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). Occupational exposure limits for selenium and its compounds are set very low, in line with other toxic metal dusts. 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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.