AMIDES 131 



Carbamide or Urea. 



Preparation. 



(a) By synthesis from ammonium cyanate. 



A solution of approximately equal parts of ammonium sulphate 

 and potassium cyanate (p. 160) are boiled together and evaporated to 

 dry ness on the water-bath. The dry residue is extracted with alcohol, 

 which dissolves the urea leaving potassium sulphate. Urea is left as a 

 residue on evaporating the alcoholic solution on a water-bath. It is 

 -crystallised from alcohol : 



(NH 4 ) 2 SO 4 + 2KOCN = 2NH 4 OCN + K 2 SO 4 

 NH 4 OCN = CO(NH 2 ) 2 . 



(U] By other synthetical methods. 



Urea is formed by the methods described for the preparation of 

 amides. 



(c) From urine. 



(i) About 25 or 50 c.c. of urine are evaporated to dryness on the 

 water-bath. The dry residue is treated with about 10 c.c. of acetone 

 allowing the solvent to boil on a hot water-bath. The acetone is 

 poured off into a clean vessel and allowed to evaporate (not in the 

 neighbourhood of a flame). Urea crystallises out in long silky needles 

 and is recrystallised from alcohol. A yield of about I gm. per 50 c.c. 

 of urine should be obtained. 



(ii) 100 c.c. of urine are evaporated to a syrupy consistency (to 

 about J) and thoroughly cooled by immersion in cold water. Excess 

 of concentrated nitric acid is added to the cold solution, the solution 

 being kept cold during the addition and stirred vigorously. Urea 

 nitrate is precipitated in crystalline form. The crystals are filtered off 

 through glass wool, or asbestos, and freed as much as possible from 

 mother liquor by pressing between sheets of paper, or the crystals are 

 placed on a porous plate and drained from mother liquor. About 4 

 gm. should be obtained corresponding to about 2 gm. of urea (163 gm. 

 nitrate contain 60 gm. urea). Urea is prepared from the nitrate 

 by mixing it with excess of barium carbonate and adding a little 

 alcohol to form a paste ; carbon dioxide is evolved, barium nitrate and 

 urea are formed. The paste is extracted with hot alcohol, the alcohol 

 filtered from barium carbonate and evaporated. Urea separates in 

 needles and is recrystallised from alcohol. 



(iii) Urea may also be prepared from urine by precipitating it as 

 urea oxalate by adding I gm. of oxalic acid to every 10 c.c. urine; 

 the yield is about I per cent. (Roaf). 



