Sodium tungstate


Sodium tungstate is the inorganic compound with the formula Na2WO4. This white, water-soluble solid is the sodium salt of tungstic acid. It is useful as a source of tungsten for chemical synthesis. It is an intermediate in the conversion of tungsten ores to the metal.

Preparation and structure

Sodium tungstate is obtained by digestion of tungsten ores, the economically important representatives of which are tungstates, in base. Illustrative is the extraction of sodium tungstate from wolframite:
Scheelite is treated similarly using sodium carbonate.
Sodium tungstate can also be produced by treating tungsten carbide with a mixture of sodium nitrate and sodium hydroxide in a fusion process which overcomes the high exothermicity of the reaction involved.
Several polymorphs of sodium tungstate are known, three at only one atmosphere pressure. They feature tetrahedral orthotungstate dianions but differ in the packing motif. The WO42− anion adopts a structure like sulfate.

Reactions

Treatment of sodium tungstate with hydrochloric acid gives the trioxide:
This reaction can be reversed using aqueous solution of sodium hydroxide

Uses

The dominant use of sodium tungstate is as an intermediate in the extraction of tungsten from its ores, almost all of which are tungstates. Otherwise sodium tungstate has only niche applications.
In organic chemistry, sodium tungstate is used as catalyst for epoxidation of alkenes and oxidation of alcohols into aldehydes or ketones. It exhibits anti-diabetic effects.
Solutions of sodium polytungstate are used in density separation, since such mixtures are less toxic than bromoform and methylene iodide, but still have densities that fall between a number of naturally coupled minerals.
Sodium tungstate is a competitive inhibitor of molybdenum. Dietary tungsten reduces the concentration of molybdenum in tissues.