i TESTING FOR NITROUS ACID 143 



Test for Nitrous Acid in Cultures 



(1) Filter six broth cultures of the Vibrio choleras 

 Asiaticse (two days old) through a small, sterile Berkefeld 

 filter into a sterile flask. 



(2) To a portion of the filtrate add a drop or two of 

 hydrochloric acid, and then a solution of the hydrochloric 

 acid salt of meta-phenylenediamine. 



The solution will become yellowish red or deep red, 

 according to the amount of nitrous acid (nitrites) 

 present (Griess's reaction). 



The red colour is due to the formation of phenylene 

 brown or Bismarck brown. 



Test broth cultures of the Proteus vulgaris in the same 

 manner. 



