122 § 86. BASES AND ACIDS WITH REAGENTS. . 



is finislied, the bulbs rinsed out with alcohol, and the am- 

 monia determined with the aid of platinic chloride (§ 47, a). 

 If the substance is very rich in nitrogen, it must be 

 mixed with an equal quantity of i:)ure sugar, in order to 

 avoid the danger of an impetuous retrograde current in 

 the bulbed tube, when the evolution of gas ceases and 

 the whole apparatus is filled with ammonia-gas. 



UREA. CH4N0O. 60. 



86. Urea is very soluble in water and alcohol, but dif-' 

 ficultly soluble in ether. 



Reactions. — When heated it gives oif ammonia. 



Nitric or oxalic acid causes the formation of a white 

 precipitate in concentrated solutions of urea. 



Quantitatiye estimation. — The volumetric method of 

 Liebig, with mercuric nitrate, is easily and quickly exe- 

 cuted and yields accurate results. 



To prepare the standard solution of mercuric nitrate, 

 heat an excess of mercury with concentrated nitric acid, 

 concentrate the solution, and, on cooling, crystals of mer- 

 curous nitrate will be deposited ; pour off the mother- 

 liquor, wash the crystals, first with dilute nitric acid, and 

 then with cold water, dissolve them in pure nitric acid, 

 and heat the sohition until a drop of it gives no precipi- 

 tate or turbidity with sodic chloride ; evaporate the solu- 

 tion on the water-bath to the consistency of a syrup, dilute 

 it with 10 volumes of water, let it stand 21: hours, and 

 filter if any precipitate is formed. 



To determine the strength of this solution, dissolve 

 10.864 grms. of pure, ignited sodic chloride in 1 litre of 

 water; dilute 10 c.c. of the mercuric solution with 90 

 c.c. of water, put 10 c.c. of this diluted solution in a small 

 beaker, add 4 c.c. of a cold saturated solution of sodic 

 phosphate, and quickly, before the precipitate caused by 

 this reagent has time to become crystalline, allow the so- 



