ALKALIS.
The common alkali metals which enter into the formation of soap are sodium and potassium. The hydroxides of these metals are usually used, except in the so called carbonate saponification of free fatty acids in which case sodium and potassium carbonate are used. A water solution of the caustic alkalis is known as lye, and it is as lyes of various strengths that they are added to oils and fats to form soap. The density or weight of a lye is considerably greater than that of water, depending upon the amount of alkali dissolved, and its weight is usually determined by a hydrometer. This instrument is graduated by a standardized scale, and while all hydrometers should read alike in a liquid of known specific gravity, this is generally not the case, so that it is advisable to check a new hydrometer for accurate work against one of known accuracy. In this country the Baumé scale has been adopted, while in England a different graduation known as the Twaddle scale is used. The strength of a lye or any solution is determined by the distance the instrument sinks into the solution, and we speak of the strength of a solution as so many degrees Baumé or Twaddle which are read to the point where the meniscus of the lye comes on the graduated scale. Hydrometers are graduated differently for liquids of different weights. In the testing of lyes one which is graduated from 0° to 50° B. is usually employed.
Caustic soda is received by the consumer in iron drums weighing approximately 700 lbs. each. The various grades are designated as 60, 70, 74, 76 and 77%. These percentages refer to the percentage of sodium oxide (Na2O) in 100 parts of pure caustic soda formed by the combination of 77-1/2 parts of sodium oxide and 22-1/2 parts of water, 77-1/2% being chemically pure caustic soda. There are generally impurities present in commercial caustic soda. These consist of sodium carbonate, sodium chloride or common salt and sometimes lime. It is manufactured by treating sodium carbonate in an iron vessel with calcium hydroxide or slaked lime, or by electrolysis of common salt. The latter process has yet been unable to compete with the former in price. Formerly all the caustic soda used in soap making was imported, and it was only through the American manufacturer using a similar container to that used by foreign manufacturers that they were able to introduce their product. This prejudice has now been entirely overcome and most of the caustic soda used in this country is manufactured here.