Sodium amalgam

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Sodium amalgam, commonly denoted Na(Hg), is an amalgam, or alloy of mercury, with sodium metal. When metallic sodium is dissolved in mercury, it reacts exothermically to produce the intermetallic compound NaHg₂, with enough heat to cause localised boiling of the mercury. This process is normally performed under dry nitrogen gas. This compound is a spongey grey mass, but is also soluble in mercury, allowing a more dilute, liquid amalgam to be produced.

Preparation

Sodium amalgam may be prepared in the laboratory simply by dissolving sodium metal in mercury, under air-free conditions.^[1] Alternatively, it may be purchased from chemical supply houses.

Uses

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.

Mercury cell electrolysis

Sodium amalgam is a by-product of chlorine manufactured by mercury cell electrolysis. In this cell, brine (concentrated sodium chloride solution) is electrolysed between a liquid mercury cathode and a titanium or graphite anode. Chlorine is formed at the anode, while sodium formed at the cathode dissolves into the mercury, making sodium amalgam. Normally this sodium amalgam is drawn off and reacted with water in a "decomposer cell" to produce hydrogen gas, concentrated sodium hydroxide solution, and mercury to be recycled through the process. In principle, all the mercury should be completely recycled, but inevitably a small proportion goes missing. Because of concerns about this mercury escaping into the environment, the mercury cell process is generally being replaced by plants which use a less toxic cathode.

References

1. ^ Tony C. T. Chang, Myron Rosenblum, and Nancy Simms (1993), "Vinylation of Enolates with a Vinyl Cation Equivalent", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv8p0479 ; Coll. Vol. 8: 479 

Further reading

• Babcock, S. H. (1939). "Sodium Amalgam". Inorg. Synth. 1: 10. doi:10.1002/9780470132326.ch4. 

External links

• Oxford MSDS