501 



UREA. 



UREA. 



603 



roasting operation : the cyanogen of the ferrocyanide was, however, 

 also oxidised at the same time and by the same means to cyanic acid, 

 which remains in solution combined with ammonia. Evaporated to 

 dryness over a water-bath the solution of cyanate of ammonia yields 

 dry urea, which is separated from residual sulphate of potash by 

 digesting in hot alcohol, containing not more than 10 or 15 per cent, of 

 water. The alcoholic solution yields, on cooling, colourless crystals of 

 urea to the extent of 33 per cent, of the ferrocyanide of potassium 

 used. Should the solution of sulphate of potash have a yellow colour, 

 it will be due to undecomposed ferrocyanide of potassium. In that 

 case sulphate of iron must be added till prussian blue no longer pre- 

 cipitates, and any excess of iron got rid of by carbonate of ammonia. 



Pure urea crystallises in colourless, inodorous, transparent, four- 

 sided prisms. It has a saline taste somewhat like that of nitrate of 

 potash. It is soluble in its own weight of water, and requires five 

 parts of cold alcohol, but only one part of boiling alcohol, to dissolve 

 it : it is less soluble in ether, and insoluble in oil of turpentine. 

 Exposed to the air it becomes somewhat moist ; melts when heated to 

 near 250 Fahr., and at higher temperatures yields ammonia, carbonate 

 of ammonia, BICRET and MELANUEIC Aero. Solution of urea is stable 

 if pure, but in urine, or in the presence of any decomposing organic 

 matter, the urea rapidly changes into carbonate of ammonia : \ 

 C 8 H,N a O, + 4HO = 2(NH,0, CO 2 ) 



If a sulphocyanate be used, a sulpho-urea will of course be pro- 

 duced : 



Urea. 



Carbonate of ammonia. 



The same change occurs if solution of urea be heated to 284 Fahr. in 

 a sealed tube. Chlorine produces cyanuric acid from fused urea, but 

 quite breaks up urea in solution : nitrous acid also oxidises it to car- 

 bonic acid, nitrogen and water. 



Urea combines with acids to form definite salts. The formation of 

 nitrate of urea (CjH.N^O., HO, N0 5 ) has already been described ; it 

 crystallises in bright transparent rhombic plates or leaflets, only slightly 

 soluble in water, alcohol, or nitric acid. HydrocMarate of urea (CjH, 

 N S S , HC1) is a deliquescent unstable salt, formed when hydrochloric 

 acid is passed into urea. A mbhydrochlorate (2C J H i N,0 J , HC1) crys- 

 tallises in long transparent blades from a solution containing the proper 

 equivalents of each compound. Oxalate of urea (C t H t ~S t O v HO, C 2 3 ) 

 falls as a crystalline, almost insoluble, precipitate when excess of strong 

 solution of oxalic acid is added to solution of urea ; it may be obtained 

 in long, thin, colourless, transparent prisms. It is probable that 

 ALLOPHANIC ACID is carbonate of urea. 



Urea also combines with metallic protoxides. If freshly precipitated 

 oxide of silver be digested in a strong solution of urea, a yellowish 

 crystalline powder is formed containing (3 AgO, C 2 H,N a 2 ). With the 

 red oxide of mercury urea forms compounds containing respectively 

 two, three, and four atoms of mercuric oxide to one of urea. The first 

 (2 HgO, C 2 H.N 2 2 ) separates out as a white powder from a solution of 

 moist mercuric oxide in urea ; the second ( 3 HgO, C 2 H,N,O a ) is a 

 gelatinous precipitate produced when solution of mercuric chloride is 

 added to solution of urea kept alkaline by potash : it becomes granular 

 and of a pale yellow colour on boiling ; the third (4 HgO, C,H 4 N,0,) 

 is prepared in the same manner as the second, mercuric nitrate being 

 used instead of chloride. 



Urea forms compounds with metallic salts. Solutions of urea and 

 nitrate of silver, mixed and evaporated in vacuo over sulphuric acid, 

 yield crystals containing (AgO, N0 5 , C 2 H,N 2 O a ) and ( (2 AgO, NO S ), 

 C,H 4 NjO 2 ). A dilute and warm solution of mercuric nitrate added to 

 a warm solution of urea, gives a granular precipitate containing (4 HgO, 

 N0 5 , CjH,N a O a ) ; if the mercuric nitrate be added in excess, and the 

 whole maintained at a temperature ranging from 104 to 122 Fahr., 

 the precipitate occurs in six-sided prisms, and has the composition 

 (3 HgO, N0 6 , C a H 4 N a O 2 ) ; if the mercuric nitrate be acidulated with 

 nitric acid, and solution of nitrate of urea be added until a precipitate 

 occurs, the filtered solution deposits shining rectangular tables of 

 (2 HgO, N0 5 , C 2 H 4 N a Oj). Double nitrates of urea and soda, baryta, 

 lime, and magnesia have also been obtained. 



