## 1 Introduction
Tungstic acid (H\({}_{2}\)WO\({}_{4}\)) can be directly precipitated from sodium tungstate solutions via acidification with mineral acids such as hydrochloric or nitric acid, according to the following reactions:  
\[\text{Na}_{2}\text{WO}_{4}+2\text{HCl}\rightarrow\text{H}_{2}\text{WO}_{4}+ 2\text{NaCl}\]  
\[\text{Na}_{2}\text{WO}_{4}+2\text{HNO}_{3}\rightarrow\text{H}_{2}\text{WO}_{4}+ 2\text{NaNO}_{3}\]  
Despite the apparent simplicity of these reactions, this method is seldom employed industrially due to significant challenges in removing residual sodium ions from the precipitate. The presence of sodium in the final tungsten trioxide product must be strictly limited, complicating purification. More commonly, calcium tungstate (CaWO4) is initially precipitated by introducing a calcium chloride (CaCl2) solution at elevated temperatures (80-90 \({}^{\circ}\)C), resulting in the formation of a fine white crystalline precipitate:  
\[\mathrm{Na_{2}WO_{4}+CaCl_{2}\to CaWO_{4}+2 NaCl}\]  
The CaWO4 precipitate, typically obtained as a pulp or paste, is subsequently decomposed by hydrochloric acid at approximately 90 \({}^{\circ}\)C:  
\[\mathrm{CaWO_{4}+2 HCl\to H_{2}WO_{4}+CaCl_{2}}\]  
According to stoichiometric calculations, the production of 249.85 kg of tungstic acid requires approximately 73 kg of hydrochloric acid. Furthermore, the actual acid consumption often exceeds this amount due to the necessity of neutralizing residual sodium carbonate (Na2 CO3) or sodium hydroxide (NaOH), which are present in solutions derived from roasting and autoclave leaching technologies, respectively. The use of inorganic acids in tungstic acid production also leads to the generation of significant volumes of saline effluents (e.g., NaCl or Na2 SO4), contributing to environmental pollution and necessitating costly wastewater treatment processes. The aim of the present study was to investigate the feasibility of producing tungstic acid from sodium tungstate solutions via a membrane-based electrodialysis process. This approach employs insoluble platinum anodes and cation exchange membranes, offering a potential alternative that eliminates the need for mineral acids and avoids the formation of saline waste streams. The specific objectives included the selection of suitable ion-exchange membranes and electrode materials, as well as the evaluation of key electrodialysis parameters and their effects on process efficiency and product quality. Electrodialysis is currently widely applied in technologies for the decomposition and concentration of aluminate solutions [23, 24], the processing of binary solutions containing sodium and zinc [25, 26], the treatment of industrial wastewater and seawater desalination [27, 28], the removal of nickel ions from solutions [29], and the recovery of acids and alkalis from sodium sulfate solutions [30]. One study [31] focused on exploring the electrodialysis behavior of sodium tungstate solutions, describing an experimental method for removing impurities from these solutions. The author proposes conducting electrodialysis of Na2 WO4 in a two-compartment electrodialysiser using a cation-exchange membrane to separate the anode and cathode chambers. This investigation was carried out with small quantities of sodium tungstate to determine the fundamental feasibility of producing tungstic acid via electrodialysis.