292 PHYSIOLOGICAL CHEMISTRY 



(b) Clinical Method. Shiff-Malfatti. This method depends 

 on the fact that when neutral solutions of ammonium salts are 

 treated with formaldehyde, urotropine (hexamethylene tetra- 

 mine) is formed and a definite amount of free acid is liberated. 



6 HCHO + 2 NH 4 ) 2 S0 4 =N 4 (CH 2 ) 6 + 2 H 2 S0 4 + 6H 2 0. 



This method is not accurate, but serves for most clinical pur- 

 poses. 



Measure 25 c.c. urine into a flask and dilute with 5 volumes 

 of water. Add 4 or 5 drops of phenolphthalein and 5 c.c. of a 

 saturated solution of potassium oxalate. Titrate with N/10 

 NaOH to a faint permanent pink. 



The formalin solution is prepared by adding 3 volumes of 

 water, a few drops of phenolphthalein, and titrating with 

 N/10 NaOH to a faint pink to neutralize the acid present in 

 the formalin solution. Add 30-40 c.c. of neutralized formalin 

 to the neutralized urine. The color disappears, for free acid 

 is set free as is shown in the above equation. Titrate again 

 to a faint permanent pink. The amount of alkali corresponds 

 to the amount of decinormal ammonia and ammonium salts 

 present in the urine. Calculate the amount of ammonia 

 N in 25 c.c. of urine, and from this the amount in your 24-hour 

 specimen. 



Calculate the per cent of the total nitrogen which is present 

 as ammonia nitrogen. 



5. Urea. 



Plimmer and Skelton Modification of the Marshall Urease 

 Method. This method consists in decomposing the urea by 

 means of the enzyme urease, which is found in the soy bean. 

 The nitrogen is converted into ammonia and the ammonia esti- 

 mated. Of the various recent modifications of the urease 

 method this is perhaps the best suited for use in a large class 

 unless exceptional laboratory facilities are available. 



The apparatus required is identical with that described in the 

 Folin ammonia determination above. 



