## 4 Conclusions  
Existing technologies for the production and processing of sodium tungstate solutions are accompanied by the use of a significant number of auxiliary reagents, including ammonia, sulfuric acid, hydrochloric acid, nitric acid, and anionites, which complicates the technological process and leads to the formation of sulfate and chloride effluents.  
Currently, membrane technologies of electrodialysis, cations, and anions are increasingly being used for water and wastewater treatment. However, the scheme of a multi-chamber electrodialyzer with ion-selective and bipolar membranes is not applicable for the electrodialysis of solutions of sodium tungstate, which in alkaline solutions forms mobile Na\({}^{+}\) cations and sedentary WO\({}_{4}\)\({}^{2-}\) anions, and when the pH of the solution decreases, polyvolframates are formed.  
In a multi-chamber apparatus, there is no transfer of an equivalent amount of WO\({}_{4}\)\({}^{2-}\) anions through the anion exchange membrane, and the magnitude of the flowing current is due to the discharge of H3O\({}^{+}\) and OH\({}^{-}\) anions at the cathode. This work uses a two-chamber electrodialysis unit with solution circulation and anolyte pH control, with separation of cathode and anode volumes by MK-40 (RF) and EDC1R (PRC) cation exchange membranes.  
A characteristic feature is the dependence of the pH change in the sodium tungstate solution on the duration of the process; the curve exhibits distinct regions of stepwise compositional changes, corresponding to the formation of intermediate polyoxotungstate species, which is consistent with previously reported data.  
At low current densities, the temperature of the circulating anolyte and catholyte remained within 25-28 \({}^{\circ}\)C, resulting in the formation of a gel-like white tungstic acid. In contrast, at higher current densities, the electrolytes were heated to temperatures exceeding 30-35 \({}^{\circ}\)C, leading to the formation of dispersed particles of yellow tungstic acid.  
The electrodialysis of sodium tungstate solutions for the production of tungstic acid offers a significant reduction in the consumption of inorganic acids typically required for the neutralization and purification of alkaline solutions.  
Process efficiency can be further improved by reducing the interelectrode gap and employing bipolar electrolyzers equipped with titanium electrodes featuring unilateral platinum coating. However, the scalability of the process is currently constrained by the available size of platinum-coated electrodes. In the Russian Federation, insoluble titanium-based plates are produced with platinum layer thickness ranging from 1 to 2.5 microns and plate dimensions ranging from 50 \(\times\) 100 mm to 310 \(\times\) 500 mm.  
Conceptualization, A.D. and B.B.; data curation, Y.T.; formal analysis, A.D.; investigation, A.D. and B.B.; methodology, G.M.; project administration, Y.T.; resources, G.M.; software, Y.T.; visualization, A.D. and B.B.; writing--original draft, B.B.; writing--review and editing, A.D. and B.B. All authors have read and agreed to the published version of the manuscript.  
The research was carried out within the framework of grant funding from the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan for 2023-2025 in the priority area \"Rational use of natural resources, including water resources, geology, processing, new materials and technologies, safe products and structures\" project No. AP19677216 \"Research and development of technologies, equipment for electrodialysis of solutions of tungstate, sodium sulfate with the regeneration of alkalis, acids\".  
Not applicable.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding authors.  
Conflicts of Interest: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.