## 3 Results and Discussion  
The schemes of a multi-chamber electrodialyzer employing cation-exchange, anion-exchange, and bipolar membranes are primarily used for water desalination [37]. These configurations are also applicable for the concentration of salt solutions such as NaCl, Na\({}_{2}\)SO\({}_{4}\), and others, where salts dissociate in water to form mobile cations and anions. However, this conventional multi-chamber electrodialyzer design--with ion-selective and bipolar membranes--is not suitable for the electrodialysis of sodium tungstate solutions. In alkaline media (pH > 8), sodium tungstate undergoes dissociation according to the following reaction scheme:  
\[\text{Na}_{2}\text{WO}_{4}=2\text{Na}^{+}+\text{WO}_{4}^{2-}\]  
During the dissociation of sodium tungstate, mobile Na\({}^{+}\) ions and relatively immobile WO\({}_{4}\)\({}^{2-}\) ions are formed. Moreover, as the pH of the solution decreases, polyoxotungstates (polytungstates) begin to form. In a multi-chamber system, the equivalent transfer of WO\({}_{4}\)\({}^{2-}\) anions through the anion-exchange membrane does not occur efficiently, and the  
Figure 4: Dependence of the voltage (**a**) and cell conductivity (**b**) on the current density electrodia-ysis. observed current is largely attributed to the migration and discharge of H\({}_{3}\)O\({}^{+}\) and OH\({}^{-}\) ions at the electrodes. The overall reaction for the production of tungstic acid via electrodialysis from a sodium tungstate solution can be represented by the following equation:  
\[2\text{Na}_{2}\text{WO}_{4}+6\text{H}_{2}\text{O}=4\text{NaOH}+2\text{H}_{2} \text{WO}_{4}+\text{O}_{2\text{(g)}}+2\text{H}_{2\text{(g)}}\]  
According to the literature [38], the speciation and degree of polymerization of tungstate ions in solution depend strongly on the pH of the medium:  
* Sequential removal of sodium salt impurities, such as silicon, phosphorus, molybdenum, arsenic, and additional silicon;
* In alkaline solutions up to pH \(\approx\) 8, normal (monomeric) tungstate ions predominate;
* In the pH range from 8 to approximately 6, hexatungstate ions (HW\({}_{6}\)O\({}_{21}\)\({}^{5-}\)) are formed;
* With a further decrease in pH, particularly in highly dilute solutions, the monohydrogen hexatungstate ion (HW\({}_{6}\)O\({}_{21}\)\({}^{5-}\)) is converted into the trihydrogen form (H\({}_{3}\)W\({}_{6}\)O\({}_{21}\)\({}^{3-}\));
* Upon the gradual neutralization of alkaline sodium tungstate solutions to pH \(\approx\) 5, a precipitate of sodium paratungstate (Na\({}_{10}\)W\({}_{12}\)O\({}_{41}\)\({}^{2}\)8H\({}_{2}\)O) is formed;
* Normal alkaline tungstates are highly soluble, whereas paratungstates exhibit significantly lower solubility;
* At pH < 4, metatungstate aquapolyions are formed, typically with a Me\({}_{2}\)O:WO\({}_{3}\) ratio of 1:4 (e.g., Na\({}_{2}\)W\({}_{2}\)O\({}_{7}\)). The number of coordinated water molecules in isopoly and heteropoly tungstate species varies depending on temperature and concentration, which in turn influences the physicochemical properties of the resulting salts. Figure 5 illustrates the variation in the acidity of the anolyte during the electrodialysis of a sodium tungstate solution with an initial concentration of 135 g/L, carried out at an anodic current density of 666.7 A/m\({}^{2}\). As seen from the pH of the sodium tungstate solution in Figure 5, distinct regions of stepwise changes in composition can be identified, which are consistent with previously reported data. The final segment of the curve, corresponding to the pH decrease from 4.0 to 1.42, reflects the progressive removal of sodium ions from metatungstate species such as  
Figure 5: Change in the pH of the tungstate solution from the duration of electrodialysis at d\({}_{\text{a}}\) = 666.7 A/m\({}^{2}\). Na\({}_{2}\)WO\({}_{7}\) and the subsequent formation of hydrated compounds, including WO\({}_{3}\)\(\cdot\)2H\({}_{2}\)O or H\({}_{2}\)WO\({}_{4}\)\(\cdot\)H\({}_{2}\)O (commonly referred to as white tungstic acid).