Sodium

The isolation of sodium from its compounds long presented a problem for chemists because of the element's reactivity. Electrolysis of a sodium chloride solution will not produce the element, for example, because any sodium produced in the reaction will immediately react with water.

The method finally developed by Sir Humphry Davy in the early nineteenth century has become the model on which modern methods for the production of sodium are based. In this method, a compound of sodium (usually sodium chloride) is first fused (melted) and then electrolyzed. In this process, liquid sodium metal collects at the cathode of the electrolytic cell and gaseous chlorine is released at the anode.

The apparatus most commonly used today for the preparation of sodium is the Downs cell, named for its inventor, J. Cloyd Downs. The Downs cell consists of a large steel tank lined with a refractory material containing an iron cathode near the bottom of the tank and a graphite anode near the top. A molten mixture of sodium chloride and calcium chloride is added to the tank. The presence of calcium chloride to the extent of about 60% lowers the melting point of the sodium chloride from 1,472°F (800°C) to about 1,076°F (580°C).

When an electrical current is passed through the mixture in the cell, sodium ions migrate to the cathode, where they pick up electrons and become sodium atoms. Chlorine ions migrate to the anode, where they lose electrons and become chlorine atoms. Since the molten sodium metal is less dense than the sodium chloride/calcium chloride mixture, it rises to the top of the cell and is drawn off. The chlorine gas escapes through a vent attached to the anode at the top of the cell. Sodium metal produced by this method is about 99.8% pure. The Downs cell is such an efficient and satisfactory method for preparing sodium that the vast majority of the metal's production is accomplished by this means.