146 DISINFECTION BY CHLORINE. atmosphere, in particular localities, is occasionally contaminated with poisonous substances, some of which are known only by their injurious effects upon the health, their quantity being so small that they do not appear in the results of the analysis of such air. Since, however, these substances appear to be acted upon by the same agents which are usually found to decompose organic compounds, they are commonly believed to be bodies of this class, and chlorine has been very commonly employed to combat these insidious enemies to health, since Guyton de Morveau, in the latter part of the last century, made use of it to destroy the odour arising from the bodies interred in the vaults beneath the cathedral of Dijon. Among the offensive and unhealthy products of putrefaction of animal and vegetable matter, sulphuretted hydrogen, ammonia, and bodies similarly constituted, are found. That chlorine breaks up these hydrogen compounds is well known, and hence its great value for removing the unwholesome properties of the air in badly drained houses, &c. Chloride of lime is one of the most convenient forms in which to apply chlorine for the purposes of fumigating and disinfecting. If a cloth saturated with the solution be suspended in the air, the carbonic acid causes a slow evolution of hypochlorous acid, which is even a more powerful disinfectant than chlorine itself. In extreme cases, where a rapid evolution of chlorine is required, the bleaching powder is placed in a plate, and diluted sulphuric acid is poured over it, or the powder may be mixed with half its weight of powdered alum in a plate, when a pretty rapid and regular escape of chlorine will ensue. 105. The discovery of chlorine and the discussions which ensued with respect to its real nature, contributed very largely to the advancement of chemical science. About the year 1770, the Swedish chemist Scheele (who afterwards discovered oxygen), first obtained chlorine by heating manganese ore with muriatic acid. The construction which Scheele put upon the result of this experiment was one which was consistent with the chemistry of that date. He supposed the muriatic acid to have been deprived of phlogiston, and hence chlorine was termed by him dephlogisticated muriatic acid. This phlogiston had long been a subject of contention among philosophers, having been originally assumed to exist in combination with all combustible bodies, and to be separated from them during their combustion. Towards the decline of the phlogistic theory, attempts were made to prove the identity of this imaginary substance with hydrogen, which shows how very nearly Scheele's reasoning approached to the truth, even with the very imperfect light which he then possessed. Berthollet's movement was retrograde when, ten years afterwards, he styled chlorine oxygenised muriatic or oxymuriatic acid, but the experiments of Gay-Lussac and Thénard, and more particularly those of Davy in 1811, proved decisively that hydrochloric acid was composed of chlorine and hydrogen, and that the effect of the black oxide of manganese in Scheele's experiment was to remove the hydrogen in the form of water, thus setting the chlorine at liberty. PREPARATION OF HYDROCHLORIC ACID. 147 HYDROCHLORIC ACID. 106. This acid is found in nature among the gases emanating from active volcanoes, and occasionally in the spring and river waters of volcanic districts. For use it is always prepared artificially by the action of sulphuric acid upon common salt 300 grains of common salt (previously dried in an oven) are introduced into a dry Florence flask (fig. 160), to which has been fitted, by means of a perforated cork, a tube bent twice at right angles to allow the gas to be collected by downward displacement. Six fluid drachms of strong sulphuric acid are poured upon the salt, and the cork having been inserted, the flask is very gently heated in order to promote the disengagement of the hydrochloric acid gas, which is collected in a perfectly dry bottle, the mouth of which, when full, may be covered with a glass plate smeared with a little grease. be closed with a perforated card. Fig. 160.