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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 |-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 1
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.
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