CHARACTERIZATION 15 



case of substances which are volatile, the mark will sooner or 

 later disappear, whereas in the case of a true fat, the mark is 

 permanent, since fats are not volatile. 



3. Fats, waxes, and lipins are all saponified by boiling 

 with alcoholic potash. In the case of most fats 2 grams can 

 be completely saponified by boiling for a quarter of an hour 

 with 25 c.c. of 3 per cent alcoholic potash. The resulting 

 mixture of potassium soap and glycerol should be completely 

 soluble in water, and, after boiling off the alcohol and acidifying 

 the solution, the free fatty acids should be precipitated. Waxes 

 being, on the whole, less easy to hydrolyse, may not have been 

 completely decomposed under these conditions, but lipins 

 would behave like fats. To distinguish between fats and 

 lipins, special tests have to be applied {vide under Lipins, p. 51). 



4. The only certain way of distinguishing between a fat 

 and a wax is to establish the presence or absence of glycerol. 

 This may be done either by heating the substance with a 

 crystal or two of potassium hydrogen sulphate, or, better, if 

 sufficient material is available, by preparing a concentrated 

 solution of glycerol free from fatty acids as follows : Saponify 

 the material as above, boil off the alcohol, take up with a 

 little water and acidify ; filter off the precipitated fatty acids 

 and evaporate the filtrate over a water bath ; extract the 

 residue with a small quantity of alcohol, which dissolves out 

 the glycerol, leaving the salts in solution. Evaporate off the 

 alcohol ; if sufficient material remains divide it into two 

 portions a and h \ a \s heated with a crystal of potassium 

 hydrogen sulphate ; the presence of glycerol is confirmed by 

 the production of acrid vapours of acrolein — 



CH2OH CHOH CHjOH = 3H2O + CH2 = CH . CHO, 

 which blackens a filter paper moistened with ammoniacal 

 silver nitrate solution. 



The second portion h is dissolved in a little water and 

 warmed in a water bath with 10 c.c. of freshly prepared 

 bromine water for 20 minutes ; any excess of bromine is then 

 evaporated off and the resulting solution is tested for the 

 presence of dihydroxy acetone, CHgOH . CO . CHjOH, as 

 follows : — 



