CHAPTER VIII
HYDROGEN ION CONCENTRATION
In the chapter on indicators, in Part I, it was noted that the
color change of the indicator bears a definite relation to the
changing hydrogen ion concentration in the solution, and that
upon this consideration rests the suitability of an indicator
for a given titration. The investigation of hydrogen ion con-
centration must necessarily precede this quantitative knowledge
of color changes and such investigations may be made with a
high degree of accuracy.
Methods.—A number of methods have been used for the
determination of hydrogen ion concentration. Of these, two
will be mentioned. These are the potentiometer method and the
indicator method. The manipulative details, necessary prc-
cautions and the sources of error of these determinations lie
outside the scope of this book. The brief discussion hero inter-
posed is provided in order to give the, student an idea of the
general principles involved in the laboratory methods and of the
importance of such measurements to the problems of the agri-
cultural chemist. For a full discussion and detailed directions
for the determinations, refer to the numerous papers in the
journals and to Clark's book, "The Determination of Hydrogen
Ions." In this book is a tolerably complete bibliography of the
various papers that have appeared on the subject.
The Potentiometer Method.—In principle, this method
depends upon measuring the electromotive force of a system in
which are placed (a) a hydrogen electrode immersed in a solution
of known hydrogen ion concentration, (/>) a hydrogen electrode
surrounded by the solution whose Pu value in to be measured and
(c) a potentiometer. From the measured o.m.f. of the system
and the known hydrogen ion concentration around the standard
electrode, the ion concentration in the unknown solution in
calculated, usirg the equation:
4 c.m.f. ••- 0.059 log (,,
'/I
' M where (' and (•' represent the hydrogen ion concent rations in the
111 two solutions.