[44 bis] Sodamide, NH2Na, (Chapter IV., Note [14]), discovered by Gay-Lussac and Thénard, has formed the object of repeated research, but has been most fully investigated by A. W. Titherley (1894). Until recently the following was all that was known about this compound:—
By heating sodium in dry ammonia, Gay-Lussac and Thénard obtained an olive-green, easily-fusible mass, sodamide, NH2Na, hydrogen being separated. This substance with water forms sodium hydroxide and ammonia; with carbonic oxide, CO, it forms sodium cyanide, NaCN, and water, H2O; and with dry hydrogen chloride it forms sodium and ammonium chlorides. These and other reactions of sodamide show that the metal in it preserves its energetic properties in reaction, and that this compound of sodium is more stable than the corresponding chlorine amide. When heated, sodamide, NH2Na, only partially decomposes, with evolution of hydrogen, the principal part of it giving ammonia and sodium nitride, Na3N, according to the equation 3NH2Na = 2NH3 + NNa3. The latter is an almost black powdery mass, decomposed by water into ammonia and sodium hydroxide.
Titherley's researches added the following data:—
Iron or silver vessels should be used in preparing this body, because glass and porcelain are corroded at 300°–400°, at which temperature ammonia gas acts upon sodium and forms the amide with the evolution of hydrogen. The reaction proceeds slowly, but is complete if there be an excess of NH3. Pure NH2Na is colourless (its colouration is due to various impurities), semi-transparent, shows traces of crystallisation, has a conchoidal fracture, and melts at 145°. Judging from the increase in weight of the sodium and the quantity of hydrogen which is disengaged, the composition of the amide is exactly NH2Na. It partially volatilises (sublimes) in vacuo at 200°, and breaks up into 2Na + N2 + 2H2 at 500°. The same amide is formed when oxide of sodium is heated in NH3: Na2O + 2NH3 = 2NaH2N + H2O. NaHO is also formed to some extent by the resultant H2O. Potassium and lithium form similar amides. With water, alcohol, and acids, NH2Na gives NH3 and NaHO, which react further. Anhydrous CaO absorbs NH2Na when heated without decomposing it. When sodamide is heated with SiO2, NH3 is disengaged, and silicon nitride formed. It acts still more readily upon boric anhydride when heated with it: 2NH2Na + B2O3 = 2BN + 2NaHO + H2O. When slightly heated, NH2Na + NOCl = NaCl + N2 + H2O (NHNa2 and NNa3 are apparently not formed at a higher temperature). The halogen organic compounds react with the aid of heat, but with so much energy that the reaction frequently leads to the ultimate destruction of the organic groups and production of carbon.
[45] As sodium does not displace hydrogen from the hydrocarbons, it may be preserved in liquid hydrocarbons. Naphtha is generally used for this purpose, as it consists of a mixture of various liquid hydrocarbons. However, in naphtha sodium usually becomes coated with a crust composed of matter produced by the action of the sodium on certain of the substances contained in the mixture composing naphtha. In order that sodium may retain its lustre in naphtha, secondary octyl alcohol is added. (This alcohol is obtained by distilling castor oil with caustic potash.) Sodium keeps well in a mixture of pure benzene and paraffin.
[46] If sodium does not directly displace the hydrogen in hydrocarbons, still by indirect means compounds may be obtained which contain sodium and hydrocarbon groups. Some of these compounds have been produced, although not in a pure state. Thus, for instance, zinc ethyl, Zn(C2H5)2, when treated with sodium, loses zinc and forms sodium ethyl, C2H5Na, but this decomposition is not complete, and the compound formed cannot be separated by distillation from the remaining zinc ethyl. In this compound the energy of the sodium is clearly manifest, for it reacts with substances containing haloids, oxygen, &c., and directly absorbs carbonic anhydride, forming a salt of a carboxylic acid (propionic).
[46 bis] It is even doubtful whether the suboxide exists (see Note [47]).
