Sodium tellurite is an inorganic tellurium compound with formula Na₂TeO₃. It is a water soluble white solid and a weak reducing agent

Reducing agent

A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...

. Sodium tellurite is an intermediate in the extraction of the element, tellurium; it is a mineral obtained from slimes and is a precursor to tellurium.

Preparation

The main source of tellurium is from copper anode

Anode

An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 slimes, which contain precious metal

Precious metal

A precious metal is a rare, naturally occurring metallic chemical element of high economic value.Chemically, the precious metals are less reactive than most elements, have high lustre, are softer or more ductile, and have higher melting points than other metals...

s as well as various tellurides. These slimes are roasted with sodium carbonate

Sodium carbonate

Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...

 and oxygen to produce sodium tellurite.

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

Electrolysis

In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

 of a tellurite solution yields purified tellurium.

Anode: 4OH^– → 2H₂O + O₂ + 4e^–

Cathode: TeO₃^2– + 3H₂O + 4e^– → Te + 6OH^–

Structure and properties

Tellurium has properties similar to sulfur and selenium. The tellurite anion is pyramidal with C_3v symmetry. The Te-O distance is 1.86 Å and the O-Te-O angle is 99.5°. The tellurite anion is a weak base. Sodium tellurite would be similar to sodium selenite

Sodium selenite

Sodium selenite is a salt, a colourless solid, and the most common water-soluble selenium compound. It has the formulas Na2SeO3 and Na2SeO35 . Respectively, these are the anhydrous salt and its pentahydrate. This hydrated salt is the more common one...

 and sodium sulfite

Sodium sulfite

Sodium sulfite is a soluble sodium salt of sulfurous acid. It is a product of sulfur dioxide scrubbing, a part of the flue gas desulfurization process...

. 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⁺ + TeO₃^2– 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^– → TeO₃^2– + 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.