In chemistry a tungstate is a compound that contains an oxoanion of tungsten or is a mixed oxide containing tungsten. The simplest tungstate ion is WO₄²⁻, "orthotungstate".^[1] 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 polyoxometalates, ("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 elements, the large tungstate ions generally contain 6 coordinate metal atoms similar to molybdenum (molybdates) and contrast to chromium (chromates) where 4 coordination predominates.^[1]

Some examples of tungstate ions that have been detected in solution in the solid state include^[2]:

• HWO₄^{− [2]}
• polymeric W₂O₇²⁻ ions of various structures in Na₂W₂O₇, Li₂W₂O₇ and Ag₂W₂O₇^[3]

• [W₇O₂₄]⁶⁻ (paratungstate A)^[2]
• [W₁₀O₃₂]⁴⁻ (tungstate Y)^[4]
• [H₂W₁₂O₄₂]¹⁰⁻ (paratungstate B) ^[2]
• α-[H₂W₁₂O₄₀]⁶⁻ (metatungstate)^[4])
• β-[H₂W₁₂O₄₀]⁶⁻ (tungstate X)^[4])

See the tungstates category for a list of tungstates.

[edit] Occurrence

Tungstates occur naturally with molybdates. Powellite is a mineral form of calcium molybdate containing a small amount of tungstate, and scheelite is a mineral form of calcium tungstate containing a small amount of molybdate. Wolframite is manganese and iron tungstate, and all these are valuable sources of tungsten.

[edit] Chemical and physical properties

Solutions of tungstates, like those of molybdates, give intensely blue solutions of complex tungstate(V,VI) analoguous to the molybdenum blues when reduced by most organic materials.^[1]

Tungstate can also be used as an oxidizing agent in the oxidation of cyclohexene. The tungstate can be constantly regenerated through the presence of hydrogen peroxide.

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

[edit] References

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