-Preparation of hydrochloric acid gas. While being filled, the bottle may Common salt in powder sometimes froths to a very inconvenient extent with sulphuric acid; it is therefore often preferable to employ fragments of fused salt, prepared by fusing the common salt in a clay crucible, and pouring on to a clean dry stone. A more regular supply of hydrochloric acid gas is obtained from 1 oz. of salammoniac in lumps, and 11 oz. (measured) of sulphuric acid. The bottle will be known to be filled with gas by the abundant escape of the dense fumes which hydrochloric acid gas, itself transparent, produces by condensing the moisture of the air; for since the gas is much heavier than air (sp. gr. 1.247), it will not escape in any quantity from the bottle until the latter is full. The odour of the gas is very suffocating, but not nearly so irritating as that of chlorine. The powerful attraction for water is one of the most important properties of hydrochloric acid gas. If a jar of hydrochloric acid gas be closed with a glass plate and inverted under water, it will be found, on removing the plate, that the gas is absorbed with great rapidity, the water being forced up into the bottle by the pressure of the external air in proportion as the gas is absorbed. A Florence flask is more convenient than a gas bottle for this experiment. It must be perfectly dry, and thoroughly well filled with the gas, which may be allowed to escape abundantly from the mouth. The tube delivering the hydrochloric acid gas must be slowly withdrawn, so that the vacancy may be filled by gas and not by air. The flask is then closed with the thumb, and opened under Fig. 161. 148 HYDROCHLORIC OR MURIATIC ACID. water, which will enter it with great violence. The experiment may also be made as in the case of ammonia (fig. 161, see page 115). The liquid hydrochloric, or muriatic acid of commerce, is a solution of the gas in water, and may be recognised by the grey fumes, with the peculiar odour of the acid, which it evolves when exposed to the air. One pint of water at a temperature of 40° F. is capable of absorbing 480 pints of hydrochloric acid gas, forming 1 pint of the solution, having the specific gravity 1-21. The strength of the acid purchased in commerce is usually inferred from the specific gravity, by reference to tables indicating the weight of hydrochloric acid contained in solutions of different specific gravities. The strongest hydrochloric acid (sp. gr. 1·21) contains 43 per cent. by weight of the gas. The common acid has usually a bright yellow colour, due to the accidental presence of a little perchloride of iron (Fe,Cl), and not unfrequently smells of chlorine. This acid is produced in enormous quantities in the alkali works, where common salt is decomposed by sulphuric acid in order to convert it into sulphate of soda, as a preliminary step to the production of carbonate of soda. The alkali manufacturer is compelled to condense the gas, for it is found to wither up the vegetation in the neighbourhood. For this purpose the hydrochloric acid gas is drawn up from the furnace through vertical cylinders filled with coke, over which streams of water are made to trickle. The water absorbs the acid, and is drawn off from below. In preparing a pure solution of the acid for chemical use on a small scale, the gas prepared as above may be passed into a small bottle containing a very little water to wash the gas, or remove any sulphate of soda which may splash over, and then into a bottle about two-thirds filled with distilled water, the tube delivering the gas passing only about inch below the surface, so that the heavy solution of hydrochloric acid may fall to the bottom, and fresh water may be presented to the gas (fig. 162). For ordinary use, an acid of suitable strength is obtained by passing the gas from 6 ounces of common salt and 10 ounces of sulphuric acid into 7 (mea sured) ounces of water until its bulk has increased to 8 ounces. The bottle containing the water should be surrounded with cold water, since the absorption of hydrochloric acid by water is attended with evolution of heat. When the concentrated solution of hydrochloric acid is heated in a retort, it evolves abundance of hydrochloric acid gas, rendering it probable that it is not a true chemical compound of water with the acid. The evolution of gas ceases when the remaining liquid contains 20 per cent. of acid (and has a sp. gr. of 1·10). If a weaker acid than this be heated, it loses water until it has attained this strength, when it distils unchanged.* Fig. 162.-Preparation of solution of hydrochloric acid. The concentrated solution forms a very convenient source from which to procure the gas. It may be heated in a flask, and the gas dried by passing through a bottle filled with fragments of pumice-stone wetted with concentrated sulphuric acid, being collected over the mercurial trough (fig. 163). *The proportion of acid thus retained by the water varies directly with the atmospheric pressure to which it is exposed during the distillation. ACTION OF HYDROCHLORIC ACID ON METALS. 