APPENDIX P 145 



constitutes tlie standard ammonium cMoride solution, and each 

 cubic centimeter contains 0.005 mg. of NH^. 



5. Standard colorimetric solution. This may be prepared by dilut- 

 ing 10 CO. of the standard ammonium chloride solution (4) to about 

 90 cc., adding 4 cc. of Xessler's reagent (3) and diluting to 100 cc. 

 This standard should be prepared simultaneously with the develop- 

 ment of the color ia the solutions to be tested. This colorimetric 

 standard contains 0.5 part of NH^ per million. 



AXALTTICAL PROCESS 



If the solution be colorless and free from salts which interfere 

 with the reagents, the determinations can be made without previous 

 distillation ; otherwise it will be necessary to distiU. a measured 

 quantity of the solution after it has been made alkaline with sodium 

 carbonate. The flask and condenser should be rinsed with ammonia- 

 free water. The condenser must be kept quite cold, with the end 

 dipping into a little ammonia^free water at the start. The distUlate 

 is made up to a definite volume. The distillation has the advantage 

 of concentrating the ammonia in the case of very weak solutions. 



Pirst test a small amount of the solution, to ascertain the dilution 

 necessary to get a good color for comparison. This may be done by 

 adding some of the Xessler reagent (3) to a few cubic centimeters 

 of the solution in a test tube. If a precipitate forms, it wiU be neces- 

 sarj- to dilute the solution and again test a small sample. The color 

 of the solution should be a light shade of pure yellow. If it has a 

 deep yellow or reddish tint, it is too strong to be used direct, and 

 further dilution is necessary. 



For the determination of the ammonia the analytical solution is 

 diluted to about 45 cc, 2 cc. of Xessler's reagent (3) is added, and 

 sufficient water is added to make 50 cc. The standard colorimetric 

 solution is prepared at the same time, and after fifteen minutes the 

 comparison is made in the colorimeter or ia Nessler tubes. 



