ESTIMATION OF UREA. 175 



7. Add to a concentrated solution of urea nitric 

 acid; crystals of urea nitrate will immediately form. 

 Add oxalic acid to urea, and the oxalate will be 

 deposited. 



Estimation of urea. 1. Dissolve in a beaker 100 

 grammes of pure mercury in about 500 grammes of 

 pure nitric acid ; add a further quantity of nitric acid 

 in drops, gently shaking occasionally until no red 

 vapours are evolved either on the addition of nitric 

 acid or on shaking ; then evaporate the solution at a 

 gentle heat until it is a colourless syrup. Care 

 must be taken that none of the liquid is lost by 

 spurting. 



2. Dilute the solution to exactly 1400 c.c., adding a 

 little nitric acid to prevent the formation of an in- 

 soluble basic salt. This forms the standard solution of 

 nitrate of mercury, each cubic centimetre of which 

 represents a centigramme of urea. To be certain of its 

 strength a test experiment should be made by es- 

 timating a known quantity about two decigrammes 

 of pure urea as described hereafter. 



3. Prepare cold saturated solutions of baryta water 

 and baryum nitrate ; mix two volumes of baryta water 

 with one volume of nitrate of baryum solution. This 

 mixture forms the baryta solution used in the analysis. 



4. On a glass plate under which is a piece of white 

 filter paper, place a number of small drops of carbonate 

 of soda. 



5. Add to 30 c.c. of urine 15 c.c. of the baryta 

 solution ; mix well with a stirring-rod, and filter through 