149 The avidity with which water absorbs hydrochloric acid is the more remarkable, because this gas can be liquefied only under a very high pressure, amounting at the ordinary temperature to about 40 atmospheres. The liquefied hydrochloric acid has comparatively little action even upon those metals which decompose its aqueous solution with great violence; quick-lime is unaffected by it, and solid litmus dissolves in it with a faint purple colour, instead of the bright red imparted by the aqueous hydrochloric acid. (These facts answer the objection that anhydrous sulphuric acid (SO) cannot be considered an acid, because it has none of the powerful acid characters of oil of vitriol, since it cannot be doubted that hydrochloric acid is, in a chemical sense, an acid in its anhydrous state, though it manifests its acid properties only when water is present.) The injurious action of hydrochloric acid gas upon growing plants is probably connected with its attraction for water. If a spray of fresh leaves is placed in a bottle of hydrochloric acid, it becomes at once brown and shrivelled. 107. Action of hydrochloric acid upon metals.-Those metals which have the strongest attraction for oxygen will also generally have the strongest attraction for chlorine, so that in respect to their capability of decomposing hydrochloric acid, they may be ranked in pretty nearly the same order as in their action upon water (p. 23). Since, however, the attraction of chlorine for the metals is generally superior to that of oxygen, the metals are more easily acted upon by hydrochloric acid than by water, the metal taking the place of the hydrogen, and a chloride of the metal being formed. Even silver, which does not decompose water at any temperature, is dissolved, though very slowly, by boiling concentrated hydrochloric acid, the chloride of silver formed being soluble in the strong acid, though it may be precipitated by adding water. Gold and platinum, however, are not attacked by hydrochloric acid, but if a little free chlorine be present, it converts them into chlorides. Iron and zinc decompose the acid very rapidly in the cold, forming chlorides of iron and zinc, and liberating hydrogen: Fe + HCl = FeCl + H. When potassium or sodium is exposed to hydrochloric acid gas, it immediately becomes coated with a white crust of chloride, which partly protects the metal from the action of the gas, but when these metals are heated to fusion in hydrochloric acid gas, they burn vividly 150 EQUIVALENT OF CHLORINE. The composition of hydrochloric acid may be exhibited by confining a measured volume of the gas over mercury (see fig. 73, page 73), and passing up a freshly cut pellet of sodium. On gently agitating the tube, the gas diminishes in volume, and after a time will have contracted to one-half, and will be found to have all the properties of hydrogen. This result confirms that obtained by synthesis, as described above, that one volume of hydrochloric acid contains half a volume of hydrogen and half a volume of chlorine. 108. Action of hydrochloric acid upon metallic oxides.—As a general rule it may be stated, that when hydrochloric acid acts upon the oxide of a metal, the results are water and a chloride of the metal having a composition which corresponds to that of the oxide. Thus, oxide of silver acted on by hydrochloric acid gives water and chloride of silver; AgO + HCl = HO+ AgCl. = Suboxide of copper (cuprous oxide) yields water and subchloride of copper (cuprous chloride); Cu,O + HCl = HO + Cu,Cl. Sesquioxide of iron gives water and sesquichloride of iron With binoxide of tin, water and bichloride of tin are obtained— Teroxide of antimony is converted into water and terchloride of antimony; SbO, + 3HC1 = 3HO+ SbC13. In cases where the corresponding chloride does not exist, or is not stable under the conditions of the experiment, a chloride is formed containing less chlorine than is equivalent to the oxygen in the oxide, and the balance is evolved in the free state. Thus, when sesquioxide and binoxide of manganese are heated with hydrochloric acid Mn,O + 3HCI = зно + 2 MnCl + Cl MnO, + 2HC1 = 2HO + MnCl + Cl since the sesquichloride and bichloride of manganese are decomposed by heat into the chloride (MnCl) and free chlorine. Chromic acid, a chloride corresponding to which is not known to exist, when heated with hydrochloric acid, yields sesquichloride of chromium and chlorine Every metallic protoxide (containing one equivalent of oxygen with one equivalent of a metal) has a corresponding chloride of a stable character, but the higher oxides less frequently form corresponding chlorides with any stability. 109. Equivalent weights of hydrochloric acid and of chlorine.-It is ascertained by experiment that 36.5 grains of hydrochloric acid are required to neutralise one equivalent (47 grains) of potash. The number 36.5, therefore, represents the equivalent weight of hydrochloric acid. When water is decomposed by chlorine (p. 142) 35.5 grains of chlorine are required to displace 8 grains (one equivalent) of oxygen, so that 35.5 is the equivalent weight of chlorine. By measuring 35-5 grains of chlorine, it is found to occupy twice the volume of 8 grains of oxygen, so that if 1 equivalent of oxygen be represented to occupy one volume, 1 equivalent of chlorine will occupy two volumes, like the equivalent of hydrogen. |