Sodium selenite

Sodium selenite is a salt, a colourless solid, and the most common water-soluble selenium compound. It has the formulas Na2SeO3 and Na2SeO3(H2O)5 (CAS#26970-82-1). Respectively, these are the anhydrous salt and its pentahydrate. This hydrated salt is the more common one. The hydrated and anhydrous salts behave identically for most purposes, although their molar masses differ.

Synthesis and fundamental reactions
Sodium selenite usually is prepared by the reaction of selenium dioxide with sodium hydroxide:
 * SeO2 + 2 NaOH → Na2SeO3 + H2O

Akin to the related salt sodium sulfite, Na2SeO3 features a pyramidal dianion SeO32–. Oxidation of this anion gives sodium selenate, Na2SeO4.

Applications
Together with the related barium and zinc selenites, sodium selenite is mainly used in the manufacture of colorless glass. The pink color imparted by these selenites cancels out the green color imparted by iron impurities.

Because selenium is an essential element, sodium selenite is an ingredient in some food supplements.

The US Food and Drug Administration approved a selenium supplement to animal diets, however the most common form is sodium selenite for pet foods. According to one article, "not much was known about which selenium compounds to approve for use in animal feeds when the decisions were made back in the 1970s .. At the time the regulatory action was taken, only the inorganic selenium salts (sodium selenite and sodium selenate) were available at a cost permitting their use in animal feed.”

Safety
Selenium is toxic in high concentrations. The chronic toxic dose for human beings is about 2.4 to 3 milligrams of selenium per day.