Sodium Selenite
Properties
| State | Solid (pentahydrate is common commercial form) |
| Color | Colorless to white |
| Solubility | Very soluble in water (850 g/L); insoluble in ethanol |
| Melting Point | 710 °C (anhydrous) |
About Sodium Selenite
Sodium selenite (Na2SeO3, 172.948 g/mol; pentahydrate Na2SeO3.5H2O at 263.03 g/mol) is the +4 oxidation-state selenium salt that dominates selenium's commercial nutritional supply. The selenite anion SeO3 2- is trigonal pyramidal — Se(IV) carries a stereochemically active lone pair, so the anion looks geometrically more like sulfite SO3 2- than the tetrahedral selenate SeO4 2- you might first guess. That shape matters in metabolism: SeO3 2- is reduced by glutathione in the liver to selenide HSe-, then converted to selenocysteine and inserted into selenoproteins via the UGA-recoding mechanism unique to this amino acid. The dietary RDA is 55 microg/day for adults, and Na2SeO3 is the form used in roughly 80% of US multivitamins, in nearly all total parenteral nutrition (TPN) bags, and in livestock feed premixes for cattle, sheep, and poultry. Outside nutrition, selenite shows up as a colorant in art glass — at 0.05 to 0.5% in a glass batch with cadmium sulfide it produces the deep red of selenium ruby glass — as a brightener additive in zinc electrowinning, and as the spot-test reagent for ascorbic acid (selenious acid is reduced to a red colloidal selenium suspension, a quick qualitative check for vitamin C in food samples).
Where you'll encounter it
If you have ever swallowed a centrum-style multivitamin, the 55 to 70 microg of selenium on the label was almost certainly delivered as sodium selenite or sodium selenate — selenite predominates in older formulations, selenate in newer ones because it is slightly less hygroscopic in the powder blend. Veterinarians inject sodium selenite plus vitamin E into newborn lambs and calves in selenium-deficient pastures (the upper Midwest, parts of New Zealand, China's Keshan belt) to prevent white-muscle disease, a fatal nutritional myopathy that develops within weeks if the dam's diet was selenium-poor during pregnancy. In an analytical lab, you encounter Na2SeO3 as a starting material for atomic-absorption selenium standards or as a reductant trap in mercury speciation work. The compound's chronic toxicity is real but manageable — long-term intake above 400 microg/day causes selenosis (garlic breath, hair loss, brittle nails) but the lethal dose is more than 1 g, comfortably above any plausible accidental exposure.
Common Uses
- Selenium source in human multivitamins delivering 50 to 200 microg per dose as Na2SeO3
- Trace selenium in total parenteral nutrition bags for hospitalized patients on long-term IV feeding
- Veterinary injectable for prevention of white-muscle disease in lambs and calves
- Selenium supplement in cattle, sheep, and poultry feed premixes at 0.1 to 0.3 ppm in finished feed
- Colorant for selenium ruby glass and red ceramic enamels at 0.05 to 0.5% in the batch
- Brightener and grain-refining additive in zinc electrowinning electrolyte
- Spot-test reagent for ascorbic acid via reduction to red colloidal selenium
- Catalyst for the Kjeldahl nitrogen digestion as a selenium-mercury accelerator pellet
Safety Information
TOXIC. GHS: H301 (acute oral toxicity, Category 3, LD50 oral rat 7 mg/kg), H331 (acute inhalation toxicity, Category 3), H372 (repeated-exposure toxicity to liver and CNS, Category 1), H400+H410 (very toxic to aquatic life with long-lasting effects). OSHA PEL for selenium and compounds is 0.2 mg/m3 TWA (as Se); ACGIH TLV is 0.2 mg/m3 TWA. The therapeutic window is narrow — 55 microg/day RDA, 400 microg/day upper tolerable intake, garlic breath and hair loss above 800 microg/day, acute toxicity above 1 mg/kg body weight. Handle with nitrile gloves, safety glasses, and powder-handling local exhaust ventilation. Keep away from children; never store in food-storage containers.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.