138 SOIL . BACTERIOLOGY 



quickly, which changes to orange, then slowly to lemon or 

 yellow, and finally becomes a greenish yellow. 



(n) Phenolsulphonic Acid. Dissolve 25 grams of pure 

 white phenol crystals in 150 c.c. of pure concentrated 

 sulphuric acid, add 75 c.c. of fuming sulphuric acid (13 per 

 cent. SOs), stir well, and heat for two hours at about 

 100 C. The reagent prepared in this way should not 

 contain any mono-acids or any tri-acids. Two c.c. of 

 this reagent give reliable results with not more than 5 

 milligrams of nitrate nitrogen. 



Chamot, Pratt, and Redfield, Jour. Amer. Chem. Soc., vol. xxxiii, pp. 

 381-384, 1911. 



(12) Fehling' s Reagents : 



(a) Copper sulphate (CuSO 4 + sH 2 O) 34-639 gm. 



Distilled water 500.0 c.c. 



(b) Sodium potassium tartrate (KNaC 4 H 4 O 6 + 



4H 2 O) 178.0 gm. 



Sodium hydroxid 50.0 gm. 



Distilled water 500.0 c.c. 



Pulverize the crystalline substances before attempting 

 to dissolve. 



Qualitative Test. Mix equal quantities of (a) and (ft), 

 about 5 c.c. of each. Add an equal amount of the solution 

 to be tested and boil. A red precipitate indicates reducing 

 sugar. 



(13) Citric Acid Reagent: 



Mercuric sulphate solution 50 gm. 



Sulphuric acid (cone.) 200 c.c. 



Distilled water 1000 c.c. 



Test for Citric Acid. To a water solution of the citric 

 acid add 2 c.c. of the mercury reagent and boil. Now add 5 



