202 Water Analysis [340 



a lamp screen, as shown in fig. 49. The burner must be turned 

 down as low as possible. The water will then evaporate slowly 

 ,„,„, and without ebullition. As it evapo- 



1^K_ ., rates it must be filled from time to 

 IjFljjl time llnt -il the whole 500 c.c. has been 



placed in the basin. The evaporation 

 is now carried on until only about 

 . ■•■ggj j^l BL_ 2 ° cc - i s 1^ m ^e dish. Then remov<  

 P the dish to a water bath until appar- 

 ently dry. When no further evapora- 

 tion takes place, heat in the air bath at 

 1 io° C. until its weight is constant. The residue, after weighing, 

 must be saved for the nitrate estimation. 



340. Estimation of Nitrogenous Matter.— The 

 nitrogen present in water may be in one or both of two forms 

 known respectively as ' free ammonia ' and ' albuminoid am- 

 monia ' — the free ammonia being either ammonia or ammoniacal 

 salts, and the albuminoid ammonia being obtained from the 

 organic nitrogen. 



For the estimation of these two constituents the following 

 solutions are necessary : 



Nessler's Solution. Dissolve 16 5 grams of potassic iodide 

 and 6 -5 grams of mercuric chloride in 400 c.c. of ammonia- 

 free water. Boil and stir until all is dissolved. Now add 

 cold saturated solution of mercuric chloride until the precipi- 

 tate of mercuric iodide just becomes permanent. Now add 

 80 grams of caustic potash or 60 grams of caustic soda, 

 allow to cool, and dilute to 500 c.c. Finally, to ensure sensi- 

 tiveness, add a few more drops of mercuric chloride solution, 

 and allow the precipitate to settle. This solution is kept in a 

 well-stoppered bottle. A little should be decanted off for 

 immediate use into an 8-oz. bottle fitted with a perforated cork 

 through which passes a pipette with 1 or 2 c.c. marks on it. 



