160 FATS, OILS AND LIPINES. [cH. VI. 



171. Heat strongly a drop or two of pure glycerol with solid 

 potassium hydrogen sulphate in a dry test-tube. The pungent 

 odour of acrolein (acrylic aldehyde) is noticed. 



CH 2 OH.CHOH.CH 2 OH = CH 2 : CH.CHO + 2H 2 O 



Glycerol. Acrolein. 



172. Treat about 5 cc. of a 0-5 per cent, solution of borax 

 with sufficient of a I per cent, alcoholic solution of phenolphthalein 

 to produce a well-marked red colour. Add a 20 per cent, aqueous 

 solution of glycerol drop by drop, until the red colour is just dis- 

 charged. Boil the solution : the colour returns, provided that an 

 excess of glycerine has not been added (Dunstan's test for glycerol). 



NOTES. i. Any ammonium salt will discharge the colour, but in this 

 case it does not return on heating. 



2. Any polyhydric alcohol is likely to give the same reaction. The sugars 

 are all polyhydric alcohols, but are distinguished from glycerol by their reducing 

 properties, etc., and by the fact that they are not volatile when distilled by 

 steam. 



3. The probable explanation of the reaction is as follows. Sodium borate 

 is partially hydrolysed in aqueous solution to boric acid and sodium hydroxide. 

 Boric acid being a weak acid is only feebly ionised and therefore the solution 

 reacts alkaline. On adding glycerol, glyceroboric acid is formed. This is a 

 strong acid and hence the reaction of the solution changes from alkaline to acid. 

 On heating, unless a large excess of glycerol be present, the glyceroboric acid 

 is hydrolysed to glycerol, and boric acid, and the solution again becomes 

 alkaline. 



The Higher fatty acids and their salts, the soaps. 



173. Shake a few drops of oleic acid with 5 cc. of water, ether 

 and alcohol respectively in separate tubes. The acid is insoluble in 

 water, but soluble in alcohol or ether. 



174. Place a drop of oleic acid on writing paper : a greasy stain 

 results. 



175. Shake the alcoholic solution of oleic acid with dilute 

 bromine water. The colour of the bromine is discharged, owing to 

 the unsaturated acid absorbing the halogen till it is saturated. 



176. Repeat the experiment with an alcoholic solution of 

 stearic acid or commercial " stearine " (a mixture of stearic and 

 palmitic acids). The colour of the bromine persists, since these acids 

 are members of the saturated series. 



