§ 86. UKEA. 123 



lution of sodic chloride to flow in from a burette with 

 constant stirring, until the precipitate disappears, and the 

 solution becomes clear again. Each cubic centimetre of 

 the sodic solution required corresponds to 0.02 grm. of 

 mercuric oxide ; on the basis of this determination, then, 

 the amount of mercuric salt in the diluted solution, and 

 the strength of the concentrated solution, can be estimated. 



Now, to bring the mercuric solution to a urea standard, 

 it must be diluted so that 1 litre will contain 72 grms. of 

 mercuric oxide ; dilute the solution almost to this point, 

 and then compare it with a solution of urea of known 

 strength. For this purpose, dissolve 2 grms. of pure 

 urea, dried at 100° C, in water, dilute the solution to 100 

 c.c. and put 15 c.c. of this solution in a beaker; in an- 

 other small beaker, wash a few crystals of sodic bicar- 

 bonate with a considerable quantity of cold water, and, 

 after pouring this oiF, add just enough water to make a 

 thin paste with the salt ; have ready also a piece of clean 

 glass, with its under surface coated with asphaltum varnish. 



itTow, allow the mercuric solution to flow slowly from 

 the burette into the solution of urea, with the addition 

 from time to time of a small quantity of dry sodic car- 

 bonate, in order to nearly, but not quite, neutralize the 

 nitric acid that is set free ; the solution must all the while 

 have a slight acid reaction. To ascertain whether all the 

 urea is precipitated, transfer a drop of the solution, on 

 the end of a glass rod, to the glass plate, and cover it 

 with a drop of the paste of sodic bicarbonate ; if no yel- 

 low color appears in a few seconds, wash the test-drop 

 back with the smallest possible quantity of water, and, 

 after adding a little more mercuric nitrate, test again with 

 the sodic bicarbonate ; the first trace of a yellow color, 

 that appears as soon as the two drops come together, in- 

 dicates that sufficient mercuric chloride has been added. 



The standard mercuric solution is to be diluted so that 

 1 cubic centimetre corresponds to 10 mgr. of urea, or to 



