24 METHODS FOR THE ANALYSIS OF URINE
I Examyk.
I 0.2 N HC1 used in titration............................. '.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.
| i Corrected titration figure=7.6-1.9........................ 5.7
i 1 0. IN organic acid per liter=80X5.7....................... 456.0
|! Remarks.—Tropaeolin 00 is preferred as indicator for the final
| j end-point. In neutral solution it gives nearly the same yellow
; * color as urine, but so much more intense that a water solution of
I j 0.002 N hydrochloric acid with the indicator can be used as a color
' J 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
i I titration the solution becomes redder than the standard. This
] \ particular advantage is possessed in much less degree by the three
j » 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 fromyellow to red, but the bromophenol blue turns from blue to a clear yellow on acidifying, and
j tj affords a very different alternative. The tropaeolin 00 end-point
I * 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 sulf ates 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 hydrogen ion concentration. The organic acids known to occur in normal and pathological urines, in amounts sufficient to be quantitatively significant in the total acid excretion of the body, belong to the class of weak acids whose salts behave in the above manner.
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