QUANTITATIVE ESTIMATION OF UREA. 397 



with nitrogenous residues in the juices of the body, e.g., carbamic acid or cyanic acid. The 

 union of these may produce urea. According to Salkowski, feeding with these substances causes 

 the breaking up of the proper proteids of the body so as to provide the necessary components. 

 Schmiedeberg is of opinion that urea is formed in the body from ammonia carbonate by the 

 removal of water ; and v. Schroder found that, when he passed blood containing ammonia car- 

 bonate through a fresh liver, the urea in the blood was greatly increased. Drechsel succeeded 

 in producing urea at ordinary temperatures by the rapid alternating oxidation and reduction of 

 a watery solution of ammonia carbonate. [We know that the greater part of the urea exists in 

 the blood, and that the renal epithelium removes it from the blood. Although it is surmised 

 that some of the nitrogenous bodies named above, more especially leucin, and perhaps also 

 kreatin, are the precursors of urea, yet we cannot say definitely how or where the transformation 

 takes place. Perhaps this is effected in the liver, and, it may be, also in the spleen ( 193).] 



Preparation. Urea may be prepared from dog's urine (especially after a diet of flesh) by 

 evaporating it to a syrupy consistence, extracting it with alcohol, and again evaporating the 

 filtrate to a syrupy consistence. The crystals which p ^-* 



separate are washed with water to remove any extractives ^ *z=r~~^ {\ 



that may be mixed with them, and dissolved in absolute [J it^~W 



alcohol. It is then filtered and allowed to crystallise ,X ^^^ > vr^^^^^ 



slowly. Or, human urine may be evaporated to one-sixth Cy ^^^^ 



of its volume and cooled to 0, and excess of strong nitric i \ - Q /V 

 acid added, which precipitates urea nitrate mixed with \~\ 'w > ^ 



colouring-matter. This precipitate is pressed in blotting- ^\^ CI /^\ ^N/ ' 



paper, then dissolved in boiling water containing animal V^ ^W 4 \ V 



charcoal, and filtered while hot. When it cools, colourless ^s*r n \ } P"] 



crystals of urea nitrate separate (fig. 252). These crystals 



are redissolved in warm water, and barium carbonate added *.^/' ^ w^ 



until effervescence ceases ; urea and barium carbonate are w 



formed. Evaporate to dryness, extract with absolute ^.^ _ 



alcohol, filter, and allow evaporation to take place, when , l &' ' 



urea separates. Perfect crystals of oxalate of urea. 



Compounds of Urea. Urea combines with acids, bases, and salts. The follow- 

 ing are the most important combinations : 



1. Urea nitrate (CH 4 N 2 0, HN0 3 ) is easily soluble in water, and not so soluble in water con- 

 taining nitric acid. It forms characteristic rhombic crystals (fig. 252, 3, 4, 5, 6). Sometimes 

 the formation of these crystals is used to determine microscopically the presence of urea in a fluid. 

 If a fluid is suspected to contain minute traces of urea, it is concentrated and a drop of the fluid 

 is put on a microscopic slide. A thread is placed in the fluid, and the whole is covered with a 

 cover-glass. A drop of concentrated nitric acid is allowed to flow under the cover-glass, and after 

 a time crystals of urea nitrate adhering to the thread may be detected with the microscope. 



2. Urea oxalate (0H 4 N 2 0) 2 , C 2 H 2 4 + H 2 0, is made by mixing a concentrated solution of 

 urea with oxalic acid. The crystals form groups of rhombic tables, often of irregular shape. 

 It is only slightly soluble in cold water, and still less so in alcohol (fig. 253). 



3. Urea phosphate (CH 4 N 2 0, H 3 P0 4 ), forms large, glancing, rhombic crystals, very easily 

 soluble in water. It is obtained by evaporating the urine of pigs fed on dough. 



4. Sodic chloride + urea (CH 4 N 2 0, NaCl + H 2 0) forms rhombic, shining prisms, which are 

 sometimes deposited in evaporated human urine. 



5. Urea + mercuric nitrate is obtained as a white cheesy precipitate, when mercuric nitrate 

 is added to a solution of urea. Liebig's titration method for urea depends on this reaction 

 ( 257, II.). 



257- QUALITATIVE AND QUANTITATIVE ESTIMATION OF UEEA. I. The qualita- 

 tive Estimation of Urea. (1) It may be isolated as such. If albumin be present, add to the 

 fluid three or four times its volume of alcohol, and, after several hours, filter. Evaporate 

 the filtrate over a water-bath, and dissolve the residue in a few drops of water. 



(2) The crystals of urea nitrate may be detected microscopically (fig. 252). 



II. Quantitative Estimation. (1) Sodic hypobromite decomposes urea into C0 2 , H 2 0, and N. 

 On this reaction depends the Knop-Hiifner method of quantitative estimation. The N rises in 

 the form of small bubbles in the mixed fluid, while the C0 2 is absorbed by the caustic soda. 

 [The reaction is the following : 



N 2 H 4 CO + 3NaBrO = 3NaBr + C0 2 + 2H 2 + N. 

 The nitrogen is collected and estimated in a graduated tube, and the amount of urea calculated 

 from the volume of nitrogen. The uric acid is also decomposed, but that can be estimated 

 separately and a correction made. We may use the apparatus of Russell and West, or Dupre, 

 or that of Charteris (fig. 254).] 



[Ureameter. Make a solution of hypobromite of soda by mixing 100 grammes NaHO in 250 

 c.c. of water, and adding 25 c.c. of bromine. It is better to be made fresh, as it decomposes by 



