Chemical Properties of Tungsten

Tungstates Formed by dissolving WO3 in NaOH solution and subsequently by cation exchange. The normal tungstates. e.g.Na2WO4,2H2O, contain tetrahedral WO42-ions. Most metal salts other than alkali metal salts are insoluble. Acidification of a solution of WO42-ions gives polymeric anions-iso-polytungstates-containing WO6 octahedra joined corner to corner to corner or edge to edge; [HW6O21]5- ,[H2W12O42]10- and[W12O39]6- are well-established. If the acidification is carried out in the presence of another oxyanion, heteropolytungstates in which MO6 or MO4 groups (M=heteroatom) are incorporated into the polyanion are formed. E.g.[FeW12O4]5- and [PW12O4]3- .It seems unlikely that free tungstic acids of any of these forms are sable. Further acidification of isopolytungstates gives WO3,2H2O, Heteropolytungstates are used to form Wcontaining catalysts by heating.

Tungsten,(w) The main ore of tungsten is wolframite (Fe,Mn)WO4; scheelite (CaWO4) and solzite (PbWO4) are also important. The concentrated ores are fused with NaOH and, after water extraction, WO3 is precipitated with acid and reduced to the metal with H2. The metal is bcc. The fuse metal is lustrous silver-white and is only attacked slowly even by HNO3-HF. Dssoleds in KNO3-NaOH or Na2O2, attacked by O2 at red-heat. The metal isused extensively in steel alloys, in electric lamp and heating filaments and in electric contaces. Tungsten carbides are very hard and are used in cutting tools. World production WO31981 52000 tonnes.

Tungsten is a typiacl element of transition element Group VI and shows oxidation states from +6 to-2 and, particularly in its oxides, forms many non-stoicheiometric compounds. There is little aqueous chemistry except that of complex oxy-anions and some complex halides. The hexahalides are moleular but lower halides are polymeric and the lowest halides show extensive W-W bonding (more than Mo). Carbonyl and phos-phine derivatives are typical low oxidation state compounds. Complexes are formed, particularly by O-and S-ligands in higher oxidation states and by P-ligands in low oxidation states. Complex cyanides are well established.