Titration of organic acids in urine

Abstract

Carbonates and phosphates are removed by adding 2 grams of calcium hydroxide to 100 c.c. of urine, and filtering after 10 minutes. 25 c.c. of the filtrate is brought to a pH of approximately 8 by adding 0.2 _N HCl with phenolphthalein as indicator, till the pink color disappears. Then 5 c.c. of 0.02 per cent. Tropeolin 00 solution are added, and the solution is titrated to a pH of 2.7 with 0.2 _N HCl, the volume being brought to approximately 50 c.c. by addition of water towards the end of the titration. The color is compared with that of 50 c.c. of a control solution with the same pH and indicator. The amount of 0.2 _N HCl required to give the endpoint with a control in which water replaces the urine is subtracted. Of the organic acids known to be present in urine in quantitatively significant amounts, the titration measures from 93 to 100 per cent. of each. It also includes very weak bases, but apparently of this class of substances only creatine and creatinine are significant; they are titrated to nearly 100 per cent. The titration figure, corrected for the amounts of these two bases, represents the organic acids.