Sodium Tungstate as Mordant

The production of colloidal tungsten hydroxide by the electrolysis of a solution of sodium tungstate has already been described. If precautions are taken to prevent the sodium hydroxide formed at the cathode from reaching the anode, for example, by means of a porous partition, it is possible to prepare the paratungstate, or other complex tungstate, from the anode solution.

The use of sodium tungstate has been recommended as a mordant, and it has been used as a fire-proofing material for flannelette, but owing to its solubility it cannot be considered satisfactory and it is not now used.

Sodium ditungstate, Na2O.2WO3, may be obtained by fusing together tungstic anhydride and sodium hydroxide or sodium carbonate, the mixture containing lNa2O:2WO3. On cooling, long needles separate, which on prolonged heating with water dissolve, yielding an alkaline solution which contains metatungstate. The dihydrate, Na2O.2WO3. 2H2O, is described by Rammelsberg as a crystalline precipitate obtained by addition of hydrochloric acid to a solution of the normal tungstate. The hexahydrate, Na2O.2WO3.6H2O, is stated by Lefort to crystallise from a solution containing the normal tungstate (2 molecules) and acetic acid (1 molecule); von Knorre, however, could only obtain the paratungstate from such a solution. The hydrate,Na2O.2WO3.12H2O, has also been described.

Sodium paratungstate is known commercially as "tungstate of soda" and may be prepared on a large scale by fusing wolframite with soda ash and lixiviating the fused mass. On nearly neutralising the boiling solution with hydrochloric acid and allowing to crystallise, large triclinic crystals of the salt separate.