Tungstate

In chemistry a tungstate is a compound that contains an oxoanion of tungsten

Tungsten

Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

 or is a mixed oxide

Mixed oxide

In chemistry, a mixed oxide is a somewhat informal name for an oxide that contains more than one chemical element in its cation, or of a single element cation that has atoms in several states of oxidation....

 containing tungsten. The simplest tungstate ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

 is WO₄²⁻, "orthotungstate". There are many other tungstate ions that contain more than one tungsten atom and belong to a large group of polyatomic ions that are termed polyoxometalate

Polyoxometalate

In chemistry, a polyoxometalate is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form a large, closed 3-dimensional framework....

s, ("POMs"), and specifically termed isopolyoxometalates as they contain, along with oxygen and maybe hydrogen, only one other element. Comparing tungsten to the other group 6

Group 6 element

A Group 6 element is one in the series of elements in group 6 in the periodic table, which consists of the transition metals chromium , molybdenum , tungsten , and seaborgium ....

 elements, the large tungstate ions generally contain 6 coordinate metal atoms similar to molybdenum (molybdate

Molybdate

In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, although the latter are only found in the solid state.The...

s) and contrast to chromium (chromate

Chromate

Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents.- Chemical properties :...

s) where 4 coordination predominates.

Some examples of tungstate ions that have been detected in solution in the solid state include:

• HWO₄⁻
• polymeric W₂O₇²⁻ ions of various structures in Na₂W₂O₇, Li₂W₂O₇ and Ag₂W₂O₇

• [W₇O₂₄]⁶⁻ (paratungstate A)
• [W₁₀O₃₂]⁴⁻ (tungstate Y)
• [H₂W₁₂O₄₂]¹⁰⁻ (paratungstate B)
• α-[H₂W₁₂O₄₀]⁶⁻ (metatungstate))
• β-[H₂W₁₂O₄₀]⁶⁻ (tungstate X))

See the tungstates category for a list of tungstates.

Occurrence

Tungstates occur naturally with molybdate

Molybdate

In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, although the latter are only found in the solid state.The...

s. Powellite

Powellite

Powellite is a calcium molybdate mineral with formula CaMoO4. Powellite crystallizes with tetragonal - dipyramidal crystal structure as transparent adamantine blue, greenish brown, yellow to grey typically anhedral forms. It exhibits distinct cleavage and has a brittle to conchoidal fracture. It...

 is a mineral form of calcium molybdate containing a small amount of tungstate, and scheelite

Scheelite

Scheelite is a calcium tungstate mineral with the chemical formula CaWO4. It is an important ore of tungsten. Well-formed crystals are sought by collectors and are occasionally fashioned into gemstones when suitably free of flaws...

 is a mineral form of calcium tungstate containing a small amount of molybdate. Wolframite

Wolframite

Wolframite WO4, is an iron manganese tungstate mineral that is the intermediate between ferberite and huebernite . Along with scheelite, the wolframite series are the most important tungsten ore minerals. Wolframite is found in quartz veins and pegmatites associated with granitic intrusives...

 is manganese and iron tungstate, and all these are valuable sources of tungsten.

Chemical and physical properties

Solutions of tungstates, like those of molybdates, give intensely blue solutions of complex tungstate(V,VI) analoguous to the molybdenum blue

Molybdenum blue

Molybdenum blue is a term applied to:*reduced heteropolymolybdate complexes, polyoxometalates containing Mo, Mo, and a hetero atom such as phosphorus or silicon...

s when reduced by most organic materials.

Tungstate can also be used as an oxidizing agent in the oxidation of cyclohexene

Cyclohexene

Cyclohexene is a hydrocarbon with the formula C6H10. This cycloalkene is a colorless liquid with a sharp smell. It is an intermediate in various industrial processes...

. The tungstate can be constantly regenerated through the presence of hydrogen peroxide.

Unlike chromate

Chromate

Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents.- Chemical properties :...

, tungstate is not a good oxidizer, but like chromate, it polymerizes in acid and depolymerizes back to tungstates in alkaline solutions.