HYDROGEN ION CONCENTRATION

.139

In practice, corrections must be applied to the above formula
in order to account for certain effects not here discussed. It is
possible also to substitute for the standard hydrogen electrode
certain other well known forms of standard electrodes, such as
the "calomel" electrode, in which case the measured e.rn.f. has
to be corrected for the difference between the potentials of the
calomel and the hydrogen electrodes.

The Indicator Method.—This method may be used with
greater convenience and at less expense for equipment than the
potentiometer method, although it should be recognised that the
latter is the fundamental method and that the indicators must
themselves have been standardized, usually by the potentiometer
method. For the test, there must be provided a series of indi-
cators of which the color corresponding to a given hydrogen ion
concentration is known, and extending over a wide range of
PJI values. A set of "buffer" solutions is prepared, these being
solutions of certain salts or acids whose hydrogen ion concentra-
tion is definite and known and which are easily reproduced. By
matching the color produced when definite quantities of suitable
indicators are added to the solution under investigation, with
those produced by the same indicators and the various buffer
solutions, the P// value of the former is determined.

The indicator** listed below are the selection of dark and Lubs
and their preparation and use are described in detail by ('lark,
in his work above cited.

TAJBLK V.-—INDKJATOKH

Color dmngc, j
ne.id t

Common name of indicator

Thymol blue (acid range)............. .i Rod-yellow

Brom phenol blue*..................... j Yellow-blue

Methyl red........................... | Ked-yollow

Brom eratol purple................,...! Yellow-purple

Brom thymol blue. , . ,. ,.......!.,....! Yellow-blue,

Phenol red......................... . . | Yellow-red

Oresol red............................ Yellow-red

Thymol blue (hanic range). . , , ,....... , . j Yellow-blue

Cresol phthalein,.................... . | CoIorleHH-red

rung*'

K ()
S.2

2.8
4.0
0.0
o.x
7.0
X. -1
S.K
«.) (\
If 0