What Is Sodium Selenate?

Elements in the periodic table belong to various families. There is the halogen family of fluorine, chlorine, bromine and iodine. There is the alkali metal family of lithium, sodium, potassium, rubidium and cesium. Selenium, from which sodium selenate is derived, is a non-metal belonging to the oxygen, sulfur and tellurium family. Comparable to sodium sulfate, decahydrate, Na₂SO₄ • 10 H₂O, is sodium selenate, decahydrate, Na₂SeO₄ • 10 H₂O. It is important to nutritionists, to ecologists and to toxicologists.

Sources

Selenium was originally discovered in 1817 by the chemist Jöns Jacob Berzelius. It was named after the Greek word "selene," meaning moon.

Selenium-containing ores contain the selenides of copper, nickel, silver, lead and are mixed in with sulfides of the same metals. Copper is the most abundant among these, so most selenium is industrially produced as a byproduct of the electrolytic refinement of copper.

Although sulfur is sometimes found in its elemental form, selenium rarely is.

Preparation of Sodium Selenate

In the electrolytic purification of copper, anode slime (also called "anode mud") is combined with soda ash, then pelletized, dried and roasted. Sodium selenate is then extracted by water from the resultant roast.

An alternate method of isolating selenium uses sulfuric acid. The end result of that process is not sodium selenate but elemental selenium.

Uses of Sodium Selenate

Selenium is a micronutrient essential to life. It plays a role in enzyme chemistry and helps regulate the thyroid gland. Consumed in excess, selenium is toxic. Fortunately, the amount of selenium the average person needs can be found in a normal, healthy diet.

A second application of sodium selenate is in glass manufacture to counter the color caused by iron impurities.

Selenium in the Diet

The National Institutes of Health provides a useful chart of selenium levels in food. By far the leader is the Brazil nut, which has nearly 10 times the level of selenium of the food listed second, tuna. The NIH chart also contains considerable information on selenium, pregnancy and lactation.

Caution should be used if considering a selenium supplement, as it is easy to consume too much. The current RDA is 55 micrograms a day. As little as 400 mcg selenium a day is capable of causing selenosis. Selenosis can cause traumatic changes, including neurological ones and even lead to death.

An Ecological Problem

Sedimentary phosphate and selenium rock often occur together. So in the mining of phosphate rock, much waste-rock containing selenium becomes exposed to the elements. The rains leach out the selenium, which finds its way into streams and bodies of water. The contaminated water can affect cattle and other animals, causing selenosis. This reality was forced on an unsuspecting public in Idaho in the late 1990s. It also affects current mining practices. Old mines undergo evaluation for damage done and remediation.

References

• Agency for Toxic Substances and Disease Registry: Public Health Statement for Selenium
• Monsanto Company: A History of Selenium and its Connection to Phosphate Mining
• Agency for Toxic Substances and Disease Registry: Selenium Production

Resources

• National Institutes of Health: What Foods Provide Selenium?
• American Chemical Society: Selenium Glass