores containing chromium(III) oxides. The ore is fused with base, typically sodium carbonate, 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 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.^[1] 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 tanning of leather, sodium dichromate is first reduced with sulfur dioxide.

In the area of organic synthesis,^[2] 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.^[4]. Similarly, 2,3-dimethylnaphthalene is oxidized by Na₂Cr₂O₇ to 2,3-naphthalenedicarboxylic acid.^[5]

Secondary alcohols are oxidized to the corresponding ketone, e.g. menthol to menthone;^[6] dihydrocholesterol to cholestanone:^[7]

3 R₂CHOH + Cr₂O₇^2- + 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 chromium compounds, sodium dichromate is considered hazardous. It is also a known carcinogen^[8].

References