Sodium tellurite explained

Sodium tellurite should not be confused with Sodium telluride.

Sodium tellurite is an inorganic tellurium compound with formula Na₂TeO₃. It is a water-soluble white solid and a weak reducing agent. Sodium tellurite is an intermediate in the extraction of the element, tellurium; it is a product obtained from anode slimes and is a precursor to tellurium.

Preparation

The main source of tellurium is from copper anode slimes, which contain precious metals as well as various tellurides. These slimes are roasted with sodium carbonate and oxygen to produce sodium tellurite.^[1]

Ag₂Te + Na₂CO₃ + O₂ → 2Ag + Na₂TeO₃ + CO₂ (400–500 °C)

This is a reaction with silver telluride. The telluride is oxidized to tellurite and the silver(I) is reduced to silver.

Purification

The electrolysis of a tellurite solution yields purified tellurium.^[1]

Anode: 4OH⁻ → 2H₂O + O₂ + 4e⁻

Cathode: TeO₃²⁻ + 3H₂O + 4e⁻ → Te + 6OH⁻

Structure and properties

Tellurium has properties similar to sulfur and selenium. In the anhydrous form Na₂TeO₃ the tellurium atoms are 6 coordinate, three Te-O at 1.87 Å and three at 2.9 Å, with distorted octahedra sharing edges.^[2] In the pentahydrate, Na₂TeO₃.5H₂O there are discrete tellurite anions, TeO₃²⁻ which are pyramidal. The Te-O distance is 1.85 - 1.86 Å and the O-Te-O angle is close to 99.5°.^[3] The tellurite anion is a weak base. Sodium tellurite would be similar to sodium selenite and sodium sulfite. Sodium tellurite is both a weak oxidizing agent and a weak reducing agent.

Tellurite-related reactions

H₂TeO₃ → H⁺ + HTeO₃⁻ pK 2.48

Telluric acid loses a proton at this pKa.

HTeO₃⁻ → H⁺ + TeO₃²⁻ pK 7.7

Hydrogen tellurite loses a proton at this pKa to become the tellurite ion. This would happen in the reaction of tellurous acid with sodium hydroxide to make sodium tellurite.

TeO₂ + 2OH⁻ → TeO₃²⁻ + H₂O

This is the reaction of tellurium dioxide with a base to make a tellurite salt.

Applications

Sodium tellurite improves the corrosion resistance of electroplated nickel layers. Solutions of sodium tellurite are used for black or blue-black coatings on iron, steel, aluminum, and copper. In microbiology, sodium tellurite can be added to the growth medium to isolate bacteria with an inherent physiological resistance to its toxicity.^[4]

References

• Cameo Chemicals. Sodium Tellurite. Retrieved March 8, 2009. Website: http://cameochemical.noaa.gov/chemical/5185.

Notes and References

1. Book: Inorganic chemistry. Egon. Wiberg. Arnold Frederick. Holleman. Nils Wiberg. translated by Mary Eagleson. Academic Press. 2001. 0-12-352651-5. 588 .
2. Masse. R.. Guitel. J.C.. Tordjman. I.. Preparation chimique et structure cristalline des tellurites de sodium et d'argent: Na₂TeO₃, Ag₂TeO₃. Materials Research Bulletin. 15. 4. 1980. 431–436. 0025-5408. 10.1016/0025-5408(80)90048-3.
3. Acta Crystallogr. B . Etude cristallographique du tellurite de sodium à cinq molécules d'eau, Na₂Te^IVO₃·5H₂O . 35 . 1979 . 1337–1340 . 10.1107/S0567740879006403.
4. Borsetti, Francesca; Toninello, Antonio; Zannoni, Davide (2003). "Tellurite uptake by cells of the facultative phototroph Rhodobacter capsulatus is a pH-dependent process." Federation of European Biochemical Societies. Volume 554, Issue 3, 20 November 2003, pp. 315–318. Elsevier B.V.