HYDROCHLORIC ACID, a liquid chemical reagent of great industrial significance sometimes known as muriatic acid or spirits of salt. It consists essentially of a solution of gaseous hydrogen chloride in water. This gas, which is less appropriately termed hydrochloric acid gas, is the compound actually evolved by the action of concentrated sulphuric acid on common salt.
In its physical and chemical properties, gaseous hydrogen chloride differs fundamentally from its aqueous solution which is hydrochloric acid. The former is a colourless gas liquefied only at a very low temperature (boiling point —83.4° C). When dry it has a very low electrical conductivity, and it retains this property even in the liquid and solid conditions. When thoroughly dry, hydrogen chloride and ammonia do not combine to form ammo nium chloride. (See DRYNESS [CHEMICAL].) Hydrochloric acid is a liquid which readily conducts the elec tric current and in doing so undergoes decomposition (electrolysis, q.v.). Hydrogen is evolved at the cathode (q.v.), whereas the anode (q.v.) gas is oxygen or chlorine, depending on the concen tration of hydrogen chloride in the solution. A 20% solution of hydrogen chloride in water boils at I io° C under 73o mm. pressure.
2NaC1 = Na2SO4 2HC1 (Common salt) (sulphuric (sodium (hydrochloric acid) sulphate acid) or salt cake) This process is accomplished commercially by mixing sulphuric acid with salt in large iron pans surrounded with brickwork so arranged that the pan can be heated underneath while the hydro chloric acid gas escapes through a flue. The hot gas is then cooled and passed into stone towers, filled with coke or earthenware shapes, down which water is made to trickle. This absorption, though apparently simple, is complicated in at least two respects. The general law governing the solution of any gas in a liquid is that the greater the concentration of that gas and the lower the temperature of the liquid the more gas will be taken up. Now the acid gas leaving the above apparatus is mixed with a consid erable and sometimes large quantity of air, and the introduction of the gas into the water generates a large quantity of heat. It is evident, therefore, that the exclusion of as much air as possible from the initial process and thorough cooling during the subse quent stages are essential; and much ingenuity has been expended in devising plant to accomplish this.
Another method is the direct burning of chlorine in an atmos phere of hydrogen gas : this gives a very pure product and forms a convenient outlet for chlorine. Properly speaking, hydrochloric acid is a gas. As this would be a most inconvenient form in which to transport and use it, it is therefore absorbed (as described) in water in which it is extremely soluble. The resulting liquor usually contains from 27 to 37% by weight of the acid; it has a pungent and irritating smell, and is dangerous to handle by those who are not aware of its properties. It is packed mostly in glass carboys surrounded by an iron protective frame. Pure hydrochloric acid in this form is colourless, but the ordinary commercial variety frequently is tinged more or less yellow on account of impurities accumulated during the process of absorption in water. Chemi cally it is extremely active and belongs to the class of "strong" acids. The aqueous solution will readily attack nearly all metals, forming the corresponding chlorides and disengaging hydrogen gas; gold and platinum are not affected; lead, silver and mercury are only partially dissolved owing to the formation of a protective coating of insoluble chloride. It is thus evident that the handling of the acid in the process of manufacture, and subsequently, is beset with some difficulty and that the acid can only be allowed to come into contact with stoneware, glass, ebonite and similar materials upon which it does not act.
It combines energetically, with the production of much heat, with strong bases such as soda, potash, ammonia and so on. It will readily dissolve metallic oxides, of which common rust (oxide of iron) is an example, and upon this property is based one of its most important commercial applications, namely, the cleaning of the surface of iron sheets previous to tinning or galvanizing. Huge quantities are used for this one purpose alone, and indeed this forms the principal item of the acid makers' trade. When mixed with nitric acid it forms the so-called "aqua regia," which has the property of dissolving gold, platinum and closely allied metals.
Hydrochloric acid can readily be converted into elementary chlorine by the action of atmospheric oxygen in conjunction with chloride of copper which in some way promotes the action. This formed the basis of the Deacon process for the manufacture of chlorine (q.v.), and provided an outlet for the large surplus of hydrochloric acid produced in the first stage of the Le Blanc soda process. As the Le Blanc process has now been replaced by the Solvay Ammonia-Soda method, which does not produce hy drochloric acid as a by-product, the former surplus of acid no longer exists and the Deacon process has fallen into disuse. The residue of the hydrochloric acid made, besides that used by the metal and galvanizing trades, is absorbed by the textile and arti ficial silk manufacturers, by dyestuff makers, and by chemical industry in general. (See CHLORINE.) See J. W. Mellor, A Comprehensive Treatise of Inorganic and Theoretical Chemistry (1922). (A. E. H.)