434 PEACTICAL PHYSIOLOGY 



the other, therefore, the method is to heat the substance containing 

 these two bodies on a water bath with a half-saturated solution 

 of sodium carbonate, until the mass has become nearly dry. The 

 soap, formed by the sodium carbonate uniting with the free 

 fatty acid, is separated from the unchanged neutral fat by dis- 

 solving the residue in water, and shaking the resulting solution in 

 a separating funnel with ether. The ether takes up the neutral 

 fat, the water takes up the soap, and the two fluids can easily be 

 separated from one another by allowing the mixture to stand, when 

 the ether rises to the top and the underlying water can be run off by 

 opening the tap. The soap solution can then be decomposed by pouring 

 it into a warm solution of 20 per cent, sulphuric acid, when the fatty 

 acid rises to the top as an oily layer and can be removed with a glass 

 rod after cooling. The neutral fat is obtained by evaporating away the 

 ether. 



Tests for Fatty Acid. Apply the following reactions to some fatty 

 acid prepared as above. 



(1) Place some fatty acid on a piece of ordinary paper, and hold the 

 paper near a flame. The fatty acid will melt and produce a greasy stain 

 on the paper. 



(2) Dissolve a small piece of fatty acid in ether, and add to the 

 solution two drops of a saturated solution of phenol-phthaleine 1 in 

 absolute alcohol, and then, drop by drop, a very dilute solution of 

 sodium hydrate until the solution becomes distinctly red. Repeat 

 this experiment with a solution of neutral fat in ether, and note that, 

 in this case, the red colour develops with much less alkali. 



(3) Place a small piece in a half -saturated solution of sodium 

 carbonate, warm and shake, when the fatty acid will dissolve, a solution 

 of soap being formed, which gives a lather on shaking. 



Divide the soap solution into two parts, a and b. 



To a add a few drops of a solution of calcium chloride a white 

 precipitate of calcium soap falls down. 



To b add some lead acetate solution a white precipitate of the lead 

 soap falls down (lead plaster). 



(4) Apply the acrolein reaction (see p. 182). It is negative. 



The fatty acids prepared by the above method usually consist of a 

 mixture of Palmitic, Stearic, and Okie. These differ from one another 

 mainly in two points, viz., in their melting points and in the ease with 

 which they can be precipitated by lead acetate. Those reactions are 

 taken advantage of in separating them from one another. 



1 An indicator which is red with alkali and colourless with acids, and which is 

 specially sensitive towards fatty acids. 



