56 LABORATORY MANUAL OF MICROBIOLOGY 



hydroxide solution, using phenolphthalein as an indicator. 

 One cubic centimeter of this solution is equivalent to exactly 1 

 milligram of carbon in carbon dioxide determinations. 



Standard (A^/6) Barium Hydroxide Solution 



To prepare one liter of a A^/6 barium hydroxide solution, 

 14.28 grams of anhydrous barium hydroxide is theoretically 

 required. However, in the process of preparation of the solution, 

 considerable barium carbonate is formed. Hence about one- 

 fourth to one-sixth more of the hydroxide is taken than is 

 required theoretically. Add 100 grams of C.P. anhydrous 

 barium hydroxide to 6 liters of distilled water. Allow to dissolve 

 for 24 hours with occasional shaking. The residue is allowed to 

 settle for 7 days, when the supernatant solution is filtered through 

 a double layer of filter paper. The solution is standardized to 

 exactly iV/6 against the oxaHc acid, using phenolphthalein as an 

 indicator. 



QUALITATIVE METHODS FOR THE DETERMINATION OF VARI- 

 OUS FORMS OF INORGANIC NITROGEN 



Nessler's Reagent for Ammonia 



1. Dissolve 50 grams of potassium iodide in a small quantity 

 of cold distilled water. (Ammonia, free; about 35 c.c.) 



2. Add a saturated solution of mercuric chloride until a slight 

 precipitate persists. 



3. Now add 400 cubic centimeters of a 50 per cent solution of 

 potassium hydroxide made by dissolving the potassium hydroxide 

 and allowing it to clarify by sedimentation before using. 



4. Dilute to 1,000 cubic centimeters, allow to settle for 1 

 week, and decant. This solution gives the required color with 

 ammonia within 5 minutes after addition. 



5. Keep the Nessler's solution in a well-stoppered bottle 

 away from the light. 



Test for Ammonia. — Add to a drop of Nessler's solution in a 

 test plate a loopful of the solution to be tested. A deep golden- 

 yellow color indicates the presence of ammonia. 



The presence of glucose in solution interferes with the ammonia 

 test. In that case, the ammonia should be removed first by 

 aeration and then tested (see p. 63). 



