Sodium amalgam

Sodium amalgam, commonly denoted Na(Hg), is an alloy of mercury and sodium. The term amalgam is used for alloys, intermetallic compounds, and solutions (both solid solutions and liquid solutions) involving mercury as a major component. Sodium amalgam is often used in reactions as strong reducing agents with better handling properties compared to solid sodium. They are less dangerously reactive toward water and in fact are often used as an aqueous suspension.

No particular formula is assigned to "sodium amalgam." Na₅Hg₈ and Na₃Hg are well defined compounds. In sodium amalgams, the Hg-Hg distances are expanded to around 5 Å vs. about 3 Å for mercury itself. Usually amalgams are classified on the weight percent of sodium. Amalgams with 2% Na are solids at room temperature, whereas some more dilute amalgams remain liquid.

Metallic sodium dissolves in mercury exothermically, i.e. with the release of heat, therefore, formation of sodium amalgam is famously dangerous for generating sparks. The process causes localised boiling of the mercury and for this reason the formation is usually conducted in a fume hood and often performed using air-free techniques, such as synthesis under anhydrous liquid paraffin. Sodium amalgam may be prepared in the laboratory by dissolving sodium metal in mercury or the reverse. Sodium amalgams can be purchased from chemical supply houses.

Since its introduction in the 1880s by students of Robert Bunsen, Julius Tafel and Hans Goldschmidt, sodium amalgam has also been used in organic chemistry as a powerful reducing agent, which is safer to handle than sodium itself. An example of its use is in the Emde degradation.

A sodium amalgam is used in the design of the high pressure sodium lamp providing sodium to produce the proper color, and mercury to tailor the electrical characteristics of the lamp.

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