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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.
This experiment involves the titration
of an acid with a base. As the titration proceeds, the hydronium
ions,
H3O
+
(aq)
, will be neutralized by the hydroxide
ions, OH
-
(aq)
, from the NaOH
(aq)
added to the solution. The
change in concentration of H3O
+
(aq)
during titration will be monitored
using a pH meter. The pH of a solution
is a measure of the [H3O
+
] in solution and is defined as
pH = - log [H3O
+
]
(2)
In an acid-base titration, the change
in the concentration of H3O
+
may be very large, for example,
from
1 M to
1 x 10
-12
M. This
represents a trillion-fold
change in concentration and it would be quite inconvenient to
