I-1
Experiment 1
Acid-Base Titrations
Discussion
Volumetric procedures are among the most common and convenient methods of analysis. The
preparation of a reactive solution of accurately known concentration is fundamental to these
methods, and the exercise serves as an introduction to the techniques of solution preparation and
titration.
The objective of this exercise is to prepare and accurately determine the concentration of a solution
of NaOH, and to use that standardized solution in the determination of the concentration of acid in a
commercially available sample. The first step is the preparation of a sodium hydroxide solution
whose concentration is approximately known. The second step is the determination of the
concentration accurately by titration with a solution containing a known concentration of a primary
standard, in this case potassium hydrogen phthalate. (Primary standards are substances which
may be obtained in a stable form of known purity and which react with other substances quickly in
a definite and known manner.)
Potassium hydrogen phthalate and sodium hydroxide react as follows:
KHC8H4O4 + NaOH → KNaC8H4O4 + H2O
or, expressed as an ionic equation:
HC8H4O4– + OH– → C8H4O42– + H2O
The complete structure of potassium hydrogen phthalate is
This reaction is a representative of an acid-base reaction. In this case the hydrogen phthalate ion is
the acid (proton donor) and the hydroxide ion is the base (proton acceptor). The stoichiometric
ratio between the hydrogen phthalate ion and the hydroxide ion, and therefore between the
potassium hydrogen phthalate and the sodium hydroxide, is obviously one to one.
The equivalent weight of an acid (also called an equivalent) is the weight in grams that
furnishes one mole of H+ ion in an acid-base reaction. Correspondingly, the equivalent weight
of a base is the weight in grams that accepts one mole of H+ ion in an acid-base reaction. (In the
reaction under consideration, what is the equivalent weight of the potassium hydrogen phthalate and
the sodium hydroxide?)
CH
CH
CH
C
C
CH
C
C
O
OH
O
O-K+or
COOH
COO-K+
