898 APPENDIX. 



cipitate formed in Liebig's quantitative determination of urea, and may be repre- 

 sented by the formula: 2N 2 H 4 CO.Hg(N0 3 ) 2 3HgO. The exact constitution of 

 these salts has not yet been determined. 



Preparation. Ammonic sulphate and potassic cyanate are mixed together in 

 aqueous solution, and the mixture is evaporated to dryness. The residue, when 

 extracted with absolute alcohol, yields urea. From urine, either by evaporating to 

 dryness. having previously precipitated the urine with normal and basic lead acetate 

 in succession and removed the lead by sulphuretted hydrogen, and then extracting 

 with alcohol ; or concentrating only to a syrup, and then forming the nitrate of 

 urea ; this is washed with pure nitric acid and decomposed with baric compound. 



Detection in solution. In addition to the microscopic appearance of the crys- 

 tals obtained on evaporation, the nitrate and oxalate should be formed and exam- 

 ined. Another part should give precipitate with mercuric nitrate, in the absence 

 of sodic chloride, but not in the presence of this last salt in excess. A third por- 

 tion is treated with nitric acid containing nitrous fumes ; if urea is present, nitrogen 

 and carbonic anhydride will be obtained. To a fourth part nitric acid in excess 

 and a little mercury are added, and the mixture is warmed. In presence of urea 

 a colorless mixture of gases (N and CO 2 ) is given off. A fifth portion is kept 

 melted for some time, dissolved in water, and cupric sulphate and caustic soda are 

 added ; a red or violet color, due to biuret, is developed. 



Quantitative determination. For this some special manual must be con- 

 sulted. 1 It will suffice here to point out that the determination is made either with 

 a solution of mercuric nitrate of known strength (Liebig) ; by decomposing the 

 urea by means of sodic hypobromite into nitrogen, carbonic anhydride, and water, 

 and measuring the nitrogen (Knop) [N 2 H 4 CO'+ SNaBrO = 3NaBr + C0 2 + 2H 2 

 H~N 2 ], or by heating the urea with caustic baryta in a sealed tube, the urea being 

 determined by the weight of baric carbonate formed (Bunsen). 



Urea is generally considered to be an amide of carbonic acid, i. e. , carbamide. 

 The amide of an acid is formed when water is removed from the ammonium salt 

 of the acid; if the acid be dibasic and two molecules of water be removed, the 

 result is often spoken of as a diamide. Thus if from ammonic carbonate (NHJ 2 

 C0 3 , two molecules of water, 2H 2 0, be removed, carbonic acid being a dibasic 

 acid, the result is urea ; thus : 



(NH 4 ) 2 C0 3 - 2H 2 = (NH 2 ) 2 CO, 

 which may be written either according to the ammonia type as 



CO ) 



H, N, or as CO 



X1 2 ) 



two atoms of amidogen (NH 2 ) being substituted for two atoms of hydroxyl 

 (HO). 



This connection between carbonic acid and urea is shown by the fact that 

 ammonic carbonate may be formed out of urea by hydration, as when urea is 

 subjected to the 1 specific ferment mentioned above. Regarded, then, as a diamide 

 of carbonic acid, urea may be spoken of as carbamide. But the theoretical deri- 

 vation of urea from ammonic carbonate by dehydration cannot be realized in 

 practice, whereas urea can readily be formed from ammonic carbamate, and 

 Kolbe is inclined to regard it, not as the diamide of carbonic acid, but as the 

 amide of carbamic acid. Ammonium carbamate, C0 2 N 2 H 6 minus H 2 0, gives 

 urea, CO, N, H 4 which, if carbamic acid be written as CO, OH, NH 2 , may be 

 written as CO, NH 2 , NH 2 , one atom of amidogen being substituted for one atom 

 of hydroxyl, and not two, as when the substance is regarded as derived from car- 

 bonic acid. Drechsel's experiments indicate a ready derivation of urea from 

 ammonic carbamate. He has obtained urea by the electrolysis of a solution of 

 this salt with rapidly alternating currents, thus removing the elements of water 



i Neubauer and Vogel, Analyse des Harns, viii. Aufl., 1881, S. 264. 



