DETECTION OF- BENZOIC ACID. 63 



evaporated to dryness on a water bath. The mass is then 

 finely pulverized, moistened with dilute sulphuric acid 

 and shaken with twice its volume of cold fifty per cent 

 alcohol. The alcohol dissolves out milk sugar, salts, and 

 benzoic acid ; only a trace of fats dissolves. The solution 

 is neutralized with barium hydrate and without filtering 

 off the barium sulphate which separates out, evaporated 

 to small volume (lo c. c.) and again acidified with 

 dilute sulphuric acid and extracted three or four times 

 with a small amount of ether. By evaporation of the ex- 

 tracts pure solid benzoic acid remains. (The residue is 

 usually mixed with a trace of fats.) 



For a quantitative determination the above residue is 

 dried at 60 degrees or over concentrated sulphuric acid, 

 weighed, and then heated upon a water bath until the ben- 

 zoic acid sublimes of?. In order to identify the benzoic 

 acid subsequently, the sublimate should be collected. 

 This may be accomplished by inverting an evaporator 

 over the other dish. Finally the dish and residue is 

 weighed back and the loss of weight, that is, the differ- 

 ence between the two weighings, indicates the weight of 

 benzoic acid. 



The presence of benzoic acid may be confirmed by sub- 

 jecting the sublimate to the following tests: (i) A small 

 amount is heated gently in a watch glass, over which is 

 inverted another glass of the same size. The acid, if pres- 

 ent, sublimes and condenses on the upper watch glass 

 in fine white gHstening flakes. (2) A small sample is 

 treated with a few drops of fuming nitric acid, and evap- 

 orated to dryness. The dry residue is mixed with sand 

 and heated strongly in a glass ignition tube. In conse- 

 quence of the formation of nitrobenzol a strong odor of 

 bitter almond oil will be noticed. 



