Chapter : 5. Compounds of Common Use
Manufacture of Sodium Hydroxide
11. Manufacture of Sodium Hydroxide.
Sodium hydroxide is the most important alkali and is made commercially by electrolysis of saturated brine solution (sodium chloride). Three kinds of cells are used :
Castner-Kellner Cell. In this cell, mercury flows along the bottom of the cell and is made cathode in outer compartments as shown in figure. The anode consists of number of graphite blocks.
Thee electrolysis of brine solution takes place. Since hydrogen has a over voltage (i.e.more energy is required to discharge H+ ion) at mercury cathode, sodium is preferentially discharged at cathode forming amalgam with mercury. Sodium amalgam flows out and is reacted with H2O to give NaOH.
2NaHg + 2H2O → 2NaOH + 2Hg + H2
The mercury is recirculated in the cell. H2H2 and Cl2 are two important by-products.
Sodium hydroxide is the most important alkali and is made commercially by electrolysis of saturated brine solution (sodium chloride). Three kinds of cells are used :
Castner-Kellner Cell. In this cell, mercury flows along the bottom of the cell and is made cathode in outer compartments as shown in figure. The anode consists of number of graphite blocks.
Thee electrolysis of brine solution takes place. Since hydrogen has a over voltage (i.e.more energy is required to discharge H+ ion) at mercury cathode, sodium is preferentially discharged at cathode forming amalgam with mercury. Sodium amalgam flows out and is reacted with H2O to give NaOH.
2NaHg + 2H2O → 2NaOH + 2Hg + H2
The mercury is recirculated in the cell. H2H2 and Cl2 are two important by-products.
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