Current efficiency sodium hydroxide

Sodium staimate and sodium hydroxide have been used, but at less efficiency, less conductivity, less current density, and more sludge. 

The clear filtrate, which has only a pale yellow color, is now run, with stirring, into the cuprous chloride solution in a 12-I. flask, while the temperature is kept at 25-30° (Note 3). A somewhat sticky precipitate first forms, which later becomes crystalline nitrogen is evolved in a continuous stream. The addition requires about thirty minutes. The mixture is then warmed on a steam bath under an efficient reflux condenser until the evolution of nitrogen ceases. It is then distilled in a current of steam (using the apparatus described in Vol. II, pp. 80-82) until no more nitrochlorobenzene passes over 9-10 1. of distillate collects during this process (Note 4). When quite cold, the water is decanted off, and the solid shaken with 1-2 1. of 1 per cent sodium hydroxide solution at 50°. The mixture is again allowed to cool, and the light yellow alkaline solution is decanted from the solid product, which is then collected on a filter, washed with a little cold water, dried, and distilled under reduced pressure. It boils completely at n6-ii7°/i2 mm. or 124-I2S°/i8 mm., and the distillate solidifies to 430-450 g. (68-71 per cent of the theoretical amount) of a pale yellow solid which melts at 44-450. 

Sodium orthoarsenate is also obtained electrolytically by the method described under calcium arsenate (p. 198). Yields up to 100 per cent, may be obtained 9 by employing a cell with a diaphragm between iron electrodes. The anolyte should contain sodium arsenite, or sodium hydroxide and arsenious oxide (equivalent to 150 g. As2Os per litre), and the catholyte sodium hydroxide (150 g. per litre). With a current density of 3 amps, per sq. dm. the current efficiency is 100 per cent. A solid crust of sodium arsenate forms around the anode. The process may be rendered continuous by circulating the anolyte and removing the precipitated arsenate. Iron or nickel electrodes are... 

Current efficiency can also be reduced by chemical by-processes to which Faraday s law does not apply. Consequently such by-proccsses cannot be quantitatively explained by this law. On electrolyzing sodium sulphate, for instance, we obtain free sulphuric acid at the anode and sodium hydroxide at the cathode both substances get mixed, however, due to diffusion and form the original sodium sulphate whereby the current efficiency proves lower. Some reduction of yields is also posible, due to mechanical losses such as evaporation of certain products of electrolysis, dispersion of them in the electrolyte or short circuits in the electrolyzer. 

The amount of energy consumed depends upon the voltage across the bath and the current efficiency of the electrolyticaJ process. Although three products alkali hydroxide, chlorine, and hydrogen are obtained when electrolyzing a solution of sodium or potassium chloride, current efficiency is usually assessed by the resultant caustic which is the main product. 

An electrolyzer of the above type was loaded with a current of 4000 amperes corresponding to the current density of roughly 5 A/sq dm at the anode. The voltage across the bath was 3.2 to 3.8 V at a temperature 45—(50 °C Current efficiency attained 90 per cent while the content of sodium hydroxide in the caustic solution reached about 120 grams per litre. 

Earlier types of electrolyzers worked with a current of 2200 A the current density at the anode was 4.7 A/sq. dm and the voltage across the bath 3.7 to 4.2 V while the temperature of the electrolyte was 60 °C. The sodium hydroxide content in the caustic solution produced was about 120 grams per litre and the remaining sodium chloride amounted to some 180 grams per litre. Current efficiency was between 92 and 94 per cent. 

In the modem type of electrolyzers the electrodes are set closer together approximately 14—20 mm apart. The cathodes are corrugated and the anodes are shaped according to the corrugations. In this way it was possible to raise current density to some 15 A/sq. dm. In order to achieve the highest possible current efficiency a part of the electrolyte is continually removed from the electrolyzer, resaturated with sodium chloride and returned to the bath. The voltage across the electrolyzer amounts to 3.5 to 3.8 V. Electrolyzers for 12 000 to 24 000 A have been constructed for which less floor space was needed. These modem electrolyzers require 230 to 240 kw-hr. of direct, current and 153 kg of NaCl to produce 100 kg of 100 per cent Bodium hydroxide. 

