134 GENERAL MICROBIOLOGY 



2. Repeat with uninoculated control. 



3. The development of a red color in ten minutes indi- 

 cates the presence of nitrites, the intensity of the color 

 depending upon the amount of nitrites present. 



(B) For ammonia. 1. Add 0.2 c.c. of Nessler's solu- 

 tion to each culture to be tested. 



2. Repeat with uninoculated control. 

 The presence of ammonia is shown by a yellow color 

 or precipitate. 



(C) For nitrates unchanged or free nitrogen liberated. 

 1. When either or both of the preceding tests are positive, 

 no further determination need be made, but if negative, 

 then one of two conditions may prevail: (a) Either the 

 nitrates have not been changed, or (6) they may have been 

 reduced to free nitrogen. To ascertain which is true, it 

 will be necessary to determine the presence or absence 

 of nitrates. 



2. Test as follows: (a) Evaporate 10 c.c. 'of each cul- 

 ture and the controls almost to dryness in an evaporat- 

 ing dish and add to the residue 1 c.c. of phenolsulphonic 

 acid. 



(6) Dilute with 10 c.c. distilled water, then add suf- 

 ficient ammonium hydroxide, diluted 1 : 1 with distilled 

 water, or concentrated potassium hydroxide solution, 

 to make alkaline. 



(c) Transfer the liquid to a 50 c.c. Nessler tube or grad- 

 uated cylinder and make up the volume to 50 c.c. with 

 distilled, water. 



A yellow color shows the presence of nitrates. 



