456 A TEXTBOOK OF PHYSIOLOGY 



In the first part of the process, a known quantity of the urine is- 

 heated in a long-necked, hard glass flask with strong sulphuric acid, 

 and a little of the sulphates of potassium and copper are added to 

 facilitate oxidation by raising the boiling-point of the acid. The 

 ammonia formed is combined with the sulphuric acid to form am- 

 monium sulphate. The heating is continued until the fluid becomes. 

 almost colourless. 



In the second part of the process, the excess of acid is neutralized 

 by strong alkali (40 per cent. NaHO), an excess of alkali being added. 

 Then the ammonia is distilled over into a standard acid solution 

 ( ^ HgSO^ containing an indicator such as methyl orange. The indi- 

 cator is required to show that there is sufficient acid present to 

 trap all the ammonia driven over. 



In the third part of the process, the excess of acid in the receiving 

 flask is ascertained by titrating with standard alkali (^ NaHO). 

 By subtracting this excess from the amount of standard acid originally 

 taken, the amount of acid combined with ammonia is found. Multi- 

 plying this amount by 0-0014, we arrive at the amount of nitrogen in 

 the sample of urine. 



Urea is the chief nitrogenous waste substance of the mammal. 

 Its formula is 



\NH 2 

 and it ma\' be regarded as carbonic acid (H 2 CO 3 ), or 



ro /OH 

 \OH 



in which the two hydroxyl groups (OH) have been replaced by 

 two aniine (NH 2 ) groups. It has the same empirical formula as 

 ammonium cyanate, from which body it was first prepared by 

 Wohler in 1828 



CONNH 4 -> CO 



the first synthesis of an organic substance out of inorganic material. 

 Carbamic acid 



\OH 



may be present in the urine as a salt, for example after experi- 

 mentally excluding the liver from the circulation. It is apparently 

 a precursor in the synthesis of urea (see p. 449). 



Pure urea consists of colourless elongated crystals. It is extremeh r 

 soluble in water, alcohol, and acetone, but insoluble in ether and 

 chloroform. It possesses the property of dissolving connective tissue, 

 and may be used in making teased preparations e.g., to isolate 

 muscle fibres. It is also a good solvent for uric acid. It combines 

 with acids to form salts, such as urea nitrate and urea oxalate 

 (Figs. 219, 220). 



