REACTIONS OF FATS 21 



sulphide, but are only sparingly soluble in alcohol and insoluble 

 in water. 



(2) Being esters of glycerol they all contain this substance 

 as may be proved by heating any fat with potassium hydrogen 

 sulphate, whereby the glycerol is broken down into acrolein, 

 which may be detected by its unpleasant odour. 



CHsOHCHOHCH.OH = CHo : CHCHO + 2H2O 



In order to show that the hydrolysis of fats gives rise to 

 glycerol some fat should be saponified as described above and 

 then acidified with hydrochloric acid ; after filtering off the 

 fatty acids, the filtrate is evaporated to small bulk over a water 

 bath and the residue is extracted with alcohol, which dissolves 

 out the glycerol leaving behind the salts. The presence of 

 glycerol in the alcoholic extract may be proved by evaporating 

 to dryness and applying the following test : two drops of the 

 residue are carefully heated to about 120° with two drops of 

 molten phenol and an equal quantity of concentrated sulphuric 

 acid ; the resulting resinous mass on cooling gives a brown 

 solid which dissolves in ammonia forming a carmine-coloured 

 solution. 



(3) All fats can be saponified by boiling with alcoholic 

 potash. For this purpose 2 grams of fat may be boiled for fif- 

 teen minutes with 25 c.c. of 3 per cent alcoholic potash. The 

 resulting mixture of potassium soap and glycerol is soluble in 

 water ; on acidifying the solution the free fatty acids are pre- 

 cipitated. 



(4) Fats leave a translucent mark on paper. Similar 

 stains may be left by substances other than fats, but in most 

 cases the stains disappear fairly rapidly as the substance 

 evaporates. 



SPECIAL TESTS FOR PARTICULAR CLASSES OF FATS. 



Elatdin Test. — This test, which is distinctive of the oleic 

 series, depends on the fact that nitrous acid converts liquid 

 olein into solid elaidin, while it has no corresponding action on 

 glycerides of linolic, linolenic, or isolinolenic acids. The test 

 may be performed as follows : — 



Ten grams of oil are shaken in a test tube with 5 grams 

 of nitric acid (sp. gr. i '38-1 "41) and i gram of mercury for 



