262 PRACTICAL PHYSIOLOGY 



The contrary condition is also true, namely, when there is an 

 excess of alkali ingested, either in the food or otherwise, ammonia 

 may practically disappear from the urine. This diminution in the 

 output of ammonia is marked when, for instance, vegetable food- 

 stuffs predominate in the diet. 



Estimation of Total Acidity and Ammonia in Urine. EXPERI- 

 MENT. Weigh out roughly 15 grms. powdered potassium oxalate 

 (neutral to phenolphthalein), place in a flask, and add from a pipette 

 25 c.c. urine and an equal volume of water. Add about 10 drops 

 1 per cent, alcoholic phenolphthalein. Mix and allow to stand for 



about a minute. Now run in caustic soda from a burette until the 



contents of the flask assume a slight pink tint. Read the burette. 

 Measure into a small beaker 5 c.c. formalin (40 per cent, formal- 

 dehyde) and roughly 5 c.c. water, and add a few drops of phenol- 

 phthalein solution. Run in - caustic soda till a slight pink 



colour is attained. -Add this mixture to the flask containing the 



N 

 neutralised urine. The pink colour disappears. Run in caustic 



soda until the colour returns, and take the reading. 

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



urine in terms of soda. Potassium oxalate is added to pre- 

 cipitate 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, 

 urotropine, thus liberating the acid which it previously neutralised. 

 The second titration, therefore, determines the amount of ammonia 



N 

 present in terms of soda. To calculate the amount of nitrogen 



in grammes present as ammonia in the volume of urine taken 

 multiply the reading of this titration in c.c. by 0-0014. 



This 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 ammo acids, which are 

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

 acid to phenolphthalein. 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 (see p. 286). 



