PHYSIOLOGICAL CHEMISTRY 315 



EXPERIMENTS IV. Reactions of Fatty Acids. Remove some of the 

 fatty acid with a clean glass rod, and place it on a piece of ordinary 

 paper ; a greasy stain will result. 



In order to purify the fatty acid allow the contents of the beaker 

 to cool, when the fatty acid will solidify and can be easily removed 

 with a penknife, and transferred to distilled water in a small beaker. 

 This removes a great part of the adherent sulphuric acid. But to free 

 it completely it is necessary to dissolve the fatty acid in alcohol, and 

 pour the resulting solution into excess of cold distilled water. The 

 fatty acid which separates is filtered off and washed with distilled water. 

 Use the purified fatty acids for the following reactions : 



A. Demonstrate that fatty acid is acid in reaction. For this purpose 

 place some alcohol in a test tube, add a few drops of an alcoholic 

 solution of phenolphthalein (an indicator which turns red with alkali, 



N 

 but is colourless with acids), and then a few drops of weak ^. caustic 



soda. Warm the resulting red solution on the water-bath, and drop 

 into it small pieces of fatty acid. The red colour will disappear. 

 Repeat the experiment with a piece of neutral fat ; the result is 

 negative. 



B. Add a piece of fatty acid to some half saturated solution of sodium 

 carbonate, and warm ; the fatty acid dissolves, carbon dioxide is 

 liberated, and a solution of soap is obtained. Neutral fat is insoluble 

 in cold sodium carbonate solution. 



C. Press out some fatty acid between filter paper until it is dry, and 

 apply the acrolein test as described in Experiment V. (p. 316). The 

 result is negative. 



D. To a solution of soap add : (a) a few drops of a solution of calcium 

 chloride a white precipitate of a calcium soap falls down ; (b) some 

 lead acetate solution a white precipitate of lead soap falls down (lead 

 plaster). 



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

 mixture of palmitic, stearic and oleic. To separate these from one 

 another, advantage is taken of the fact that they differ in the readiness 

 with which they form salts (soaps) with lead acetate. 



ADVANCED EXPERIMENT. To separate the Solid from the Fluid Fatty 

 Acids. Melt the fatty acids in a beaker, and add to the resulting fluid about four 

 times its bulk of 70 per cent, alcohol. Place the beaker on the boiling- water bath 

 for a few minutes, and then filter quickly through a folded filter. Allow the 

 filtrate to cool, when the solid acids will separate out as a crystalline mass, whereas 

 the oleic acid will remain in solution. The two can then be separated by filtration. 

 The further separation of stearic from palmitic acid is a laborious process, and 

 consists of the addition of an alcoholic solution of lead acetate in small quantities 



