Sodium is an alkali metal (Group 1, symbol Na). It is a highly reactive element characterized by its strong tendency to lose its single outermost electron to form stable chemical compounds. Due to this extreme reactivity, elemental sodium is not found freely in nature. Instead, it is locked into salts and minerals, such as common table salt (sodium chloride). Manufacturing pure elemental sodium requires an industrial process to isolate the metal from its chemically bound state.
Why Sodium Is Hard to Isolate
The fundamental challenge in obtaining elemental sodium stems from its high position on the chemical reactivity series. Sodium exhibits a powerful drive to exist as a positive ion, \(\text{Na}^{+}\), having shed its valence electron. This makes it an extremely strong reducing agent, readily giving up electrons to other substances. Consequently, it forms robust ionic bonds with elements that easily accept electrons, such as chlorine in sodium chloride (\(\text{NaCl}\)).
Traditional methods used to extract less reactive metals, such as heating a metal oxide with carbon, are ineffective for sodium. Carbon cannot supply the energy needed to break the strong ionic bonds and force the sodium ion to regain its electron. The metal is too electropositive to be liberated through a standard chemical reduction reaction. Reversing this natural chemical affinity requires substantial energy and a non-chemical approach.
This chemical challenge meant the element was not isolated until the early 19th century. Sir Humphry Davy first produced metallic sodium in 1807 by applying a powerful electrical current to molten sodium hydroxide. This demonstrated that only electrolysis could overcome the powerful chemical forces holding the sodium ion in its compound. This principle remains the basis for industrial production today.
Producing Sodium Through Electrolysis
The modern industrial process is fused-salt electrolysis, primarily conducted using a specialized apparatus called the Down’s cell. The raw material is abundant sodium chloride, which must be melted so the ions can move freely. However, the melting point of pure sodium chloride (\(801^\circ\text{C}\)) is too high for efficient industrial operation.
To mitigate the high temperature, manufacturers introduce additives, typically calcium chloride (\(\text{CaCl}_2\)) or a mixture including barium chloride (\(\text{BaCl}_2\)), to the electrolyte. These substances act as a flux, lowering the melting point to about \(580^\circ\text{C}\) to \(600^\circ\text{C}\). This temperature reduction improves economic viability by reducing energy costs. Calcium ions do not interfere because sodium is slightly easier to reduce, ensuring only sodium metal is deposited.
Within the Down’s cell, a direct electric current is passed through the molten salt mixture. The cell features an iron ring or steel cylinder that acts as the cathode, which is the negatively charged electrode. At the cathode, the positively charged sodium ions migrate and gain an electron in a reduction reaction, forming pure liquid sodium metal: \(\text{Na}^+ + \text{e}^- \rightarrow \text{Na}\).
The anode is typically a graphite block (the positively charged electrode). Negatively charged chloride ions migrate to the anode and lose an electron in an oxidation reaction, forming chlorine gas: \(2\text{Cl}^- \rightarrow \text{Cl}_2 + 2\text{e}^-\). This reaction produces chlorine gas (\(\text{Cl}_2\)) as a valuable industrial byproduct. The total cell reaction is the decomposition of sodium chloride into its constituent elements: \(2\text{NaCl} \rightarrow 2\text{Na} + \text{Cl}_2\).
A screen or diaphragm separates the liquid sodium metal from the chlorine gas. This separation is necessary because the highly reactive products would immediately reform sodium chloride if allowed to mix. The elemental sodium is less dense than the molten salt mixture, so it floats and is continuously collected through a cooled riser pipe.
Applications of Elemental Sodium
Metallic sodium is primarily used as a powerful reducing agent in numerous industrial chemical processes. Its high reactivity is indispensable for manufacturing specialized sodium compounds, such as sodium peroxide, sodium cyanide, and sodium azide, which are employed in diverse fields from bleaching to air bag inflation systems.
In metallurgy, elemental sodium is used to produce high-purity metals like titanium and zirconium. Sodium is introduced to reduce metal halide compounds, stripping away the halogen to yield the pure metal. This application leverages sodium’s strong electron-donating capability.
Another application is in sodium vapor lamps used for street lighting. When vaporized, elemental sodium emits a distinct yellow-orange light when an electric current is passed through it. Furthermore, an alloy of sodium and potassium, known as \(\text{NaK}\), is used as a liquid heat transfer fluid in certain nuclear reactor designs due to its low melting point and excellent thermal conductivity.