Electrolysis with the Downs cell

 
Sodium metal and chlorine gas are starting materials for the inudstrial production of a range of substances ranging from plastics to sodium fluoride found in toothpaste. Niether sodium nor chlorine, however, occur naturally in nature, but we do have a plentyfull source in the form of salt (NaCl). The sea is full of it and we can easily get to it.
 


The Downs cell is used in industry to produce sodium metal. The cell consists of a central carbon anode surrounded by a cylindrical iron cathode. An iron mesh screen is used to prevent chlorine gas, formed at the anode, from coming into contact with sodium metal formed at the cathode.

The sodium metal rises in the molten mixture and is collected in a water tight storage vessel. The high current passing through the cell provides sufficient heat to keep the mixture molten. Now sodium chloride melts at approximately 800°C this requires a significant amount of energy to be supplied. Not only is the issue of viability important but at this temperature molten sodium forms a thick fog inside the reaction vessel which is difficult to recover. To lower the melting temperature, calcium chloride is added to the sodium chloride in a 1:2(NaCl:CaCl2) part mixture. The mixture has a much lower melting temperature, around 600°C which prevents the formation of the sodium fog.
The Downs cell operates continuously with the addition of NaCl periodically. The high current of 30,000A keeps the mixture molten.

The reaction that occurs at the carbon anode is
2Cl-(l) => Cl2(g) + 2e

On the cathode, the iron mesh, electrons are forced onto the most reactive sodium cations, the reaction is as follows
Na+(l) + e=> Na(l)

 
What is the purpose of adding CaCl2 to the mixture?
What is the purpose of the iron mesh in the Downs cell?
What is the reaction at the carbon anode?
What is the reaction at the iron cathode?
The elecrtrolyte mixture of 1:2(NaCl:CaCl2) should allow
 
Balance for charge by adding electrons tot he most positive side. Balance for hydrogen by adding Hydrogen ions to the hydrogen deficient side. Balance for oxygen by adding water to the oxygen deficient side. Balance for Ag atoms Balance for oxygen by adding water to the oxygen deficient side. Balance for hydrogen by adding Hydrogen ions to the hydrogen deficient side. Balance for charge by adding electrons tot he most positive side. Home
Each oxygen atom has an oxidation number of -2