Sodium chromate

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Sodium chromate
Names
IUPAC name
Sodium chromate
Systematic IUPAC name
Sodium chromate
Other names
Chromic acid, disodium salt
Chromium disodium oxide
Rachromate
Properties
Na2CrO4
Molar mass 161.97 g/mol
Appearance Yellow hygroscopic solid
Odor Odorless
Density 2.698 g/cm3
Melting point anhydrous
792 °C (1,458 °F; 1,065 K)
decahydrate
20 °C (68 °F; 293 K)
Boiling point Decomposes
31.8 g/100 ml (0 °C)
84.5 g/100 ml (25 °C)
126.7 g/100 ml (100 °C)
Solubility Slightly soluble in ethanol, methanol
Solubility in methanol 0.344 g/100 ml (25 °C)
Vapor pressure ~0 mmHg
Hazards
Safety data sheet Sigma-Aldrich (anhydrous)
Sigma-Aldrich (tetrahydrate)
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
52 mg/kg (rat, oral)
Related compounds
Related compounds
Potassium chromate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Sodium chromate is the inorganic compound with the formula Na2CrO4.

Properties

Chemical

At acidic pH, sodium chromate converts to sodium dichromate:

2 Na2CrO4 + 2 HCl → Na2Cr2O7 + 2 NaCl + H2O

Addition of concentrated sulfuric acid yields chromium trioxide:

Na2CrO4 + H2SO4 → CrO3 + Na2SO4 + H2O

Physical

Sodium chromate is a bright yellow hygroscopic solid, soluble in water.

Availability

Sodium chromate is sold by various chemical suppliers.

In some countries, the sale of Cr(VI) compounds is regulated.

Preparation

Can be prepared by roasting chromium(III) oxide in air in the presence of sodium carbonate:

2 Cr2O3 + 4 Na2CO3 + 3 O2 → 4 Na2CrO4 + 4 CO2

Sodium chromate can be produced from stainless steel from dinnerware or cooking implements, which contains a minimum of 10.5% chromium by mass. First, the entire piece of stainless steel is dissolved in conc. hydrochloric acid. The resulting solution should be dark green due to the chromium(III) ion. Sodium carbonate (not sodium hydroxide) is added to the solution to neutralize all remaining acids and precipitate a mixture of iron and chromium hydroxides, which are then filtered and washed. The washed mixture is mixed with sodium hypochlorite solution, which will oxidize the chromium(III) ions to chromate (CrO42-) ions, bringing them into solution as sodium chromate. The solution is filtered, concentrated and the sodium chromate is recrystallized from the solution, then dried in a desiccator.

Projects

Handling

Safety

Sodium chromate, like all hexavalent chromium compounds, is highly toxic and carcinogenic on ingestion or inhalation. Handling it without gloves can cause dermatitis, and can also be absorbed through the skin in small amounts, usually if wet. Aqueous solutions are notorious for staining most materials.

Always wear gloves and goggles when handling it, and a dust mask or respirator when handling it as a powder to avoid inhalation of it, which could be fatal.

Storage

Sodium chromate should be kept in closed bottles, with a proper label and a hazard symbol.

Disposal

Sodium chromate can be safely reduced to the less harmful Cr(III) oxide with a reducing agent, such as ascorbic acid, or potassium/sodium sulfites/metabisulfites/tiosulfates.

References

Relevant Sciencemadness threads