PHYSIOLOGICAL CHEMISTRY 431 



This determination gives the creatin and creatinin together, as creatin is con- 

 verted by the acid into creatinin. The difference between this result and that for 

 creatinin previously determined represents the amount of creatin present. 



Ammonia. In metabolism ammonia is the precursor of urea, being 

 transformed to urea by the liver. The transformation to urea is never 

 absolutely complete, so that urine always contains ammonia. In man 

 under ordinary conditions the output of nitrogen as ammonia varies 

 between 3 and 5 per cent, of the total nitrogen. When measured in 



N 

 terms of the equivalent amount of ^K alkali, the ammonia is roughly 



equal to, or rather less than, the total acidity of the urine. When the 

 nitrogen of the diet is low, the ammonia of the urine is decreased in 

 absolute amount, but forms a greater percentage of the total nitrogen 

 (see page 415). 



The principal cause of a rise in the ammonia of the urine is the 

 presence of acids, which require neutralisation, and so prevent the con- 

 version of some ammonia into urea, the amount of ammonia rising more 

 rapidly than the total acidity. From this cause the ammonia nitrogen 

 may rise in pathological cases till it forms 40 per cent, of the total 

 nitrogen. A similar effect is produced in dogs (carnivora) by the 

 administration of hydrochloric acid. On the other hand, hydrochloric 

 acid given to rabbits (herbivora) causes only a slight rise in the ammonia 

 excretion. These animals rely on mineral bases to neutralise the acid, 

 with the result that they are much more easily poisoned with mineral 

 acid than dogs are. Similarly, ammonium chloride given to man and 

 to dogs is mainly excreted unchanged, because the hydrochloric acid of 

 the salt prevents the conversion of ammonia to urea, which takes place 

 when ammonium carbonate is given ; on the other hand, a large pro- 

 portion of the ammonia of ammonium chloride given to rabbits is 

 converted to urea. 



EXPERIMENT VIII. Estimation of Total Acidity and Ammonia in 

 Urine. Weigh out roughly 15 grm. powdered potassium oxalate (neutral 

 to phenol phthalein), 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 

 phenol phthalein. Mix and allow to stand for about a minute. Now 



run in y^ 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, formaldehyde) 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 



