6o MILK 



Benzoates. — This is detected by a modification 

 of Mohler's method by Von der Heide and 

 Jakob as given by U. S. Bureau of Chemistry. 



The residue that is to be tested for benzoic 

 acid is dissolved in a little water, the solution 

 mixed with from i to 3 c.c. of normal sodium hy- 

 droxid and evaporated to dryness. To this resi- 

 due is added from 5 to 10 c.c. of concentrated sul- 

 furic acid and a small crystal of potassium nitrate 

 and the mixture heated either for ten minutes 

 in a glycerol bath between 120° and 130° or for 

 twenty minutes in boiling water. If heated in 

 the glycerol bath the temperature must not be 

 permitted to go over 130°. Metadinitrobenzoic 

 acid is formed. After cooling i c.c. of water is 

 added, the liquid made decidedly ammoniacal, 

 boiled to break up ammonium nitrite, and some 

 fresh colorless ammonium sulfid solution added 

 so that the liquids do not mix. A brown ring at 

 junction indicates benzoic acid. The liquids 

 being mixed, the color diffuses and on heating 

 changes to greenish-yellow. The last reaction 

 distinguishes benzoic acid from salicylic and 

 cinnamic acid as these latter form amino-deriva- 

 tives which are not destroyed by heating. 

 Phenolphthalein interferes with this process. 



Salicylic Acid. — The other portion of the 

 ether-extract may be evaporated and tested for 



