432 PRACTICAL PHYSIOLOGY 



to the flask containing the neutralised urine. The pink colour dis- 



N 

 appears. Kun in y^ caustic soda until the colour returns, and take 



the reading. 



The first reading of the burette gives the total acidity of 25 c.c. 



N 

 urine in terms of y^ soda. Potassium oxalate is added to precipitate 



the calcium in the urine as calcium oxalate, as the formation of calcium 

 phosphate would otherwise interfere with the end-point. 



On the addition of neutral formaldehyde the ammonia in the urine 

 combines with the formaldehyde forming a neutral compound, urotro- 

 pine, thus liberating the acid which it previously neutralised. The 

 second titration, therefore, determines the amount of ammonia present 



N 

 in terms of y^ soda. To calculate the amount of nitrogen in grammes 



present as ammonia in the volume of urine taken multiply the reading 

 of this titrabion in c.c. by O0014. 



The method of determining ammonia is of sufficient accuracy for 

 clinical purposes. The amount of ammonia is always higher by this 

 method than by the more accurate methods which follow. This is 

 due to the fact that formalin combines with amino acids, which are 

 normally present in urine in minute traces, and thus renders them 

 acid to phenol phthalein. This source of error is small, unless amino 

 acids are present in excessive amount, as in cystinuria. The difference 

 between the result of this method and that of one of the methods 

 which follow affords a measure of the amount of amino acids present 

 in the urine. 



Estimation Of Ammonia. Folin's Method. Ammonia is liberated from 

 its salts when a solution of the latter is made alkaline with a soluble carbonate, 

 which, unlike a hydroxide, does not decompose the other nitrogenous bodies. By 

 bubbling a fast current of air through such a mixture the ammonia is carried 

 away and may be collected and measured by passing this air through standard 

 acid. Excess of sodium chloride in the mixture not only preserves it against 

 putrefaction, but encourages the evolution of the ammonia. 



The technique of the method is as follows : 25 c.c. urine are placed in a large 

 test tube a (Fig. 247) 2-3 cm. diam. and 20-30 cm. long), and mixed with 8-10 

 grms. sodium chloride and 5-10 c.cm. petroleum (to prevent excessive frothing), 

 and lastly with 1 grm. sodium carbonate. The test tube is closed by an india- 

 rubber stopper through which pass two tubes, the one for the air inlet passing to 

 the bottom of the test tube, the other connecting the top of the test tube with a 

 wide tube (U tube) 6 containing a loosely packed cotton-wool plug (to catch any 

 particles of fixed alkali which might be sucked over with the air current). 

 This safety tube is connected with a second test tube c (of the same size as the 

 first) containing 15 c.c. N/10 H 2 S0 4 + 5 c.c. water, the tubing being so arranged 

 that the air bubbles through the acid. A third tube or bottle d, arranged in the 



