|
|
|
|
|
In the commercial preparation of sodium, molten NaCl is electrolyzed in a specially ed cell called the Downs cell :
Schematic of a Downs cell used in the industrial production of sodium
The electrolyte medium through which current flows is molten NaCl. Calcium chloride, CaCl2, is added to lower the melting point of the cell medium from the normal melting point of NaCl, 804oC, to around 600oC. The Na(l) and Cl2(g) produced in the electrolysis are kept from coming in contact and reforming NaCl. In addition, the Na must be prevented from contact with oxygen because the metal would quickly oxidize under the high-temperature conditions of the cell reaction. The electrolysis reaction is:
2NaCl --> 2Na(l) + Cl2(g).
The cell is designed to collect sodium at the cathode and chlorine at the anode without allowing these two products to react with each other. The melting point of sodium chloride is about 800oC, so a mixture whose mole ratio is about one mole of NaCl to three moles of CaCl2 is employed to reduce the melting point to about 600oC. The cell does not produce calcium metal because the electrowinning of sodium occurs at a less negative cathode potential than does the electrowinning of calcium. (In the absence of NaCl, electrolysis of CaCl2 would indeed produce calcium metal at the cathode, and calcium metal is produced in just this way.)
Modern Downs cells operate at 25 to 40 kA and at potentials of 7 to 8 volts. The actual difference in reversible potential between sodium and chlorine under industrial conditions is only 3 to 4 volts, almost the same as the difference in their aqueous standard potentials (4.0745 V). The rest of the applied potential (nearly half) is used to overcome the iR drop due to the cell resistance and the overpotential for chlorine evolution. The Downs cell is the major production process for sodium metal and is a minor source of industrial chlorine.
