Sodium selenite

September 27, 2021
I’m highly toxic, but useful in glassmaking.
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Image of Sodium selenite 3D Image of Sodium selenite

Sodium selenite (Na2SeO3) is a toxic inorganic salt that is highly soluble in water and insoluble in organic solvents. The crystal structure of the anhydrous salt has been variously reported as tetragonal or monoclinic.

In 1889, Irish chemists Sir Charles A. Cameron* and John Macallan produced Na2SeO3 and the previously unknown selenium oxychloride1 (SeOCl2) by heating selenium oxide (SeO2) with sodium chloride. This finding was reported in a pioneering series of articles on selenium chemistry.

In an improved 1929 synthesis, J. B. Krak at the Roessler and Hasslacher Chemical Co. (Perth Amboy, NJ) prepared Na2SeO3 by evaporating a solution of sodium hydroxide and selenious acid (H2SeO3) at 60–100 °C. Commercial Na2SeO3 is now made by heating an aqueous solution of SeO2 and NaOH to form its pentahydrate2, which is crystallized and then warmed to 40 °C to liberate the anhydrous salt.

Na2SeO3 and other selenite salts are used to “neutralize” green impurities during the manufacture of colorless glass. Iron-containing impurities in the natural silicas (e.g., sand) used to make glass absorb red and blue wavelengths of light, giving it a green appearance. Selenites have no color; but when they are dissolved in the glass mixture, they appear pinkish, counteracting the green hue and making the glass colorless.

Sodium selenite hazard information

Hazard class*GHS code and hazard statement
Acute toxicity, oral, category 2H300—Fatal if swallowedChemical Safety Warning
Skin corrosion/irritation, category 2H315—Causes skin irritation Chemical Safety Warning
Skin sensitization, category 1H317—May cause an allergic skin reactionChemical Safety Warning
Serious eye damage/eye irritation, category 2AH319—Causes serious eye irritationChemical Safety Warning
Acute toxicity, inhalation, category 2H330—Fatal if inhaledChemical Safety Warning
Short-term (acute) aquatic hazard, category 2H401—Toxic to aquatic lifeChemical Safety Warning
Long-term (chronic) aquatic hazard, category 2H411—Toxic to aquatic life with long-lasting effectsChemical Safety Warning

*Globally Harmonized System (GHS) of Classification and Labeling of Chemicals.  
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MOTW update

Tetrodotoxin was the Molecule of the Week for April 5, 2010. It is an extremely toxic natural substance—the poisonous principle in pufferfish and other marine animals. Tetrodotoxin, a nerve blocker, has been tested as a local anesthetic, but its concentration range must be limited to prevent it from becoming systemically toxic. It is not used currently for clinical purposes.

Anesthesiologist Daniel S. Kohane and his team at Boston Children’s Hospital and Harvard Medical School developed a self-assembling drug delivery system that releases the nerve-blocker more slowly and with less toxicity than direct injection. In experiments with rats, they used modified versions of peptides from the same voltage-gated sodium channels that tetrodotoxin targets to deliver the drug. The slower release allowed the toxin to block the rats’ sciatic nerves >3 times as long as free tetrodotoxin, without harmful effects.

See this week's MOTW update below.

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Sodium selenite 
fast facts

CAS Reg. No.10102-18-8
Selenious acid, sodium salt (1:2)
Empirical formulaNa2O3Se
Molar mass172.94 g/mol
AppearanceColorless crystals or white powder
Melting point720° C (dec.)a
Water solubility900 g/L

a. Decomposition temperatures as low as
320 °C are reported.

Over the years, readers have noted that ionic substances are not actually molecules. This is correct, but we use "molecules" in the broadest sense to include them in Molecule of the Week.—Ed.

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