The first commercial process for sodium production used the electrolysis of sodium hydroxide, but it had the disadvantage that the current efficiency was less than 50%. At present this process is the only way of producing metallic sodium. The cell reaction is... 

The 140 liters of solution obtained from an alkali-chlorine cell, operating for 10 hours with a current of 1250 amps., contained on the average 116.5 g. of sodium hydroxide per liter. Determine the current efficiency with which the cells were operating. 

The first experiments showed that an appreciable amount oS sodium cinnamate was transferred to the anodic compartment when the salt was electrolyzed. To avoid this the reduction was carried out in tlu presence of an excess of sodium hydroxide. The best conditions found were as follows Cathode sheet load (area 260 sq. cm.) anode, sheet lead. The cathodic solution was prepared by adding 30 grams of cinnamii acid to a warm solution of 8.1 grams of sodium hydroxide dissolved ir 400 cc. water. The solution was transferred to a porous cup, heated tsodium hydroxide in 100 cc. of warm water was then slowly added. The additio of the excess of alkali caused the precipitation of a part of the sodiuiv-cinnamate. The solution was stirred during the electrolysis until th( suspended matter dissolved. The temperature was kept at about 60° C A current density of 5.7 amperes per sq. dm. was maintained unti 90.7 per cent of the theoretical current had passed it was then reduced to 2.4 amperes per sq. dm. until 9.8 per cent excess had passed. Thi current efficiency therefore was 83 per cent. 

Some of these features are illustrated in Figures 14-18. A rather typical literature plot of current efficiency vs, sodium hydroxide concentration for perfluorosulfonate membranes is shown in Fig. 14. Nation 427 is a 1200-EW sulfonate membrane with fabric reinforcement. Poor hydroxide rejection occurs at catholyte concentrations above 10 wt % but a minimum is seen at higher concentrations, wtih increasing current efficiency from 28 to 40% caustic (9-14 M). The current efficiency of a 1200-EW homogeneous perfluorosulfonate film is shown in more detail over this concentration region in Fig. 15. Sodium ion transport number niol F ), which is equivalent to caustic current efficiency, is plotted vs. both brine anolyte and caustic catholyte concentration. These values were determined using radiotracer techniques, which have proven to be rapid and accurate methods for the determination of membrane performance. " " " A rather sharp maximum is seen at 14 M NaOH, and the influence of brine con-... 

The measurement and control of transport properties for ion exchange membranes is the key element in optimizing the operating conditions for modern chlor-alkali membrane cells. Ideally, a membrane should allow a large anolyte-catholyte sodium ion flux under load, while at the same time the hydroxide ion and water fluxes are kept minimal. Under these conditions, high current efficiency and low membrane resistance can be realized simultaneously in a cell producing concentrated caustic and chlorine gas. 

The properties of the membrane hydrolyzed with 10 % sodium hyroxide were measured, and a saturated sodium chloride solution was electrolyzed using this membrane. The results are given in Table I. For comparison, the sulfonyl chloride-type membrane was treated in n-butyl alcohol at 110 °C for 3 hours without introducing air. The treated membrane was subjected to hydrolysis treatment in a methanol solution containing 10 % of sodium hydroxide. Electric resistance of the membrane was 450 —cm, and the current efficiency could not be measured. 

Using the treated membrane electrolysis of sodium chloride solution was carried out under the same electrolysis conditions as 1). The treated surface of the membrane was faced to the cathode side in the electrolyzer. When 6.5 N sodium hydroxide solution was obtained as catholyte, the current efficiency was 93% and the cell voltage was 3,85v. On the other hand, the ion exchange membrane not treated by phosphorous pentachloride and triethylamine showed the current efficiency of 52% and the cell voltage of 3.68v when 6.5 N sodium hydroxide was obtained as catholyte. 

In electrolysis of sodium chloride solution under the same conditions as mentioned before, 8,0 N sodium hydroxide was obtained as catholyte at the current efficiency of 95 % and the cell voltage of 4.1 V. 

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