Electrodes rinsing

R Overall electric resistance of the membrane stack (O) Re Reynolds number (defined in Table III dimensionless) RErs Electric resistance of electrode rinsing solution (O)... 

OF BULK SOLUTIONS (J ,J IN D AND C COMPARTMENTS, AND ELECTRODE-RINSING SOLUTION (fiERS) ... 

The ion removal medium is generally a sodium chloride solution, while the ideal electrode-rinsing solution contains sodium sulfate rather than... 

FIG. 12 Schematic layout of a transport depletion ED stack used to demineralize whey c, cationic membrane ERS, electrode-rinsing solution n, neutral membrane. 

Figure 5.7 Flow scheme ofelectrodialysis reversal in acontinuous operating mode with the feed solution also used as electrode rinse.

The operation of an electrodialysis unit requires one or more pumps to circulate the diluate, the concentrate, and the electrode rinse solution through the stack. The energy required for pumping these solutions is determined by the volumes of the solutions to be pumped and the pressure drop. It can be expressed by ... 

The energy consumption due to the pressure loss in the electrode rinse solution can be neglected in most practical applications because the volume of the electrode rinse solution is very small compared to the volumes of the diluate and concentrate. 

Determine the endpoint potentiometrically with a pH meter equipped with a standard glass electrode and a calomel electrode modified as follows Discard the aqueous potassium chloride solution contained in the electrode, rinse and fill with the supernatant liquid obtained by shaking thoroughly 2 g each of potassium chloride and silver chloride (or silver oxide) with 100 mL of methanol, then add a few crystals of potassium chloride and silver chloride (or silver oxide) to the electrode. 

When the measurement is finished, the selector must be switched to zero position, and the electrodes rinsed with distilled water and stored away. The glass electrode must be kept in water or dilute hydrochloric acid, while the salt bridge of the calomel electrode should be left dipped into concentrated potassium chloride. When finishing for the day, the pH-meter is switched off, otherwise it should be left on, rather than switched on and off frequently. 

Accumulation or basic precipitates in the catholyte compartment can be prevented by acidification or softening of the electrode-rinse solution. The OH ions generated at the cathode can transfer into the multiple compartments and cause precipitates to Ruin, Therefore, isolating cumpartmsnts that are flushed with acidified or softened catholyte solutions often are used. A high solution velocity is used in the catholyte compartment to sweep out the gas as it is formed. 

The reactions that occur at the electrodes are normally incidental and occasionally detrimental to the desired separations that take place in the repeating cell pairs of an ED stack. The H+ and OH ions generated at the electrodes can cause undesired reaction with the components of the feedwater and lead to membrane fouling. Moreover, migration of some feed components into the electrode rinse streams can cause precipitation on the electrodes or the spacers and membranes close by. Therefore, it is prudent to carefully consider such possibilities and isolate the electrodes to minimize undesired reactions. 

Kedem et al demonstrated that gas evolution could be avoided by the use of powdered activated carbon in a common electrode rinse stream.19 The large surface of a carbon particle became charged when it came in contact with the anode or with another positively charged carbon particle. This charge was neutralized by sorption of anions from the rinse solution. When the suspension reached the cathode, the charge on the particle was reversed and the sorbed anions were exchanged for cations. With a 2.5% suspension of carbon in 0.02 to 0.2 normal solution, a current of 10-20 mA/cm2 could be sustained for weeks with no gas evolution. 

Figure 8.14 The use of cation-exchange membranes at both electrodes conserved the Na2 C03 in the common electrode rinse stream of a photographic developer regeneration process.

The reference electrodes employed were either a SCE or a Ag/3M NaCI (aq) electrode. The reference electrode was usually separated from the bulk electrolyte by a sintered glass-irit arrangement The films were deposited on either gold-plated stainless steel or platinum electrodes and growth was terminated when a fixed charge had been passed. The films were then peeled from the electrode, rinsed thoroughly and dried All films were dried prior to elemental analysis at 120°C in dynamic vacuum for 24 hours. 

In electrodialysis, electrolytes are transferred through solutions and membranes by an electrical driving force. Electrodialysis is used to change the concentration or composition of solutions, or both. The process usually involves multiple, thin compartments of solutions separated by membranes that allow passage of either positive ions (cations) or negative ions (anions) and biock the passage of the oppositely charged ions. But the process may be operated with only one membrane that separates two electrode-rinse solutions to purify one of the solutions (e.g., production of salt-free sodium hydroxide). 

This reaction gives rise to both gas (Hj) and high values of pH. The gas can blanket the electrode and increase the resistance of the stack. High values of pH can cause precipitation of any pH-sensidve materials (e.g., calcium caibonate) in the electrode-rinse stream, which can raise resistance and even make the stack inoperative. 

Curating costs include costs for electrical energy (for stacks, pumping, lighting, and auxiliaries), membrane replacement, chemicals (e.g., acids for electrode-rinse streams), maintenance supplies and labor, operating labor, overhead items, amortization of investment, taxes, and msurance. 

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