Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/523

NITKIC ACID.] CHEMISTRY 511 Nitric Acid, HN0 3 . This acid is now usually prepared by distilling sodium nitrate or Chili saltpetre, a salt which occurs abundantly in South America, with concentrated sulphuric acid in the proportions indicated by the equation 2NaNO 3 + H 2 S0 4 = 2HNO 3 + Na 2 S0 4 . When potassic nitrate is employed, it is advantageous to use double the quantity of sulphuric acid; thus KNO 3 + H 2 S0 4 = HN0 3 + KHS0 4 , as the heat required to produce the reaction KN0 3 + KHS0 4 = HNO 3 + K 2 S0 4 is so great that a considerable amount of the acid is de composed. The acid passes over as an almost colourless liquid in the middle of the process, but is coloured at the beginning and end by decomposition products. To obtain the pure acid, it is mixed with an equal bulk of concentrated sulphuric acid, and the mixture distilled; the first portions of the distillate are collected, and a current of dry air passed for several hours through the liquid, which is gently warmed and sheltered from strong daylight. Pure nitric acid is a colourless, mobile, fuming liquid, of the specific gravity 1*53 at 15 C. ; it solidifies at about 55 C. It is an extremely unstable substance, and cannot be distilled without experiencing partial decomposition ; it is also decomposed when exposed to sunlight, becoming yellow, and oxygen being evolved. It begins to boil at 86 C., but the temperature rises gradually, and oxygen and red fumes of oxides of nitrogen are evolved ; when the boiling point reaches about 123C., an aqueous acid, having a specific gravity of about 1 42, and containing about 68 per cent, of nitric acid, distils unchanged, and weaker and stronger acids may alike be reduced to this strength by boiling. As in the case of other aqueous acids, however, the composition of nitric acid of constant boiling point varies with the pressure under which ebullition takes place. The addition of water to nitric acid causes the develop ment of heat. The following table shows the extent to which this occurs, the amount of heat developed on adding n molecules of water to a molecule of the acid being given in the second column of the table, whilst the third exhibits the amount developed on adding a quantity of water equal to that already present : n. XH0 3 , nH 2 O. HN0 3 nH 2 0, H 2 O. 0-5 2019 units. 1284 units. i-o 3303 1-5 4185 1572 " 2-0 4844 2-5 5331 1383 3 5757 4 6316 5 6719 653 " 10 7372 139 20 7511 -14 40 7497 -15 80 7482 + 29 100 7477 160 7511 + 74 ",, 320 7585 These numbers do not appear to furnish any evidence of the formation of a distinct hydrate on adding water to nitric acid. Nitric acid is a monobasic acid, and furnishes an impor tant class of salts called nitrates, such as potassium nitrate, KNO 3 , copper nitrate, Cu(N0 3 ) 2 , and bismuth nitrate, Ei(NO 3 ) 3 . In addition to these normal nitrates, a number of so-called basic nitrates are known which may be regarded as normal salts of a distinct acid, formed by the combina tion of a molecule of nitric acid with a molecule of water ; such are the basic bismuth nitrate BiX0 4 and the basic lead nitrates Pb 2 H 2 N 2 O 8 and Pb 3 N 2 8 . The normal nitrates are best prepared by dissolving the metallic oxides, hydrox ides, or carbonates in diluted nitric acid ; they are all soluble in water. The most important nitrate is that of potassium, which is employed in the manufacture of gun powder. Nitric acid is an extremely powerful oxidizing agent, and one of the most corrosive substances known ; it rapidly destroys all animal textures and most vegetable products, and even if diluted it stains the skin, wool, and all albu minous substances a bright yellow colour. Many hydro gen ized carbon compounds are converted by it into so-called nitro-derivatives, one or more atoms of hydrogen being dis placed by the monad compound radicle XO 2 ; the hydro carbon benzene, for example, when added to the concen trated acid yields nitrobenzene Most metals, excepting gold, platinum, rhodium, iridium, titanium, and perhaps a few others, are more or less readily acted on by nitric acid and converted into nitrates ; but the non-metallic elements and metalloids iodine, sulphur, selenium, tellurium, phosphorus, arsenic, antimony, amor phous boron and carbon, and tungsten are oxidized by it, and furnish iodic, sulphuric, selenious, tellurous, phos phoric, arsenic, antimonic, boric, carbonic, and tungstic acids. The action of nitric acid on metals, however, is much influenced by temperature and concentration. An acid of the specific gravity 1 25 to T35 is usually the most active. The pure concentrated acid is without action on bismuth, iron, tin, and many other metals at ordinary tem peratures. Thus, a piece of bright iron is at once attacked by an acid of about the specific gravity 1 35, but it maybe preserved in acid of the specific gravity 1 45 without losing its brilliancy; what is more remarkable, however, is that by plunging it into the more concentrated acid it is rendered passive to the action of the weaker acid, for on removing it and at once introducing it into the weaker acid, no action is observed, although on diluting the acid below 1 35 the iron is attacked. In order, however, to understand the behaviour of nitric acid with metals, it is necessary that we should first con sider the action of metals upon acids generally. There is little doubt that in all cases the metal simply displaces the hydrogen of the acid, forming a salt ; and if, under the con ditions under which the experiment is made, the acid has no tendency to enter into reaction with the hydrogen which is displaced, whilst it is in the nascent state, hydrogen is also evolved ; but if the acid can enter into reaction with the nascent hydrogen the products of this secondary reaction are obtained instead. For example, zinc readily dissolves in cold diluted sulphuric acid, forming zinc sulphate, and hydrogen is evolved since it is without action on sulphuric acid under these conditions But when zinc and concentrated sulphuric acid are heated together, zinc sulphate and sulphur dioxide are obtained, and no hydrogen is evolved. In this case, the hydrogen is displaced from the sulphuric acid under conditions which are favourable to its action upon the acid, and it deprives the acid of a portion of its oxygen, forming sulphurous acid, which, is at once resolved into water and sulphur dioxide. The behaviour of nitric acid with metals is precisely similar to that of zinc with heated concentrated sulphuric

acid, nitric acid being a substance which with the greatest