FATS AND LIPOIDS 55 



scum upon the top of the fluid. Fatty acids in appearance closely 

 resemble neutral fats, being generally almost colourless. 

 Fatty acids may be distinguished from neutral fats 



1. By their reaction. This is usually tested as follows: Some 

 alcoholic phenolphthalein solution is added to alcohol, and one drop 

 of weak alkali (1 per cent, sodium carbonate) added. The addition 

 of neutral fats will not discharge the red colour thus produced, fatty 

 acids will. 



2. Fatty acids dissolve in cold sodium carbonate to form a soap; 

 neutral fat is insoluble in cold sodium carbonate. 



3. The acrolein test. The acrolein is derived from glycerine. 

 Neutral fats therefore give this test, fatty acids do not. To perform 

 the test, the substance is mixed with acid potassium sulphate (KHS0 4 ) 

 by grinding in a mortar. Upon heating the mixture acrolein is evolved, 

 if the test is positive, and may be recognized by its acrid smell and by 

 the fact that it blackens a piece of paper dipped in an ammoniacal 

 solution of silver nitrate. 



It is important to be able to distinguish different forms of fat. 

 The chief chemical methods employed are 



1. Melting-point. This varies with different animals. Mutton fat, 

 44 to 51 C., is generally higher than pig fat, 36' to 46 0. 



2. Specific gravity. Some melted fats will float in alcohol at room 

 temperature; others will not. For example, margarine, which often 

 contains light vegetable oils, will float, pure butter will sink. 



3. Acid value. The amount of free fatty acid contained in the fat. 



4. Iodine value. The percentage amount of iodine absorbed by 

 a weighed quantity of fat. This depends upon the amount of un- 

 saturated fatty acids present, such as olein. 



5. The amount of volatile fatty acids present (Reichert-Meissl value). 

 Obtained by saponifying the melted fat with alcoholic potash, and 

 treating the soap thus obtained with sulphuric acid. Volatile fatty 

 acids will distil off. These are collected in standard ^ alkali, and 

 their amount determined by titration. Pure butter has a much 

 higher value in volatile acids than has margarine. 



Soaps are the compounds of fatty acid and a base. When the 

 base is sodium, ordinary washing soap is obtained ; when it is potassitim, 

 " soft " soap is produced. Those are soluble in water, forming 

 a " soapy " colloidal solution. They arc " salted out " by full satura- 

 tion with ammonium sulphate. With solutions of calcium or mag- 

 nesium salts they yield a dense white precipitate; this accounts for 

 the difficulty, familiar to everyone, of washing with a " hard " water. 



When boiled with a mineral acid (20 per cent, sulphuric), the fatty 

 acid is displaced from combination, and collects at the top as a white 

 scum or oily fluid. 



If lead acetate be added to a soap solution, a white precipitate of 

 lead soap (lead plaster) is obtained. 



