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Membranes 2022, 12, 602 8 of 18 by two membranes, a feed compartment that circulates the brine water, and anodic and cathodic chambers where electrochemical reactions are implemented. The various manipulations were performed under a constant voltage and a flow rate in the electrolysis cell, maintaining them at room temperature to prevent excessive oxidation of hypochlorite to chlorate according to the following reaction [15]: 3ClO− 2Cl−+ClO3− The concentration of hypochlorite ions in the product solution was measured by titra- tion with 0.1 M sodium thiosulfate (Na2S2O3) solution, using the mixture of potassium iodide, hydrochloric acid, and aqueous starch solution as an indicator. The following equa- tions were applied to calculate the concentration of hypochlorite ions and the chlorometric degree of the sample according to the standard method [26]: ClO− = C2 × V2 2 V1 Chlorometric degree (◦Ch) = ClO− × 22.4 C2: Sodium thiosulfate concentration. V1: Sample volume. V2: Sodium thiosulfate volume. 3. Results and Discussion 3.1. The Operating Mechanism of the Hypochlorite Zero-Gap Electrolysis Cell For the electrochemical production of hypochlorous acid and sodium hydroxide, the NaCl solution serves as both an electrolyte and a reagent. The detailed schematic description of the electrolysis cell and the mechanism of the electrochemical redox re- action generating HOCl and NaOH are shown in Figure 2, according to the following chemical reactions. The proposed mechanism for the generation of hypochlorite ions involves the follow- ing set of redox reactions occurring at the electrodes [23]: At the anode: At the cathode: Cl− → 12Cl2 + e− (1) Cl2 + H2O → HClO + H++Cl− (2) 2H2O→O2 +4H+ +4e− (3) H2O+e− →H2 +OH− (4) Inside the chambers, Cl− and sodium (Na+) ions migrated toward the attracting anode and cathode, respectively [23]. As shown in the equations, water (H2O) and Cl− are oxidized at the anode to produce oxygen (O2) (Equation (3)), hydronium ions (H+) (Equation (2)), and molecular chlorine (Cl2) (Equation (1)). On the surface of the anodic electrode, Cl2 gas molecules react with adjacent water molecules and produce hypochlorous acid molecules (Equation (2)). Based on preliminary tests, it can be noted that the combination of two NaOH and HClO outputs produces a NaClO mixture with a chlorometric degree lower than 0.5. This can be explained by the fact that the mixture remains basic, which is unfavorable to the formation of HClO according to the stability diagram of hypochlorites presented in Figure 3.PDF Image | Zero Gap Electrolysis Cell for Producing Bleach
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