US Patent 7794686 Method for making ammonium metatungstate

A method for producing ammonium metatungstate from ammonium paratungstate includes preparing an ammonium paratungstate mixture including solid ammonium paratungstate and water. The mixture is contacted with a cation exchange material to lower a pH of the mixture to a pH range wherein metatungstate ion is stable and an insoluble tungstic acid is not formed. The pH of the mixture may be maintained in the pH range until the ammonium paratungstate mixture is converted into an ammonium metatungstate solution.