DETERMINATION OF UREA 87 



Nessler Solution. From completely saturated caustic soda 

 solution, containing about 55 gms. of sodium hydroxide per 100 

 c.c., decant the clear supernatant liquid and dilute to a concen- 

 tration of 10 per cent. (It is worth while to determine by titration 

 that a 10 per cent solution has been obtained within an error of 

 not over 5 per cent.) Introduce into a large bottle 3500 c.c. of 

 10 per cent sodium hydroxide solution, add 750 c.c. of mercuric 

 potassium iodide solution, to be described below, and 750 c.c. of 

 distilled water, giving 5 liters of Nessler solution. 



The Nessler solution so obtained contains enough alkali in 15 

 c.c. to neutralize 1 c.c. of the diluted sulphuric acid mixture and 

 to give a suitable degree of alkalinity for the development of the 

 color given by ammonia at a volume of 50 c.c. (In other Nessler- 

 izations, as in urine analysis, when there is no acid to be neutralized 

 10 c.c. of the Nessler reagent per 100 c.c. of Nesslerized ammonia 

 solution is the correct amount.) 



Mercuric Potassium Iodide Solution. Transfer 150 gms. of 

 potassium iodide and 110 gms. of iodine to a 500 c.c. Florence 

 flask; add 100 c.c. of water and an excess of metallic mercury, 140 

 to 150 gms. Shake the flask continuously and vigorously for seven 

 to fifteen minutes or until the dissolved iodine has nearly dis- 

 appeared. The solution becomes quite hot. When the red iodine 

 solution has begun to become visibly pale, though still red, cool 

 in running water and continue the shaking until the reddish color 

 of the iodine has been replaced by the greenish color of the double 

 iodide. The whole operation usually does not take more than 

 fifteen minutes. Now separate the solution from the surplus 

 mercury by decantation and washing with liberal quantities of 

 distilled water. Dilute the solution and washings to a volume of 

 2 liters. If the cooling is begun in time, the resulting reagent is 

 clear enough for immediate dilution with 10 per cent alkali and 

 water, and the finished solution can at once be used for Nessleriza- 

 tions. 



DETERMINATION OF UREA 



Principle. The principle of this method consists in the 

 hydrolysis of urea by means of urease or heat and the isolation of 

 the ammonia by aeration or distillation. It is thus evident that 

 there are four combinations, any one of which will give satisfactory 

 results. 