[47] A compound, Na2Cl, which corresponds with the suboxide, is apparently formed when a galvanic current is passed through fused common salt; the sodium liberated dissolves in the common salt, and does not separate from the compound either on cooling or on treatment with mercury. It is therefore supposed to be Na2Cl; the more so as the mass obtained gives hydrogen when treated with water: Na2Cl + H2O = H + NaHO + NaCl, that is, it acts like suboxide of sodium. If Na2Cl really exists as a salt, then the corresponding base Na4O, according to the rule with other bases of the composition M4O, ought to be called a quaternary oxide. According to certain evidence, a suboxide is formed when thin sheets or fine drops of sodium slowly oxidise in moist air.
[48] According to observations easily made, sodium when fused in air oxidises but does not burn, the combustion only commencing with the formation of vapour—that is, when considerably heated. Davy and Karsten obtained the oxides of potassium, K2O, and of sodium, Na2O, by heating the metals with their hydroxides, whence NaHO + Na = Na2O + H, but N. N. Beketoff failed to obtain oxides by this means. He prepared them by directly igniting the metals in dry air, and afterwards heating with the metal in order to destroy any peroxide. The oxide produced, Na2O, when heated in an atmosphere of hydrogen, gave a mixture of sodium and its hydroxide: Na2O + H = NaHO + Na (see Chapter II., Note [9]). If both the observations mentioned are accurate, then the reaction is reversible. Sodium oxide ought to be formed during the decomposition of sodium carbonate by oxide of iron (see Note [26]), and during the decomposition of sodium nitrite. According to Karsten, its specific gravity is 2·8, according to Beketoff 2·3. The difficulty in obtaining it is owing to an excess of sodium forming the suboxide, and an excess of oxygen the peroxide. The grey colour peculiar to the suboxide and oxide perhaps shows that they contain metallic sodium. In addition to this, in the presence of water it may contain sodium hydride and NaHO.
[49] Of the oxides of sodium, that easiest to form is the peroxide, NaO or Na2O2; this is obtained when sodium is burnt in an excess of oxygen. If NaNO3 be melted, it gives Na2O2 with metallic Na. In a fused state the peroxide is reddish yellow, but it becomes almost colourless when cold. When heated with iodine vapour, it loses oxygen: Na2O2 +I2 = Na2OI2 + O. The compound Na2OI2 is akin to the compound Cu2OCl2 obtained by oxidising CuCl. This reaction is one of the few in which iodine directly displaces oxygen. The substance Na2OI2 is soluble in water, and when acidified gives free iodine and a sodium salt. Carbonic oxide is absorbed by heated sodium peroxide with formation of sodium carbonate: Na2CO3 = Na2O2 + CO, whilst carbonic anhydride liberates oxygen from it. With nitrous oxide it reacts thus: Na2O2 +2N2O = 2NaNO2 +N2; with nitric oxide it combines directly, forming sodium nitrite, NaO + NO = NaNO2. Sodium peroxide, when treated with water, does not give hydrogen peroxide, because the latter in the presence of the alkali formed (Na2O2+ 2H2O = 2NaHO + H2O2) decomposes into water and oxygen. In the presence of dilute sulphuric acid it forms H2O2 (Na2O2 + H2SO4 = Na2SO4 + H2O2). Peroxide of sodium is now prepared on a large scale (by the action of air upon Na at 300°) for bleaching wool, silk &c. (when it acts in virtue of the H2O2 formed). The oxidising properties of Na2O2 under the action of heat are seen, for instance, in the fact that when heated with I it forms sodium iodate; with PbO, Na2PbO3; with pyrites, sulphates, &c. When peroxide of sodium comes into contact with water, it evolves much heat, forming H2O2, and decomposing with the disengagement of oxygen; but, as a rule, there is no explosion. But if Na2O2 be placed in contact with organic matter, such as sawdust, cotton, &c., it gives a violent explosion when heated, ignited, or acted on by water. Peroxide of sodium forms an excellent oxidising agent for the preparation of the higher product of oxidation of Mn, Cr, W, &c., and also for oxidising the metallic sulphides. It should therefore find many applications in chemical analysis. To prepare Na2O2 on a large scale, Castner melts Na in an aluminium vessel, and at 300° passes first air deprived of a portion of its oxygen (having been already once used), and then ordinary dry air over it.