Properties of Acids and Alkalis.—The name acids is given to a class of substances, mostly soluble in water, having an acid or sour taste, and capable of turning blue litmus solution red. All acids contain one or more atoms of hydrogen capable of being replaced by metals, and when such hydrogen atoms are completely replaced by metals, there result so-called neutral or normal salts, that is, neutral substances having no action on litmus solution. These salts can also be produced by the union of acids with equivalent quantities of certain metallic oxides or hydroxides, called bases, of which those soluble in water are termed alkalis. Alkalis have a caustic taste, and turn red litmus solution blue. In order to explain what is called the law of equivalence, I will remind you of the experiment of the previous lecture, when a piece of bright iron, being placed in a solution of copper sulphate, became coated with metallic copper, an equivalent weight of iron meanwhile suffering solution as sulphate of iron. According to the same law, a certain weight of soda would always require a certain specific equivalent weight of an acid, say hydrochloric acid, to neutralise its alkaline or basic properties, producing a salt. The specific gravities of acids and alkalis in solution are made use of in works, etc., as a means of ascertaining their strengths and commercial values. Tables have been carefully The point at which neutralisation of an acid by alkali or vice vers just takes place is ascertained very accurately by the use of certain sensitive colours. At first litmus and cochineal tinctures were used, but in testing crude alkalis containing alumina and iron, it was found that lakes were formed with these colours, and they become precipitated in the solution, and so no longer sensitive. The chemist was then obliged to resort to certain sensitive coal-tar colours, which did not, as the dyer and printer knew, form lakes with alumina and iron, such as methyl orange, fluorescein, Congo red, phenolphthalein, and so forth. For determining the alkalimetric strength of commercial sodas, a known weight of the sample is dissolved in water, and a few drops of a solution of methyl orange are added, which colour the solution yellow or orange. Into this solution is then run, from a burette or graduated tube, a standard solution of an acid, that is, a solution prepared by dissolving a known weight of an acid, say hydrochloric acid, in a known volume of water. The acid is run in gradually until the yellow colour changes to pink, at which point the volume of acid used is noted. Knowing the weight of acid contained in this volume of standard acid, and having regard to the law of equivalence mentioned above, it is an easy matter to calculate the amount of alkali equivalent to the acid used, and from this the alkali contained in the sample. Sulphuric Acid.—The first process for manufacturing sulphuric acid or vitriol was by placing some burning sulphur in a closed vessel containing some water. The water absorbed the acid formed by the burning sulphur. It was next discovered that by mixing with the sulphur some nitre, much more sulphuric acid could be produced per given quantity of brimstone. At first large glass carboys were used, but in 1746 the carboys were replaced by chambers of lead containing water at the bottom, and in these lead chambers the mixture of sulphur and nitre was burnt on iron trays. Next, although gradually, the plant was divided into two portions—a furnace for burning the sulphur, and a chamber for receiving the vapours. The system was thus developed into the one followed at the present time. The sulphur, or, in most cases, cupreous iron pyrites (a combination of iron and copper with sulphur), is burned in specially constructed kilns or furnaces, and the hot gases, consisting essentially of sulphur dioxide with the excess of air, pass through flues in which are placed cast-iron "nitre pots" containing a mixture of nitre (sodium nitrate) and vitriol. The gases thus become mixed with nitrous fumes or gaseous oxides of nitrogen, and, after cooling, are ready for mixing with steam or water spray in the lead chambers in which the vitriol is produced. These oxides of nitrogen enable the formation of sulphuric acid to take place more quickly by playing the part of oxygen-carriers. Sulphuric acid is formed by the union of oxygen with sulphur dioxide and water; the oxides of nitrogen combine with the oxygen of the air present in the chambers, then give up this oxygen to the sulphur dioxide and water or steam to form sulphuric acid, again combine with more oxygen, and so on. The exact processes or reactions are of course much more complicated, but the above represents what is practically the ultimate result. It is evident that the gases leaving the last lead chamber in which the formation of vitriol is effected, must still contain nitrous fumes, A considerable amount of sulphuric acid is now made by the so-called "contact process," in which sulphur dioxide and Nitric Acid.—This acid is usually prepared by distilling a mixture of sodium nitrate and vitriol in cast-iron retorts or pots, the nitric acid being collected in stoneware vessels connected one with another, or, as is more generally the case at the present time, in condensing apparatus consisting of stoneware pipes or coils cooled by water. The effluent gases are passed through a scrubber in order to free them from the last traces of acid before discharging them into the atmosphere. Hydrochloric Acid.—The greater part of the hydrochloric acid manufactured in Great Britain is obtained as an intermediate product in the Leblanc alkali process, which will presently be described, being produced by heating common salt with vitriol. A large quantity is, however, also produced by the so-called direct process of Hargreaves & Robinson, which is, in principle, the same method as that employed in the Leblanc process, except that the intermediate product, vitriol, is not separated. It consists essentially in passing the hot gases from pyrites kilns, as used in the manufacture of vitriol, through large cast-iron vessels containing common salt heated to a high temperature. Various physical conditions must be complied with in order to make the process a success. For example, the salt is used in the form of moulded hard porous cakes made from a damp mixture of common salt and rock salt. The cast-iron vessels must be heated uniformly, and the hot pyrites kiln gases must be passed downwards through the salt in order to ensure uniform distribution. The hydrochloric acid is condensed in stoneware pipes connected with towers packed with coke or stoneware. Alkali: Leblanc Process.—The manufacture of vitriol, as I have described it to you, is the first step in the Leblanc process. The next stage consists in the manufacture of sodium Mechanical furnaces are now used to a large extent for the salt-cake process. They consist broadly of a large revolving furnace-hearth or bed, on to which the mixture of salt and vitriol is charged, and on which it is continuously agitated, and gradually moved to the place of discharge, by rakes or the like, operated by suitable machinery. The next stage of the Leblanc process is the manufacture of "black ash," or crude sodium carbonate. This is usually done Caustic soda is manufactured from solution of carbonate of soda by causticising, that is, treatment with caustic lime or quicklime. It will have been noticed that one of the chief reagents in the Leblanc process is the sulphur used in the form of brimstone or as pyrites for making vitriol in the first stage; this sulphur goes through the entire process; from the vitriol it goes to form a constituent of the salt-cake, and afterwards of the calcium sulphide contained in the black ash. This calcium sulphide remains as an insoluble mass when the carbonate of soda is extracted from the black ash, and forms the chief constituent of the alkali waste, which until the year 1880 could be seen in large heaps around chemical works. Now, however, by means of treatment with kiln gases containing carbonic acid, the sulphur is extracted from the waste in the form of hydrogen sulphide, which is burnt to form vitriol, or is used for making pure sulphur; and so what was once waste is now a source of profit. Ammonia-Soda Process of Alkali Manufacture.—This process Alkali is also produced to some extent by electrolytic processes, depending upon the splitting up of a solution of common salt into caustic soda and chlorine by the use of an electric current. |