1176 PHYSIOLOGY 



sulphuric acid and can be distilled off and received into a vessel containing a known 

 amount of decinormal acid. By titrating this acid after the operation we can determine 

 the quantity of ammonia which has been produced. To carry out this method 5 c.c. 

 of urine are heated with 20 c.c. sulphuric acid and a small quantity of copper sulphate 

 and potassium sulphate. The copper sulphate is to aid the oxidation of the organic 

 substances, the potassium sulphate is to raise the boiling-point of the mixture. The 

 boiling is continued for half an hour. The flask is then cooled and half filled with dis- 

 tilled water. A special form of distillation tube (Fig. 540) is now attached by a rubber 

 cork which fits tightly, but just before this is done an excess of strong caustic soda 

 sufficient to neutralise the concentrated sulphuric acid is run in under the acid. The 

 other end of the distillation tube is at once arranged to dip under the surface of a 



measured quantity of standard acid (e. g. 10 c.c. H 2 S0 4 ), diluted with water, and con- 

 tained in a 600 c.c. Erlenmeyer flask. The flask is then shaken and heated. In about 

 a quarter of an hour the ammonia is completely distilled off, and its amount can be 



determined by titrating the acid in the flask with NaOH, methyl orange being used 

 as indicator. 



UREA. The method usually adopted for estimating the urea is that devised by 

 Hiifner. It depends on the fact that urea is decomposed by an alkaline hypobromite 

 with the production of C0 2 and nitrogen. In the presence of an excess of alkali the CO 2 

 is absorbed, and the nitrogen may be collected and measured, and serves as an index 

 of the amount of urea present. The reaction which occurs is as follows : 



CO(NH 2 ) 2 + SNaBrO -f 2NaOH = SNaBr -f N 2 -f Na 2 CO 3 -f 3H 2 O. 



60 gnn. 22-4 litres = 28 grm. 



1 grm. 372 c.c. 



Actually however only 354-33 c.c. nitrogen are evolved by 1 grm. urea. 



The disadvantage of this method is that other substances, such as ammonia, creati- 

 nine, and uric acid, give off a certain amount of their nitrogen with sodium hypobromite, 

 so that the method is not strictly accurate, though enough so for most clinical purposes. 

 In actually carrying out the method 5 c.c. cf urine are treated with 25 c.c. of freshly 

 prepared solution of sodium hypobromite, and the nitrogen evolved is collected in a 

 graduated tube over water. 



Urease Method. A still simpler method is to employ urease, a ferment contained 

 in soy bean, which splits urea with hydrolysis into ammonia and carbonic acid. Five c.c. 

 of urine with 25 c.c. of water, and half a grm. of powdered soy bean are placed in a 

 cylinder, which is kept at about 40 C. Air is drawn through the mixture and then 



through 25 or 50 c.c. of sulphuric acid for half to one hour. One gnn. anhydrous 



sodium carbonate is then added to break up any ammonium salts, and air drawn through 

 as before for another half hour. Titration of the acid then gives the amount of ammonia 

 liberated, from which, after subtraction of the ammonia originally present in the urine, 

 the percentage of urea may be calculated. 



Folirfs Method. In Kjeldahl's method all the nitrogenous constituents of the urine 

 are converted into ammonia by boiling with strong sulphuric acid. This conversion 

 occurs with extreme readiness in the case of urea, so that by using a weaker acid and 

 carefully regulating the temperature the hydrolysis may be confined practically to the 

 urea itself. This is the principle of FolhYs method of estimating urea. 



Five cubic centimetres of urine are measured into a 200 c.c. Erlenmeyer flask. Five 

 cubic centimetres of concentrated hydrochloric acid, 20 grm. crystallised magnesium 

 chloride, a piece of paraffin the size of a small hazel nut, and finally 2 or 3 drops of a 

 1 per cent, solution of alizarin red in water are added. A special safety tube is then 

 inserted into the neck of the flask and the mixture boiled until each returning 

 drop from the safety tube produces a very perceptible bump. The heat is then 



