SAPONIFICATION ii 



cent alkaline solutions which readily give a lather, and can, 

 therefore, be used as soaps,* and hence the process by which 

 they are made from fats is called saponification. Although 

 alkali metal salts of other organic acids do not exhibit the 

 characteristics of soap, the term saponification is commonly 

 extended to include all cases of the decomposition of an ester 

 into the corresponding alcohol and the salt of the acid, even 

 though that salt may have none of the characteristic proper- 

 ties of a soap. 



The saponification of a fat on a small scale t in the labora- 

 tory may be effected as follows : Boil the fat under a reflux 



* The sodium and potassium salts of oleic acid and of the higher fatty 

 acids, such as palmitic and stearic acids, when dissolved in water, are, to 

 a large extent, hydrolysed into free fatty acid and caustic soda, according 

 to the equation — 



CijHjjCOONa + H2O = Ci,H35COOH + NaOH 

 Sodium stearate Stearic acid 



The stearic acid combines with some of the unhydrolysed soap to form 

 an insoluble acid salt, giving rise to an opalescent or turbid solution. It is 

 this insoluble acid salt which is responsible for the formation of a lather on 

 shaking such a solution. The detergent or cleansing action of soap is 

 dependent on the above reaction, since the caustic soda detaches the greasy 

 dirt which then becomes enveloped in a layer of soap solution from the 

 lather, and is so carried away. 



In this connection it is interesting to note the similar effect of soap on 

 the formation of emulsions. 



An emulsion may be defined as a mixture, under special conditions, of 

 two otherwise immiscible liquids. Thus, for example, if olive oil is shaken 

 up with water, the two liquids rapidly separate as soon as the shaking ceases. 

 If, however, a little soap solution or some other substance such as gum 

 acacia, tragacanth, s?.ponin (see p. 261), or white of egg be added and the 

 shaking repeated, an emulsion results owing to the oil particles being en- 

 veloped in a layer of soap or other substance which prevents their coalescing. 

 Milk is an example of a naturally occurring emulsion ; so also is latex, 

 contained in plants. 



If pure olive oil, free from oleic or other acid, is shaken up with caustic 

 soda no emulsion is produced ; on the other hand, olive oil which has been 

 kept some time and contains free oleic acid, when shaken up with caustic 

 soda does produce an emulsion, thus showing that the emulsifying agent 

 is not the free alkali but the soap produced in the second case from the 

 soda and the oleic acid. 



This may be also illustrated by Biitschli's experiment, which consists 

 in placing a drop of old olive oil containing 9 per cent of oleic acid on a 

 little o-o6 per cent aqueous solution of sodium carbonate. If examined 

 under the microscope it will be seen to consist of a fine honeycomb structure, 

 consisting of particles of oil, the whole apparently exhibiting amoeboid move- 

 ments ; these latter are due to difference in surface tension. 



+ For commercial soap manufacture, see p. 33. 