Compound ureas. As already indicated, urea is formed by combina- 

 tion of ammonia (NH 3 ) and cyanic acid (C,HN0 2 ) : 



NH 3 + C,(H)NO a = C,(H.)N.O^ 



Ammonia. Cyanic acid Urea (ordinary), 



(cyanate of hydrogen). 



If in the place of cyanic acid or cyanate of hydrogen as, for the sake 

 of analogy, it may be more conveniently termed the cyanate of some 

 radical, other than hydrogen, be taken, an urea will be obtained con- 

 taining the radical of the cyanate used, in the place of one of the four 

 equivalents of hydrogen in common urea. Thus : 



MIj + C,(C,H S )5IO. = C,(C a H 3 , H 3 )N a O i 



NH, 



Cyanate of methyl. 

 C a (C.H s )NO a 



Cyanate of allyl. 

 i C 1 (C 11 H t )KO. j 



Cyanate of phenyl. 



Methyl-urea. 



Allyl-urea. 

 (C,,H S , H 3 )N,0, 



NH 3 



NH, 



C 2 (C 6 H 5 )NS 2 = 



.;, H 3 )N 2 S S 



Sulphocyanate Sulphallyl-urea 



of allyl. (thiosinnamine). 



C a (C 12 H 5 )NS 2 = C,(C 1 ,H s ,H 8 )N !1 S i 



Sulphocyanate Sulpho-phenyl-iirea. 



of phenyl. 



And if now in the place of ammonia itself, a derivative of ammonia 

 (monamines, &c.) be acted upon by the cyanate, ureas are formed in 

 which two, three, or even the four equivalents of hydrogen in common 

 urea, are replaced by compound radicals. Thus : 



NII 2 ,C 4 U 5 + C 2 (C 4 H 5 )N0 2 = C S (2C,H 5 , H a )N,O, 



Piperidine. Cyanate of ethyl. Ethyl-piperyl-urea. 



N(C.H a ) s + CjtC.HONO, = C,(C t H 5 ) t N,0, 



' Triethylamine. Cyanate of ethyl. Tetrethyl-urea. 



Compound ureas are also prepared by the action of water on cyanates 



of radicals. Thus : 

 2C,(C 2 H 3 )N0 2 + 



2HO 



SCO, ' + C 2 (2C 2 H 3 , 



PhenyUurea 

 (carbanilamide). 



Cyanate of methyl. Water. Carbonic acid. Dimethyl-urea. 



2C a (C,H B )NO > + 2HO = 2C0 2 + C 2 {2C,H a , H 2 )N 2 O S 



Cyanate of allyl. Diallyl-urea (siuapoline). 



The hydrogen in urea, at least one equivalent of it, may also be 

 replaced by negative radicals. The general method of formation of 

 these ureides, so called in allusion to their amidic analogy, is the reac- 

 tion of urea on the chloride of the negative radical, hydrochloric acid 

 being eliminated. Thus are obtained : 



Acetyl-urea or acetureidc . . C,(C 4 H 3 O S> H 3 )N 2 O 2 



Butyryl-urca or butyrureide . . C 2 (C 8 H,O 2> H 3 )N 8 O a 



Valeryl-urea or valerureide . . C 2 (C 10 H 9 8> H 3 )N 2 O a 



Benzoyl-urea or benzureide . . C 2 (C 14 II 5 O 2I H 3 )N 2 O 2 



Constitution of urea. Urea is now generally considered to be a 

 diamine ; that is, diammonia, in which two equivalents of hydrogen 

 are replaced by the diatomic radical carbonyl (C 2 2 ") : 



C 2 <V 

 H a 

 H, 

 Obviously,.any of the compounds already described in this article 



could be formulated on the typical diagram N, < R R , R being any 



( R R 



radical ; the formula of several ureas will, in fact, be found so written 

 in the article ORGANIC BASES. 



Estimation of urea. The amount of urea in a specimen of urine is 

 often required to be known by the physiologist and pathologist. It 

 was originally determined in the state of nitrate or oxalate, a known 

 quantity of urine being taken and treated in the manner already indi- 

 cated ; results so obtained, however, have been shown to be very 

 inaccurate. Ragsky and Heintz proposed to take advantage of the 

 facts, that urea is decomposed into ammonia and carbonic acid when 

 boiled with strong acids, and that ammonia may be estimated by bichlo- 

 ride of platinum, the weight of which would indicate the urea present 

 in the quantity of urine acted upon ; this method, however, is not 

 universally applicable. Bunsen takes a weighed quantity of urine, 

 mixes it with an ammoniacal solution of chloride of barium, seals it 



