HANDBOOK FOR BIO-CHEMICAL LABORATORY. 47 



Urea, CH 4 N 2 or H a N.CO.NH 2 . 



Preparation. Evaporate ^ to 1 litre of urine to syrupy 

 consistency on the water-bath, allow to cool in ice-water, treat 

 while stirring with 3 vols. nitric acid of sp. gr. 1.3 which 

 has previously been boiled and then coolod to C. Allow 

 this mixture to stand for a few hours at C. The crystals 

 of urea nitrate which form are transferred to a filter of glass 

 wool and washed several times with small amounts of ice-cold 

 pure concentrated nitric acid, then dissolved in as small a 

 quantity of hot water as possible, and reprecipitated by cone. 

 HN0 3 . These crystals are first drained, then dissolved in hot 

 water, and the solution decolorized by a little chlorine water, 

 and then treated with a small quantity of pure barium carbon- 

 ate until no more dissolves and effervescence ceases and the 

 solution is neutral. Now evaporate the solution to complete 

 dry ness on the water-bath, pulverize the residue, and extract 

 it with cold absolute alcohol, which dissolves the urea, but not 

 the barium nitrate. The filtered alcoholic solution is decolor- 

 ized when necessary by animal charcoal and evaporated at a 

 medium heat to point of crystallization. 



Synthetical. Heat 250 grms. coarsely powdered potassium 

 ferrocyanide in a capacious porcelain dish over a naked flame 

 with constant stirring until all is changed into a white pow- 

 der, leaving no lumps having a yellow nucleus. Care must 

 be taken not to apply too strong a heat, as otherwise the pow- 

 der will turn brown. The mass thus obtained is finely 

 powdered and thoroughly mixed with one half (-J) its weight 

 of dried finely powdered manganese dioxide, and the mixture 

 heated on an iron dish under the draught, constantly stirring 

 until deflagration begins and the mass becomes doughy. The 

 heat is continued until a portion of the mass placed in water 

 and acidified with HC1 fails to give a blue precipitate with 

 ferric chloride. Then allow to cool, lixiviate with cold water, 

 add to the solution f of the weight of the drie_d potassium 



