398 ADDITIONS. 



A standard urea solution for titrating the mercuric 

 nitrate solution may be made by drying urea over 

 sulphuric acid, weighing out 4 grams and adding 

 water to make up 200 c.c. 10 c.c. of this just 

 give the end-reaction with 20 c.c. of standard mer- 

 curic nitrate solution. This however is only the 

 case when the mercuric nitrate solution is neut- 

 ralized by sodium carbonate immediately after 

 being added to the urea solution. As this point is 

 neglected in the method given in the text, it may 

 be neglected in preparing the standard solution. 



Liebig's method. 



Take 40 c:c. of urine and add 20 c.c. of the baryta 

 mixture a copious precipitate of barium phosphate, 

 sulphate, carbona,te will be formed. Shake to mix 

 well the fluids and filter through a dry filter. The 

 object of this is to remove the phosphates, these give 

 a precipitate with mercuric nitrate. 



Fill a burette with the standard solution of mercuric 

 nitrate. On a glass plate lying on a dull black surface, 

 place a number of drops of a saturated solution of 

 normal sodium carbonate. 



Preliminafry estimation. In a beaker place exactly 

 15 c.c. of the filtered urine-mixture, into this run 

 slowly the mercuric nitrate solution from the burette. 

 Stir with a glass rod, and from time to time, add a 

 drop of the mixture to one of the drops of sodium 

 carbonate; as soon as a yellow colour is thus produced, 

 note the amount of mercuric nitrate which has been 

 added. The' yellow colour is produced when all the 

 urea has been precipitated and there is an excess of 

 mercuric nitrate ; this with sodium carbonate gives a 



