ELEMENTARY CHEMICAL PHYSIOLOGY 



257 



Estimation of Urea. Many methods have been introduced for the 

 estimation of urea. As an approximate estimation the hypobromite 

 method is f airly good and has the considerable merit of being rapidly 

 carried out. If accurate work is required on the whole the urease 

 method will prove the most serviceable (see page 280). 



Hypobromite Method. As stated this method only gives, as 

 usually carried out, an approximation, as urea yields variable 

 amounts of nitrogen unless the strict- ^ 



est of conditions are adhered to, and 

 further certain other urinary constitu- 

 ents also yield a certain amount of 

 nitrogen. With all its limitations, 

 however, the method has a certain 

 value. 



From the consideration of the for- 

 mula representing the changes which 

 take place (p. 256) it will be noted 

 that when urea is decomposed by the 

 hypobromite two gases, carbonic 

 dioxide and nitrogen, are liberated. 

 As a strongly alkaline solution of 

 hypobromite is always used the free 

 alkali combines with the carbonic 

 acid at once so that nitrogen alone is 

 measured. 



Theoretically 1 grm. urea gives off 

 372 c.c. nitrogen at and 760 mm., 

 but in practice only about 354 c.c. 

 are given off. 



There are various forms of appar- 

 atus used for collecting the liberated 

 nitrogen. That of Dupre (Fig. 185) 

 consists of an inverted burette (a) 

 placed in a cylinder of water, and to 

 the neck of which is connected a T- 

 piece (/). With the side tube of this 

 the generating bottle is connected by 

 india-rubber tubing, and the other 

 tube is closed with a piece of tubing 

 and a clip. To make the estimation, 

 25 c.c. of the freshly prepared alkaline solution of sodium hypo- 

 bromite 1 are placed in the generating bottle (o) and 5 c.c. urine in a 

 small tube, which is then carefully placed in the generating bottle 

 without allowing the two fluids to mix. The cork of the generating 

 bottle is then inserted, and the meniscus of the water both inside and 

 outside the burette brought to the same level at the zero mark, the 



1 Add 25 c.c. bromine to a well-cooled solution of caustic soda made by 

 dissolving 100 grms. NaOH in 250 c.c. water. If only a small amount is 

 required addition of 2 c.c. bromine to 23 c.c. 40 per cent. NaOH suffices. 



T 



FIG. 185. Dupre's urea 

 apparatus. 



