Physical Properties of Sodium

Sodium is a silver-white metal, rapidly tarnished by atmospheric oxidation, the process being attended by a greenish phosphorescence. In thin layers by transmitted light the metal has a brownish-yellow colour. The silver-white colour can be retained by distillation in a current of hydrogen, and preservation of the distillate in a sealed tube. Another method involves cleansing the surface with alcohol, and immersion of the bright metal in a saturated solution of naphthalene in light petroleum. Other protectors from oxidation are vaseline-oil, anhydrous ether, and petroleum freed from oxygen by distillation in hydrogen or carbon dioxide.

Sodium usually crystallizes in cubes, but it has also been obtained in quadratic octahedra. At - 20° C. it is hard, at 0° C. very ductile, at ordinary temperatures it can be moulded by the fingers, and at 50° C. it is soft. For its melting-point Rengade gives 97.90° C. Other values are: Regnault, Hagen, and Vicentini and Omodei, 97.6° C.; Kurnakoff and Pushin, Quercigh, and Guertler and Pirani, 97.5° C.; Masing and Tammann, 97° C.; and Holt and Sims, 92° C. The effect of pressure on the melting-point has been investigated by Tammann.

Several values are recorded for the boiling-point of sodium. Carnelley and Williams give values between 861° and 954° C.; Ruff and Johannsen give 877.5° C.; Hansen gives 742° C. at 760 mm. pressure and 418° C. at 0 mm. pressure; Perman gives 742° C. at 769.6 mm. pressure; and Krafft and Bergfeld give 140° C. in the vacuum of the cathode-light. In thin layers the vapour is colourless, but in thicker layers purple; at red heat it is yellow. The tension of the vapour varies between 1.2 mm. at 380° C. and 80 mm. at 570° C.; von Rechenberg found 1.94 mm. at 418° C. Its fluorescence has been studied by Dunoyer.

Attempts to determine the vapour density have been made by several experimenters, but the results have been unsatisfactory owing to the interaction of the vapour and the platinum, silver, iron, porcelain, or glass of the containing vessel.

The molecular weight was determined by Kraus to be 23 by dissolving the metal in liquid ammonia; other investigators give values for the molecular formula varying between Na_0.5 and Na₆.

The value of the specific heat at 0° C. is given by Griffiths as 0.2829. The specific heat from -185° to 20° C. is 0.2345, according to Nordmeyer and Bernoulli; from -34° to 7° C. Regnault gives 0.2943; from -79.5° to 17° C. Schiiz gives 0.2830; and from 0° to 157° C. Bernini gives values varying between 0.2970 and 0.333. For the specific heat of the solid at the melting-point, Rengade gives 0.3266. According to Iitaka, the specific heat of the solid is 0.330, and of the liquid 0.347, the corresponding values for the atomic heat being 7.59 and 7.98.

The latent heat of fusion per gram is, according to Joannis, 31.7 cal.; but Bernini gives 17.75 cal., and Rengade 27.23 cal.

Sodium is a good conductor of heat, and as a conductor of electricity it stands next to silver, copper, and gold. Its electric conductivity has been studied by several investigators.

The mean value of the density is 0.978. Gay-Lussac and Thenard give the density at 15° C. as 0.972 (water at 15° C.=1); Hackspill gives 0.9723 at 0° C.; Schroder 0.985 compared with water at 3.9° C.; Braumhauer 0.9735 at 13.5° C. and 0.9743 at 10° C. compared with water at the same temperatures; Vicentini and Omodei give 0.9519 for the solid at the melting-point and 0.9287 for the liquid; Ramsay 0.7414 for the liquid at the boiling-point; Dewar 0.9724 at 0° C. and 1.0066 at the temperature of liquid air (water at 0° C. =1); and Richards and Brink 0.9712 at 20° C.

The value last mentioned for the density gives 23.70 as the atomic volume. Vanstone found for the specific volume of the solid at 17° C. the value 1.0342; for the liquid at 110° C. 1.0794, and at 237° C. 1.1247.

According to Cohen and Wolff, sodium exists in two allotropic forms, designated α and β. The β-modification is slightly less dense than the α-form, and is produced by cooling the molten metal rapidly. At 97.22° C. it is transformed into the α-variety, the change having a measurable velocity, and being attended by the evolution of heat.