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G-1
EXPERIMENT G: pH TITRATION CURVES
OBJECTIVES
The dissociation of an acid (HA) in water can be represented as
HA
(aq)
+ H2O
(l)
H3O
+
(aq)
+ A
-
(aq)
(1)
In this experiment you will determine the extent of acid dissociation by measuring the concentration of a
product, the hydronium ion. To do this, you will learn how to use a pH meter, and how to carry out an acid-
base titration, which will allow you to determine the unknown concentration of a weak acid. With the data you
will collect in this experiment, you will carry out volumetric analyses and construct a titration curve. You will
obtain a titration curve for a weak acid (extent of dissociation is small). By comparing this curve to that for a
strong acid (extent of dissociation is large), you will identify differences between the behaviors of solutions of
strong and weak acids. You will also learn how to determine the value for the dissociation constant of a weak
acid, Ka, from a pH titration curve. By examining the change of pH in the vicinity of the equivalence point of
the titration, you will learn how to select appropriate acid-base indicators.
THEORETICAL CONSIDERATIONS AND EXPERIMENTAL METHOD
I. Titration of an Acid with a Strong Base.
The chemistry of acids and bases is fundamental to many everyday processes, including the workings of your
body. For example, the carbonic acid - hydrogen carbonate equilibrium is primarily responsible for maintaining
a relatively constant pH level in your blood stream. For the purposes of Experiments I and J, we will use the
Brønsted-Lowry concept of acids and bases: acids are proton donors and bases are proton acceptors.
The Brønsted-Lowry definitions are not limited to aqueous solutions. Since we will always work with aqueous
solutions, our definitions are nearly the same as those of Arrhenius: acids produce protons when dissolved
in water, and bases produce hydroxide ions when dissolved in water. The three acid-base definitions
and examples of important acid-base volumetric calculations can be found in your textbook.
