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IONS AND SALT ACTION. 75
with alkaline reaction. Complete investigations on this subject have
been carried out by Sorensen.1
He found the value 10
~ 14,14
for K
at 18° C. Ch is determined in either of two ways. The best method,
the electromotive, is based upon the electromotive force of gas chains,
as developed by Nernst;2
namely, if platinum foil covered with
platinum black is introduced into a watery solution and this saturated
with hydrogen, then a difference of electrical potential is produced between
the platinum and the solution and this potential is theoretically propor-
tional to the concentration of the hydrogen ions in the solution. We
cannot give any further detail as to this theory or to the performance
of the measurement of the difference in potential.3
If the concentration
of the hydrogen ions Ch is expressed in gram ions per liter by the figure
10~p
, then according to the suggestion of Sorenson the name hydrogen
ion exponent and the symbol Pr- is used f°r the numerical value of the
exponents of this potence. The relationship between pH and the electro-
motive force -k at the contact between the platinum and the solution
can be expressed graphically by a straight line; hence it follows that if
r is known then / H can be very easily found (the exponential line).
The other method used by Sorensen 4
for the determination of CH
is a colorimetric method and depends on the use of indicators. After
much investigation 20 indicators are recommended, of which certain ones
require strictly fixed methods of use. As soon as more than a qual-
itative approximation is required then the shade of color produced by
the indicator must be compared with a shade of color produced by the
same indicator in a solution of known concentration of H ions. Such
standard solutions which allow of a variation in the concentration of the
H ions at one’s pleasure have been given by Sorensen, and the original
article gives a table of curves from the corresponding value for pH which
can be read off, when the composition of a standard solution is known. The
figure j H for the standard solutions is determined by aid of the electro-
motive method. Standard solutions are selected so that they serve as
natural protectors against too sudden changes in pn (so called buffer)5.
As above stated the dissociation constant according to Sorensen
for water is 10
-1414
at 18° C. or CH- Coh = 10~ 1414
. In neutral reac-
tion CH = C H and therefore CH = 10~7-07
or pn = 7.07. Smaller values
for ;>h correspond to acid and greater values to alkaline reaction.
Hasselbach 6 has suggested a modification of Sorensen’s method
1
Bioch. Zeitschr., 21, 131 (1909) also Ergebn. d. Physiol. Vol. 11.
2
Zeitschr. f. physik. Chem., 4, 129 (1889).
3
In regard to the determination see the work of Sorensen cited on page 74.
4 Sorensen, Enzymstudien, Bioch. Zeitschr., 21, 253.
5
Ibid., 167.
6
Bioch. Zeitschr., 30, 317 (1910).
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