At this point it is well to make it clear to the reader that there are two bodies commonly called soda, and two called potash. One of each pair is caustic and one mild.
By a simple chemical test it is easy to distinguish a mild from a caustic alkali. When a little dilute acid is added to the former, there is a vigorous effervescence caused by the escape of carbon dioxide, but no gas is given off when a caustic alkali is treated in the same way. The liberation of carbon dioxide on the addition of acids shows that the mild alkalis are carbonates.
Washing Soda is so well known, that very little description of its external characteristics is necessary. It is a crystalline substance, easily soluble in water. The crystals, when freshly prepared, are semi-transparent; but after exposure to air for some time, they are found to lose their transparency and to become coated with an opaque white solid which crumbles easily. This change in appearance is accompanied by a loss in weight.
Crystals of soda melt very easily on the application of heat and, on continued heating, the liquid seems to boil. When this operation is carried out in a vessel attached to a condenser, the vapour that is given off from the melted soda condenses to a clear colourless liquid which, on examination, proves to be water. When no more water collects in the receiver, the vessel contains a dry, white solid, which by any chemical test that may be applied is shown to be the same as washing soda, but it contains no water of crystallization and has a different crystalline form. This substance is anhydrous sodium carbonate, or soda ash as it is called in commerce. When soda ash is mixed with water, it combines with about twice its own weight of that liquid, forming soda crystals again.
Washing soda, then, contains nearly two-thirds of its weight of water. Some of this water is given off spontaneously when the soda is exposed to air; the water may even be said to evaporate. This accounts for the loss of weight observed and also for the formation of the white layer of partially dehydrated soda over the surface of the crystal. The property of losing water in this way is common to most crystals containing a high percentage of water of crystallization. The phenomenon is known as “efflorescence.” It may here be observed that crystals of washing soda which have become coated over in this way contain relatively more soda than those which are transparent.
Natural Soda. In Egypt, Thibet, and Utah, there are tracts of country where the soil is so impregnated with soda that the land is desert. The separation of the soda from the earth is a simple operation, for it is only necessary to agitate the soil with water and, after the insoluble matter has settled down, to evaporate the clear solution until the soda crystallizes out.
In addition to alkali deserts, there are also alkali lakes. Those in Egypt are small, nevertheless, about 30,000 tons of soda per annum are exported from Alexandria. Owens Lake in California is said to contain sufficient soda to supply the needs of North America; while in the East African Protectorate, beneath the shallow waters of Lake Magadi (discovered in 1910), there is a deposit of soda estimated at 200,000,000 tons.
The Leblanc Process. At the present time, the greater part of the world’s supply of soda is made from common salt by two processes. The older of these, which is known as the Leblanc process, was introduced in France towards the end of the eighteenth century. In those days soda was very dear, for the main supply came from the ashes of seaweeds; wherefore the French Academy of Sciences, in 1775, offered a prize for the most suitable method of converting salt into soda on a manufacturing scale. The prize was won by Nicholas Leblanc, who in 1791 started the first soda factory near Paris. These were the days of the French Revolution; the “Comité de Sûreté Général” abolished monopolies and ordered citizen Leblanc to publish the details of his process.
Fig. 12. SALT CAKE FURNACE