Sodium hydride, NaH

By heating sodium contained in an iron vessel inside a sealed glass tube in an atmosphere of hydrogen, Moissan obtained sodium hydride in white crystals which condensed on the cooler part of the glass tube. The temperature of reaction is about 360° C., and is a factor of importance, since for sodium hydride the interval between the temperature of formation and that of decomposition is small. Much larger quantities of the hydride can be prepared by passing a rapid current of hydrogen over the surface of sodium heated to such a temperature above 350° C. as produces a yellow glow. The hydride is carried off as a white smoke, and after electrical precipitation is filtered through glass-wool. The presence of metallic calcium facilitates its formation.

The density of the hydride is 0.92, and the vapour-tension for each interval of 10° between 300° C. and 410° C. is 15, 17, 21, 27, 38, 55, 87, 136, 201, 285, 396, and 540 mm. respectively. Sodium hydride is the most stable of the alkali-metal hydrides, and caesium hydride the least. The sodium derivative is unaffected by dry air, but decomposes in presence of traces of moisture. Although insoluble in organic solvents such as carbon disulphide, carbon tetrachloride, benzene, and turpentine, it dissolves in the alkali-metals and their amalgams.

The affinity of sodium hydride for oxygen renders it a powerful reducer. At 230° C. it ignites in oxygen, and also burns in air under the influence of heat. It is not affected by liquid oxygen, and does not combine with liquid chlorine at - 35° C. Water converts it into the hydroxide, and hydrochloric acid into the chloride, both reactions being accompanied by evolution of hydrogen. Under the influence of slight heat it combines with the oxygen of carbon dioxide, depositing carbon. With oxidizers such as potassium chlorate, it unites with explosive energy.

From the heats of solution in water of the hydride and of sodium, de Forcrand has calculated the heat of formation of the hydride to be in accordance with the equation

[Na]+(H) =[NaH] + 16.60 Cal.

Almost thirty years before the date of Moissan's work, a very unstable, crystalline, silver-white compound was described by Troost and Hautefeuille. They obtained it by sorption of hydrogen by means of sodium above 300° C., the metal sorbing 237 times its volume of the gas. That their product was a definite chemical compound is doubtful; it was probably a solid solution of hydrogen in sodium, analogous to that formed by palladium.