CHLORINE. Chlorine, Cl, atomic weight 35.4, at ordinary temperature and pressure is a gas of greenish yellow color and has an irritat ing and pungent odor. Tice specific gravity of the gas is about 2.48. Underpressure chlorine is easily liquefied. The specific gravity of the liquid at 15° C. is 1.425 (Lange), and the vapor tension at this temperature is 5.75 atmospheres. The boiling point of the iiquid is —33.6° C. (Regnault), and its freezing point Is 102° C. The critical temperature of chlorine is 146° C. and the critical pressure is 93.5 atmospheres. The coefficient of expansion of liquid chlorine rows with increasing tempera ture and at 90 C. is about equal to that of gases.
Chlorine is easily soluble in water. The solution, especially under the influence of sun light, will form hydrochloric acid under libera tion of oxygen. Chlorine forms compounds math practically all elements; in many respects its resembles oxygen. It shows great affinity for hydrogen; equal volumes of chlorine and hydrogen mixed and exposed to the direct sun light will combine under explosion, producing hydrochloric acid. If hydrocarbons are mixed with chlorine, and a flame is applied to the mixture, it will catch fire and the chlorine will combine with the hydrogen whereas the carbon will be deposited on the walls of the vessel, Chlorine attacks organic matters and destroys the lower animal and vegetable life. For human beings it is not poisonous but very obnoxious as it attacks the mucous membranes.
History.— Chlorine was discovered in 1774 by Scheele while he was treating manganese dioxide with hydrochloric acid, but up to 1809 it was considered a compound substance. At that time Gay-Lussac pointed out that it was probably an element. Investigations were made by Davy, who confirmed Gay-Lussac's opinion and gave the gas its present name. As early as 1805 chlorine was liquefied by Northmore and later on in 1823 by Faraday. Faraday heated chlorine hydrate crystals (CI, 10H2O) in one branch of a sealed and bent glass tube, while the other branch was exposed to a freezing mixture. The chlorine hydrate was decom posed and liquid chlorine was obtained at the other end of the glass tube. In the laboratory chlorine is generally made by treatment of hydrochloric acid with manganese dioxide. Sometimes it is made by decomposition of hydrochloric acid by nitric acid. Commercially the process was formerly used to a very large•extent. Weldon decomposed hydro chloric acid with manganese dioxide but added lime to the waste liquor and blew air through the mixture which enabled him to recover the manganese dioxide and use it over and over again. Later on, the °Deacon° process was largely used in which process the hydrochloric acid is decomposed by air in the presence of catalysers at a temperature of about 450° C. The disadvantage of this process is the great dilution of the produced gas so that it cannot be used when the chlorine is to be liquefied.
Commercial Method of Manufacture.
To-day by far the largest quantity of chlorine is produced electrolytically. Thisprocess fre
quently is called Alkali Process' in order to indicate the simultaneous production of chlorine and alkali. It consists of decompos ing a solution of sodium chloride or potassium chloride by passing an electric current of high amperage through the electrolytic cell. The anode is generally of carbon or iron oxide and the cathode of iron. Sometimes mercury is used for the cathode. Chlorine forms at the anode and caustic soda or caustic potash at the cathode. The products of the electrolysis caustic soda and chlorine or caustic potash and chlorine must be kept strictly apart in order not to combine again. The chlorine produced by the electrolytic method has generally over 96 per cent purity, the balance being mainly car bon dioxide and air.
Commercial Uses of By far the largest quantity of chlorine is used in the manufacture of chloride of lime or bleaching powder, which is formed when slacked lime is exposed to the action of chlorine gas. Up to the time that liquid chlorine was produced com mercially nearly all shipments of chlorine were made in the form of chloride of lime and even to-day the bulk of the chlorine which is used for bleaching of textiles, etc., is shipped in this form. Prior to the commercial production of liquid chlorine most chemical works etc., using chlorine in the manufacture of their products produced it on their premises, but since that time many users of the gas purchase the liquid which is easily transported. The first com mercial plant to liquify chlorine was built in Germany in 1888 by Knietsch of the Badische Company. Knietsch used a compressor in which a body of sulphuric acid was placed between the piston and the chlorine so that the chlorine was compressed without getting in touch with the piston or the stuffing box. The compressed gas coming from the compressor entered the condenser where it was cooled and thus liquefied. In order to be suitable for ship ment in steel containers the liquid chlorine had to be absolutely free from water so that it would not attack the steel. The liquid is filled into steel cylinders or tank cars. Knietsch heated the sulphuric acid in order to reduce the absorption of chlorine 'by the acid. This system is still used by most manufacturers, al though sometimes it is used in modified form. The Linde Ice Machine Company, Germany, did not use any compressor to liquify the chlorine, but simply exposed the gas to a temperature of —45 to C. at atmospheric pressure. This system is also in use commercially but not to a large extent. A third system was de veloped in the United States of America. In this system sulphuric acid is pumped to a high tower from where it is allowed to drop down in vertical tubes entraining the chlorine ac cording to the injector principle. The chlorine is then again carried into a condenser where it is cooled and liquefied. This system was in stalled at the large government plant at Edge water Arsenal, during the World War of 1914-18.