182 PHYSIOLOGICAL CHEMISTRY 



line reaction of urine. Ingesting organic acids such as citric, 

 malic, etc., which are burned to carbonates, actually increases 

 the alkalinity of the urine, thus bearing out the above con- 

 clusions. 



As might be expected, the total titratable acidity of the urine 

 varies greatly. An average 24 hour specimen on a mixed diet 

 will require 150-400 c.c. N/10 alkali to neutralize it, using 

 phenolphthalein as indicator. These figures may be exceeded in 

 either direction. 



There appear to be no consistent and characteristic variations 

 in acidity in disease. 



The hydrogen-ion concentration of urine may be determined 

 by the gas chain method, or by the use of a series of indicators 

 such as that worked out by Henderson. The average value is 

 NX10- 6 , but it ranges between NXlO' 5 ' 1 and NxlO" 7 - 4 . 



/NH 2 



Urea. 0=0 . By far the largest part of the nitrogen 



\NH 2 



excreted by the body is in the form of urea. Other nitrogenous 

 substances are uric acid, ammonia, creatinine and creatine, hip- 

 puric acid, allantoin, amino acids and a variety of other sub- 

 stances. Of the total nitrogen contained in all these substances, 

 urea contains on an average 85-90%. On a high protein diet 

 this percentage may increase, and on a low protein diet it may 

 fall even as low as 60%. Also the total amount of urea varies 

 with the amount of protein in the food. A variation of from 

 8 or 10 grams to 30 grams per day will cover most cases. 



Urea is a colorless, odorless compound which crystallizes in 

 long needles. It is almost tasteless, but produces a sensation of 

 coolness when placed on the tongue. It is readily soluble in 

 water and alcohol but insoluble in ether. With nitric or oxalic 

 acid, it forms urea nitrate or oxalate respectively, one molecule 

 of nitric uniting with one of urea, one of oxalic with two of 

 urea. These salts crystallize easily and are useful in the isola- 

 tion and purification of urea. 



On heating dry urea, it decomposes, forming biuret, the sub- 





