Ammonium Metatungstate Prepared by Electrodialysis Method

Ammonium metatungstate (AMT) is a tungsten compounds with high molecular weight and water-soluble, which is easily dissolved in water, the solubility of it can reach 100g/mL at room temperature. It can be prepared from the material of ammonium tungstate by the method of Bipolar Membrane Electrodialysis (BEMD).

Electrodialysis method uses the effect of electric field, makes water which is in the two sides of bipolar membrane dissociated into H⁺ and [OH]^-, then directional moved to both sides of electrode, and entered into the liquid chamber (the salt chamber) and alkali chamber respectively. Electrodialysis method is widely used in the field of chemical, light, metallurgy, papermaking industry and so on.

The preparation process of AMT by the method of electrodialysis which is using ammonium tungstate or ammonium paratungstate as raw materials is performed as bellows:
1. Add ammonium paratungstate into anode chamber, when an electric current passes by, water dissociated into two forms--H⁺ and [OH]^- and directional moved to both sides of electrode under the action of strong electric field, and then respectively enter the liquid chamber (the salt chamber) and alkali chamber;
2. H⁺ ions enter the salt chamber to combine with WO₄^- and then will not go into alkali chamber ever again; ammonium ions move to cathode chamber through the exchange membrane, thus cause the concentration of ammonium metatungstate increased;
3. OH^- moves into alkali chamber through the anion exchange membrane to combine with ammonium ions which is continually moved in and no longer went into the anode chamber;
4. With the process of electrolysis carrying out, ammonium ions decreased and hydrogen ions increased continually in the salt chamber, thus cause the pH value of the solution in salt chamber decreased continuously;
5. Finally, ammonium metatungstate solution is released from the cathode chamber, and AMT generated.