60 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 1 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 1 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 
