AMINO-ACIDS, PROTEINS, UREA AND URIC ACID 95 



than neutralize the excess of the sulphuric acid. The sodium hydrox- 

 ide reacts with the ammonium sulphate, setting free the ammonia. 

 This ammonia is then distilled into standard acid and the amount 

 of standard acid which is thus neutralized by the ammonia is deter- 

 mined by titration against a standard alkali. From the amount of 

 standard acid neutralized the amount of ammonia is calculated. 

 From this amount of ammonia, the amount of nitrogen is calculated, 

 and this amount of nitrogen is the whole of the nitrogen contained 

 in the original protein. 



On the average, as previously stated, proteins contain 16 per cent 

 nitrogen. Therefore, multiplying the amount of nitrogen by the 

 factor 6.25 gives us the amount of protein in the original substance. 

 This factor, 6.25, is the one commonly used for converting nitrogen 

 into protein, and agricultural analyses in general are calculated with 

 this factor which may be understood unless otherwise stated. Some 

 proteins contain more than 16 per cent nitrogen, e.g. the proteins 

 of wheat gluten contain 17.6 per cent nitrogen. The factor for con- 

 verting into protein would be, then, 5.68. The following typical 

 analysis may be given as an illustration of the calculation. 



(c) Calculation of a Kjeldahl Nitrogen Determination. 



Milk taken = 5.000 g. 



N/io HCl in receiver = 25.0 c.c. 



N/10 NaOH used in titration = 6.1 c.c. 

 N/10 HCl neutralized = 18.9 c.c. 



I c.c. N/10 HCl contains 



.00364 g. HCl 

 .'. 18.9 X .00364 = 0.0688 g. HCl 



HCl : NH3 : : .00364 : .0017 

 .-. .0688 g. HCl = 0.0321 g. NH3 



NH3: N : : 17 : 14 



.•. .0321 g. NH3 = 0.0265 g- N. 



Protein factor = 6.25 



.'. .0265 X 6.25 =0.1656 g. Protein 



.1656 g. Protein 

 in 5.0 g. milk = 3.31 per cent = protein in milk 



(4) Color Reactions of Proteins. (See list of proteins at end of 

 this experiment.) (a) Millon's Reaction. Millon's Reagent is pre- 

 pared as follows: Dissolve one part by weight of mercury in two 



