NITROGEN AND AMMONIA 237 



Digestion Mixtures Used by Other Workers. 1 ml. cone, sul- 

 furic acid, 0.625 g. potassium sulfate, and 0.075 g. selenium; dilute 

 about 5 times. — Levy (1936). 



1% selenium dioxide and 1% copper sulfate (CuSO4.5Ho0) in 

 concentrated sulfuric acid and water (1:1) ; after digest is colorless, 

 cool, add 1 drop saturated potassium persulfate, and continue diges- 

 tion for 15 min. after the solution becomes water clear. — Borsook 

 andDubnoff (1939). 



3 g. copper sulfate, 1 g. potassium sulfate, and 0.1 g. selenium 

 dioxide to 300 ml. cone, sulfuric acid. — Needham and Boell ( 1939) . 



Cone, sulfuric acid and water (1:1) ; when fuming starts, add 1 

 drop of 30% hydrogen peroxide. After digest is clear, cool, add a 

 couple of drops of saturated potassium persulfate, and continue 

 heating; destroy peroxides by adding water, after cooling, add 

 another Hengar granule, and place in an oven at about 120° for 

 30 min. — Levy and Palmer (1940). 



Cone, sulfuric acid and water (1:1) saturated with potassium 

 sulfate and made 0.1% with respect to copper selenite. The latter is 

 prepared by mixing a strong copper sulfate soln. with one of sodium 

 selenite and collecting the precipitate. — Tompkins and Kirk (1942). 



18 A^ sulfuric acid containing 0.1% selenium dioxide and 0.1% 

 copper sulfate; use 30% hydrogen peroxide if needed. — Hawes and 

 Skavinski (1942). 



I g. copper sulfate, 1 g. potassium sulfate, 1 g. selenium dioxide, 

 and 100 ml. 50% sulfuric acid; add saturated potassium persulfate 

 to the cooled mixture if required. — Boell ( 1945) . 



Levy Nesslerization Method for Nitrogen 



SPECIAL REAGENTS 



Digestion Mixture. See above. 



Nessler Reagent (Folin and Wu). Add 15 g. potassium iodide and 



II g. iodine to 10 ml. distilled water and introduce an excess of 

 mercury ( 14-15 g.). Shake well for 7-15 min. or until the dissolved 

 iodine has nearly all disappeared. Cool in running water when the 

 solution begins to become pale and continue shaking until the 

 greenish color of the double iodide (HgIo.2KI) appears. Decant 

 the solution and wash out vessel with distilled water. Dilute the 

 solution and washings to 200 ml. Add 75 ml. of this solution and 



