496 



PHYSIOLOGICAL CHEMISTRY 



of the Folin procedure in such a way as to render the latter, applicable to urines 

 containing sugar, to remove certain interfering substances such as allantoin and 

 make the separate determination of ammonia unnecessary. The method is, how- 

 ever, time-consuming and the other methods mentioned are on this account to be 

 preferred for the analysis of diabetic urines. 



Procedure. Place 5 c.c. of urine in a 200 c.c. Erlenmeyer flask and add to it 

 5 c.c. of concentrated hydrochloric acid, 20 grams of crystallized magnesium chlo- 

 ride, a piece of paraffin the size of a hazel nut, and 2-3 drops of a i per cent aqueous 



solution of " alizarin red." Insert a Folin safety 

 tube (Fig. 159) into the neck of the flask and 

 boil the mixture until each drop of reflow from 

 the safety tube produces a very perceptible bump; 

 the heat is then reduced somewhat and con- 

 tinued one and one-half hours. The contents 

 of the flask must not remain alkaline, and to 

 obviate this, at the first appearance of a red- 

 dish tinge in the contents of the flask a few drops 

 of the acid distillate are shaken back into the 

 flask. At the end of one and one-half hours the 

 contents of the vessel are transferred to a i -liter 

 flask with about 700 c.c. of distilled water, about 

 20 c.c. of 10 per cent potassium hydroxide or so- 

 dium hydroxide solution is added and the mix- 

 ture distilled into a known volume of N/io sul- 

 phuric acid until the contents of the flask are 

 nearly dry or until the distillate fails to give an 

 alkaline reaction to litmus, showing the absence 

 of ammonia. The time devoted to this process 

 is ordinarily about an hour. Boil the distillate a 

 few moments to free it from CO 2 , then cool and 

 titrate the mixture with N/io sodium hydroxide, 

 using "alizarin red" as indicator. 



A "check" experiment should always be made 

 to determine the original ammonia content of 

 the urine and of the magnesium chloride, if it 

 is not absolutely pure, which of course should be 

 subtracted from the total amount of ammonia 

 as determined by the above process. 



6. Hypobromite Methods. Principle. The hypobromite methods are based 

 upon the fact that urea is decomposed by a solution of sodium hypobromite with 

 the formation of carbon dioxide and gaseous nitrogen according to the following 

 equation: 



CO(NH) 2 + 3 NaOBr = N 2 +CO 2 +2H 2 0+3NaBr 



The carbon dioxide is retained by the alkaline solution and the nitrogen gas is col- 

 lected and measured. On account of its simplicity this has been the most widely 

 used of clinical methods for the determination of urea. It possesses, however, sev- 

 eral essential inaccuracies among which may be mentioned the fact that all of the 

 urea nitrogen is not liberated, while on the other hand substances always present 



FIG. 159. FOLIN 's UREA 

 APPARATUS. 



