Sodium dichromate

Sodium dichromate
Names


IUPAC name Sodium dichromate
Other names Chromic acid disodium salt
Identifiers
CAS Number	10588-01-9^Y 7789-12-0 (dihydrate)^N
3D model (Jmol)	Interactive image
ChEBI	CHEBI:39483^Y
ChemSpider	23723^Y
ECHA InfoCard	100.031.070
EC Number	234-190-3
PubChem CID	25408
RTECS number	HX7750000 HX7750000 (dihydrate)
UNII	C9G6VY6ZZ4^N
UN number	3288
InChI InChI=1S/2Cr.2Na.7O/q;;2+1;;;;;;2-1^Y Key: KIEOKOFEPABQKJ-UHFFFAOYSA-N^Y InChI=1/2Cr.2Na.7O/q;;2+1;;;;;;2-1/rCr2O7.2Na/c3-1(4,5)9-2(6,7)8;;/q-2;2*+1 Key: KIEOKOFEPABQKJ-RXLKZJBDAW
SMILES [Na+].[Na+].[O-][Cr](=O)(=O)O[Cr]([O-])(=O)=O
Properties
Chemical formula	Na₂Cr₂O₇
Molar mass	261.97 g/mol (anhydrous) 298.00 g/mol (dihydrate)
Appearance	bright orange
Odor	odorless
Density	2.52 g/cm³
Melting point	356.7 °C (674.1 °F; 629.8 K)
Boiling point	400 °C (752 °F; 673 K) decomposes
Solubility in water	73 g/100 mL at 25 °C
Solubility in other solvents	soluble in methanol, ethanol
Refractive index (n_D)	1.661 (dihydrate)
Hazards
Safety data sheet	ICSC 1369
EU classification (DSD)	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
Lethal dose or concentration (LD, LC):
LD₅₀ (median dose)	50 mg/kg
Related compounds
Other anions	Sodium chromate Sodium molybdate Sodium tungstate
Other cations	Potassium dichromate Ammonium dichromate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N (what is ^YN ?)
Infobox references

Sodium dichromate is the inorganic compound with the formula Na₂ Cr₂ O₇. Usually, however, the salt is 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. 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):

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:

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# 13517-17-4 ) 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

Dichromate and chromate salts are oxidizing agents. For the tanning of leather, sodium dichromate is first reduced with sulfur dioxide.

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