24 METHODS FOR THE ANALYSIS OF URINE 



Example. 



0.2 N HC1 used in titration 7.6 c.c. 



Correction found in blank analysis 0.7 c.c. 



Creatinine correction for 500 mg. creatinine N 

 per liter urine. Correction = 500 c.c. = 1 . 2 c.c. 



Total correction 1.9 c.c. 



Corrected titration figure = 7.6 1.9 5.7 



0.1 N organic acid per liter = 80X5.7 456.0 



Remarks. Tropaeolin 00 is preferred as indicator for the final 

 end-point. In neutral solution it gives nearly the same yellow 

 color as urine, but so much more intense that a water solution of 

 0.002 N hydrochloric acid with the indicator can be used as a color 

 standard. Very dark urines may need greater dilution. Another 

 advantage of this indicator is that its maximum acid color is not 

 reached even at pH 2.7, so that if too much HC1 is added in the 

 titration the solution becomes redder than the standard. This 

 particular advantage is possessed in much less degree by the three 

 indicators mentioned below as alternatives. 



Other indicators that may be used are methyl orange, tetra- 

 bromophenolsulfonephthalein (bromophenol blue, Clark and 

 Lubs), and dimethylaminoazobenzene. To some eyes the color 

 change of one of these dyes may be more readily detected than that 

 of tropaeolin 00. The two azo dyes are not much different in color 

 from tropaeolin 00, both changing from yellow to red, but the bromo- 

 phenol blue turns from blue to a clear yellow on acidifying, and 

 affords a very different alternative. The tropaeolin 00 end-point 

 appears, however, the most satisfactory. 



Theoretical Basis of Method. The method is based on the 

 following previously known facts: 



1. Relatively little strong mineral acid is required to change 

 the hydrogen ion concentration of a water solution from 10" 3 

 to 2X10~ 3 if the only electrolytes present are alkali salts of strong 

 acids, such as sulfates and chlorides. 



2. If the salt of a weak acid is present, however, the addition 

 of nearly a full molecule of hydrochloric acid for each molecule of 

 such salt is necessary in order to cause the above change in hydro- 

 gen ion concentration. The organic acids known to occur in 

 normal and pathological urines, in amounts sufficient to be quan- 

 titatively significant in the total acid excretion of the body, belong 

 to the class of weak acids whose salts behave in the above manner. 



