MANUFACTURE OF SOAP 5 



it decomposes with the formation of oenanthol, a substance 

 having a very unpleasant odour. Castor oil is largely used in 

 the dye industry ; for this purpose it is converted into the so- 

 called turkey red oil, used for alizarin dyeing, by treatment 

 with sulphuric acid and neutralization of the resulting sulphonic 

 acid with soda. 



For the manufacture of soap the following fats and 

 oils are used : tallow fat, palm oil, palm-kernel oil, coco- 

 nut oil and olive oil ; the fats are boiled with caustic soda 

 until saponification is complete, whereupon the mixture is sat- 

 urated with common salt. The soap, being insoluble in strong 

 salt solution, rises to the surface leaving the glycerol and salt 

 in the aqueous layer below ; the latter is then run off and the 

 scum, which is allowed to harden in moulds, is known as hard 

 soap. Soft soaps are prepared by boiling the cheaper oils, 

 such as hemp-seed oil, cotton-seed oil or linseed oil with caustic 

 potash ; when saponification is complete the mixture is allowed 

 to set to a semi-solid without the addition of sodium chloride ; 

 the resulting mixture contains all the glycerol together with 

 the excess of alkali and a quantity of water. 



The whole of the glycerol of commerce is obtained from 

 fats ; it is used largely for the manufacture of dynamite. 



CONSTITUTION OF FATS. 



Fats belong to that class of organic compounds which are 

 known as esters, an ester occupying in organic chemistry ex- 

 actly the same position as salts do in inorganic chemistry. 



When an inorganic base such as caustic soda or potash 

 reacts with an acid, either organic or inorganic, one or more 

 of the replaceable hydrogen atoms in the acid is replaced by 

 the metal and the resulting product is known as a salt ; 

 thus : 



NaOH + H S0 4 = NaHSO 4 + H 2 O 

 or KOH + CH 3 COOH = CH 3 COOK + H 2 O 

 Acetic acid Potassium 

 acetate 



* 

 If now the inorganic base be replaced by its organic 



analogue, an alcohol, a similar reaction ensues, but the result- 

 ing compound is called an ethereal salt or ester. 



