144 PHYSIOLOGICAL AND PATHOLOGICAL CHEMISTRY. 



beaker. The sulphuric acid must be somewhat more than 

 equivalent to the caustic potash used (12 g. of concentrated 

 sulphuric acid poured into 250 cc. of water or 60 cc. of 20 per 

 cent, sulphuric acid 1 and 200 cc. of water). The fatty acids 

 separate as an oily layer. When all of the soap solution has 

 been added, let cool or cool in water, break up the layer of 

 fatty acids, pour off the aqueous fluid and preserve for 

 further examination for glycerin. Break up the fatty acids 

 into small pieces with a glass rod, place them on the filter and 

 wash with distilled water until the wash-water no longer 

 gives any reaction for sulphuric acid. 2 Then put the fatty 

 acids in an evaporating-dish, place this on the water-bath, 

 and heat till they are melted. Let cool perfectly, and free 

 the cake of fatty acids thus obtained from the adhering water 

 by means of absorbent paper. These fatty acids form a mix- 

 ture of oleic acid, C 18 H 34 2 (fluid fatty acid), palmitic acid, 

 C 16 H 32 2 , and stearic acid, C 18 H 36 2 (solid fatty acids). 



Reactions with Small Quantities of the Fatty Acids. 



1. Action on paper, same as in the case of fat. 



2. Conduct towards the alkaline solution of rosolic acid 

 or phenolphthalein. The rosolic acid solution turns yellow, 

 and the phenolphthalein solution becomes colorless. Even 

 a fairly large amount of tenth-normal sodium hydroxide 

 solution may be added without restoring the red color: the 

 fatty acids react acid. 



3. Conduct on heating with monopotassium sulphate: no 

 acrolein is formed. 



1 By 20 per cent, sulphuric acid is always meant one that contains 200 

 grams of concentrated sulphuric acid in one liter. 



2 The fatty acids thus prepared are not perfectly pure. They always 

 contain some potassium sulphate and some soap. If it is desired to have 

 the fatty acids free from these substances, they must either be repeatedly 

 melted with water, or, more simply, extracted with ether, the ether ex- 

 tract shaken with water, and the ethereal solution distilled or evaporated. 



