Sodium Metatungstate, Na₂W₄O₁₃

Sodium Metatungstate, Na₂W₄O₁₃.10H₂O, is obtained by methods analogous to those described for the potassium salt. The salt crystallises in colourless glistening tetragonal bipyramids, with axial ratio a:c = 1:0.9930, isomorphous with ammonium, cadmium, and manganese metatungstates, and of density 4.04. The crystals are very efflorescent, and if placed over sulphuric acid lose most of their water. When heated they lose 7H₂O per molecule at 120° C., and at 240° C. only 1H₂O remains, the residue after heating at this temperature being still soluble in water. The remaining water is removed at red heat, and the residue is insoluble.

One part of water at ordinary temperature dissolves 10.69 parts of salt, and the resulting solution has density 3.019, whilst at 19° C. one part of the salt dissolves in 0.195 part of water. The solubility increases greatly with temperature. The equivalent conductivities of solutions containing one gram-equivalent in v litres at 25° C. have been determined as follows:

v =	32	64	128	256	512	1024
Λ =	89.3	98.7	107.6	116.6	126	134.1

According to Schon cotton which has been soaked in a solution of sodium metatungstate immediately becomes blue in daylight, but the colour disappears in the dark.

A colloidal form of sodium metatungstate is obtained when a solution of normal sodium tungstate is treated with hydrochloric acid in the proportion of 2Na₂WO₄:3HCl, and the mixture subjected to dialysis. According to Graham colloidal tungstic acid is obtained by this method, but all the soda cannot be removed by dialysis; and according to Sabaneeff the solution remaining in the dialyser always contains 1Na₂O:4WO₃. On evaporation of this solution an amorphous product is obtained which appears identical in properties with the crystalline metatungstate. Biltz and Vegesack, after prolonged dialysis, obtained a solution containing 2Na₂O:11WO₃ and expressed the opinion that colloidal tungstic acid was present.