## 2 Materials and Methods  
Sodium tungstate solutions were prepared using the reagent Na2 WO4 2H2O (sodium tungstate dihydrate) of analytical grade, in accordance with standard 18289-78 [32], with the composition shown in Table 1.  
Sodium tungstate (Na2 WO4) is a white powder, soluble in water, and may contain impurities such as N, SO4, Cl, Fe, Mo, As, and Pb not exceeding 0.01%. The density at a concentration of 38% and a temperature of 20 \({}^{\circ}\)C is 3.00 g/cm\({}^{3}\). The pH value is between 9 and 11. The electrical conductivity of the sodium tungstate solution depends on the concentration and temperature of the solution. For a solution with a concentration of 0.1 mol/L, the conductivity is 0.01-0.1 S/m [33]. According to literature sources, the formation of sodium paratungstates in solutions is possible during the electrodialysis process. Sodium paratungstate is an inorganic substance, a complex salt of sodium and tungstic acid with the formula Na\({}_{5}\)[HW\({}_{6}\)O\({}_{21}\)]. Under normal conditions, it is a white crystalline substance, soluble in water, and poorly soluble in organic solvents. A crystalline hydrate with the formula Na\({}_{5}\)[HW\({}_{6}\)O\({}_{21}\)]\(\cdot\)13.5H\({}_{2}\)O exists and is stable in air [33]. The initial reagent Na\({}_{2}\)WO\({}_{4}\)\(\cdot\)2H\({}_{2}\)O was analyzed using an X-ray diffractometer X, Pert MPD PRO (PANalytical, Singapore), and JED-2300 AnalysisStation JEOL (Singapore). The results of the analyses are shown in Figure 1.  
Figure 1: Images of Na\({}_{2}\)WO\({}_{4}\)\(\cdot\)2H\({}_{2}\)O obtained using X-ray diffractometer X (**a**) and spectrum from JED-2300 AnalysisStation JEOL (**b**). \begin{table}
\begin{tabular}{c c} \hline
**Indicator Name** & **Mass \%** \\ \hline Mass fraction of Na\({}_{2}\)WO\({}_{4}\)\(\cdot\)2H\({}_{2}\)O, \%, not less than & 99.0 \\ \hline Mass fraction of water-insoluble substances, \%, not more than & 0.01 \\ \hline Mass fraction of nitrogen (N) from nitrates, nitrites, etc., \%, not more than & 0.01 \\ \hline Mass fraction of sulfates (SO\({}_{4}\)), \%, not more than & 0.01 \\ \hline Mass fraction of chlorides (Cl), \%, not more than & 0.003 \\ \hline Mass fraction of iron (Fe), \%, not more than & 0.0005 \\ \hline Mass fraction of molybdenum (Mo), \%, not more than & 0.002 \\ \hline Mass fraction of arsenic (As), \%, not more than & 0.0005 \\ \hline Mass fraction of heavy metals (Pb), \%, not more than & 0.001 \\ \hline pH of a 5\% solution of the reagent & 8–10 \\ \hline \end{tabular}
\end{table} 
Table 1: Composition of the initial sodium tungstate. A simplified scheme of an electrochemical cell (Figure 2) was used to carry out the electrodialysis of sodium tungstate solutions for the purpose of producing tungstic acid and regenerating the NaOH solution. The electrodialyzer unit was constructed from rectangular plates and frames made of plexiglass. The installation parameters of the electrodialyzer are shown in Table 2.  
Cation-exchange membranes of the MK-40 (Russia) [34] and EDC1R (China) [35] brands were used. According to the manufacturers, the electrical resistance of the membranes is Ravg = 10.0 \(\Omega\cdot\)cm\({}^{2}\) for MK-40 and Ravg = 5.0 \(\Omega\cdot\)cm\({}^{2}\) for EDC1R. Figure 3 shows the electrodialayer setup with the solution circulation and pH control of the resulting tungstic acid (H2 WO4) solution.