ACIDITY OF THE URINE. 677 



We now determine the acidity simply by acidimetric methods, titrat- 

 ing with N/10 caustic alkali, using phenolphthalein as an indicator 

 (NAEGELI, HOBER, FOLIN). On account of the color of the urine and the 

 presence of ammonium salts and alkaline earths, this method cannot 

 yield entirely exact results. The greatest error is due to the alkaline 

 earths, which, on titration with caustic alkali, precipitate as earthy 

 phosphates in variable amounts and of variable composition. This 

 error can be prevented, according to FOLIN, by the addition of neutral 

 potassium oxalate, which precipitates the lime, and in this way the dis- 

 turbing action of the ammonium salts is also inhibited. Perfectly 

 accurate results are not obtained by this method, but it is the best of 

 those which have been suggested. 



It is performed as follows: 25 cc. of urine are placed in an Erlenmeyer 

 flask (about 200 cc. capacity), treated with 1-2 drops of J-per cent 

 phenolphthalein solution, and shaken with 15-20 grams of powdered 

 potassium oxalate and immediately titrated with N/10 caustic soda 

 with constant shaking until a pronounced pale-rose color appears. 

 VOZARIK l titrates the diluted urine without the addition of oxalate 

 and uses phenolphthalein as indicator. 



The acidity, as determined by titration, varies considerably under 

 physiological conditions, but calculated as hydrochloric acid it amounts 

 in man to about 1.5-2.3 grams in the twenty-four hours. 



By titration we learn the amount of hydrogen present which can 

 be substituted by a metal, i.e., the acidity in the ordinary older sense, 

 but not the true acidity, the ion acidity, which is given by the concentra- 

 tion of the hydrogen ions of the urine. For similar reasons, as previously 

 indicated in treating of the alkalinity of the blood-serum (page 272), 

 the ion acidity cannot be determined by titration, while it can be deter- 

 mined according to the principle of the electrometric gas-chain method 

 as there given. Such estimations have been made by v. RHORER and by 

 HOBER. For normal urine v. RHORER found as a minimum 4X10"" 7 , 

 as a maximum 76X10"" 7 , and as an average 30X10" 7 . HOBER found 

 4.7X10" 7 , 100X10" 7 , and 49X10" 7 , respectively. On an average the 

 urine therefore contains 30-50 grams of hydrogen ions in 10 million 

 liters. HENDERSON 2 has obtained much lower values, namely 10.10"" 7 

 as the average of 50 investigations, and has rather great differences 

 for different persons. From the comparative estimation of the titration 



1 In regard to the degree of acidity and its estimation see Naegeli, Zeitschr. f . 

 physiol. Chem., 30; Hober, Hofmeister's Beitrage, 3; Folin, Amer. Journ. of Physiol., 

 9; Vozarik, 1. c.; de Jager, Zeitschr. f. physiol. Chem., 55; and Ringer, ibid., 60; Grim- 

 bert and Morel, Compt. Rend., 154. 



2 v. Rhorer, Pfliiger's Arch., 86; Hober, 1. c. See also Jolles, Bioch. Zeitschr., 

 13; Henderson, Bioch. Zeitschr., 24. 



