Acids, Alkalis and Salts

by George Henry Joseph Adlam

Fig. 5. PREPARATION OF NITRIC ACID

Nitric Acid. Chili-nitrate is always used for making nitric acid. It is the more abundant of the two naturally occurring nitrates, and therefore cheaper; moreover, weight for weight, it yields more nitric acid than the corresponding potassium compound. A mixture of sodium nitrate and sulphuric acid is heated in a large cast-iron retort (C, [Fig. 5]). The retort is entirely surrounded by flame and hot gases to prevent the condensation of the acid on the upper parts. If this precaution were not taken, the acid would dissolve the iron and the life of the retort would not be long; moreover, the product would contain ferric nitrate as an impurity. The vapour of the acid is led away by the tube D into a series of two-necked earthenware receivers called bonbonnes (E), and there condenses to a liquid. The lower figure shows how the leading tube of the retort is protected from corrosion by the clay tube a, b; and how it is connected to the first receiver by the glass tube e, which is luted on at f. The percentage strength of the acid which distils over depends upon that of the sulphuric acid used and on the purity of the sodium nitrate.

Pure nitric acid is a colourless liquid 1·559 times as heavy as water, volume for volume. It fumes strongly in air, and is a very corrosive liquid. The pure acid of commerce is obtained by distillation of a less concentrated acid. It is 68 per cent. pure. It is rendered free from dissolved oxides of nitrogen by blowing air through it. When kept exposed to light, the colour changes at first to yellow and then to brown, because light causes a certain amount of decomposition.

Red fuming nitric acid owes its colour to the great quantity of oxides of nitrogen dissolved in it. It is made by distilling sodium nitrate that has been thoroughly dried with the strongest sulphuric acid; the distillation is carried out at a high temperature, with the express purpose of decomposing some of the nitric acid to furnish the oxides of nitrogen. Sometimes a little powdered starch is also added to facilitate the formation of these oxides. This variety of nitric acid is particularly active and is used in many operations, especially in making dyes, explosives, and other organic chemicals.

Nitric acid has all the general properties of an acid, that is, it has a sour taste even in very dilute solution, it changes the colour of litmus to red, and dissolves carbonates and many metals.

When the vapour of nitric acid is passed through a red-hot tube, and also when a nitrate is strongly heated, oxygen gas is given off. Analysis shows that the oxygen combined in pure nitric acid amounts to 76 per cent. of its weight, while that in sodium and potassium nitrates is 56 and 50 per cent. respectively. Nitric acid and the nitrates are, therefore, highly oxygenated compounds; moreover, under favourable circumstances, they are rather easily broken up.

Pure nitric acid will set fire to warm, dry sawdust, and a piece of charcoal or sulphur thrown on the surface of molten nitre takes fire spontaneously and is quickly consumed, giving out a very vivid light. The explanation of this is that the supply of oxygen is abundant; it is also readily available and concentrated in a small space. We can vary the experiment. When a mixture of 75 parts by weight of finely-powdered saltpetre, with 15 of charcoal dust and 10 of ground sulphur, is ignited, it burns very vigorously, and is soon consumed. This mixture is, indeed, home-made gunpowder.

Explosives. Gunpowder was discovered in very early times by the Chinese, but for many years the secret of its composition did not get outside the Great Wall. In the fifth century A.D., it was apparently re-discovered at Constantinople, and that city was for a long time defended by the use of what is known in history as Greek Fire, an incendiary mixture very similar to, if not actually the same as, gunpowder. But again the secret of its composition was jealously guarded, and it was not until the thirteenth century that it was discovered, apparently for the third time, and introduced to Western Europe by Roger Bacon. It was used in siege cannon early in the fourteenth century and in field guns at Crécy; but it was apparently not employed for blasting until about 1627, although in 1605, Guy Fawkes and his fellow-conspirators were able to obtain it in large quantity.

From the battle of Crécy in 1346 to the beginning of the South African campaign in 1889, gunpowder was the only explosive used in warfare. “Villainous saltpetre” has therefore played a very important part in shaping the course of events in the world’s history. At the present day, gunpowder has become “old-fashioned.” In warfare, it has been superseded by “smokeless” powders of much greater power; while for mining operations, explosives with a much greater shattering effect have long since taken its place.