AMIDES 135 



The bottle must now be brought to its original temperature by placing 

 it for a few minutes in a fresh supply of cold water. As soon as it is 

 cool, the burette is raised till the level of the water is the same inside 

 and outside and the level is read. The difference in the readings gives 

 the volume of nitrogen evolved. By making the levels inside and 

 outside the burette the same, this volume is measured at the atmos- 

 pheric pressure. The temperature of the water and the barometric 

 pressure and the tension of aqueous vapour at that temperature are 

 ascertained and the volume of nitrogen is corrected to the volume at 

 and 760 mm. by the formula : 



V x 273 x (B - T) 



(273 + t) x 760 



where V = volume of gas evolved, B = barometric pressure, T = tension 

 of aqueous vapour (p. 608). 



The amount of urea corresponding to this volume is given by the 

 equation : 



CO(NH 2 ) 2 + 3 NaBrO + aNaOH = 3 NaBr + N 2 + Na^COg + 3 H 2 O. 



60 gm. 22-4 litres ( = 28 gm.) 



i gm- 373 c.c. 



from which the amount of urea in 5 c.c. of the solution is calculated ; 

 hence the amount in 100 or 1000 c.c. 



Actually, however, only 354 c.c. nitrogen are evolved 'by I gm. of 

 urea so that the method is not quite accurate ; this should be allowed 

 for. 



(2) By Hydrolytic Methods. 



The most accurate methods of estimating urea are by hydrolysis. 

 Urea is rapidly hydrolysed by alkali, but more slowly by hydrochloric 

 acid. 



(i) By Acid. 



The hydrolysis by acid proceeds rapidly and is complete in 

 about I hour if the hydrolysis be effected at a temperature of about 

 150-160 as was shown by Folin. This method has been particularly 

 useful in the analysis of urine and is described on p. 553. 



