Experiment 3
CHM 280L
“Quantitative Analysis”
1
Experiment 3.
Potentiometric Titration and the Gran Plot
In this experiment, you will monitor the extent of a titration reaction by measuring the pH of the
solution.
You will also determine the acid dissociation constant (K
a
) of a weak acid by
constructing a Gran Plot.
ALWAYS WERE EYE PROTECTION IN THE LABORATORY
BE SURE YOUR LEGS AND FEET ARE COVERED
Titration of a Weak Acid with a Strong Base
HA
↔
H
+
+ A

Consider the dissociation of a weak acid (HA) in water to from H
+
and A

.
By definition, a weak
acid dissociates very slightly, therefore, an equilibrium is reached with most of the acid in the
HA form.
The extent of the reaction is described by the equilibrium constant, which in this case
is called the acid dissociation constant K
a
, and K
a
id typically very small.
Eq. 1
]
[
]
][
[
HA
A
H
K
a
−
+
=
This equation may be rearranged so that:
Eq. 2
]
[
]
[
]
[
−
+
=
A
HA
K
H
a
Next, if we consider the case where we titrate the weak acid with a strong base, NaOH, the acid
is neutralized upon the addition of base as follows:
NaOH + HA
→ H
2
O + Na
+
+ A

For this case, Gran showed in 1950 that [HA] and [A

] could be replaced with the volumes of
base added as follows:
Eq. 3
( )
add
add
eq
a
V
V
V
K
H
−
=
+
]
[
where V
add
is simply the total volume of NaOH added in mL at any point during the titration, and
V
eq
is the volume of NaOH added to reach the endpoint or equivalence point.
K
a
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View Full DocumentExperiment 3
CHM 280L
“Quantitative Analysis”
2
Multiplying both sides by V
add
gives the following equation:
Eq. 4
eq
a
add
a
add
V
K
V
K
V
H
+
−
=
+
]
[
Notice that this is the equation for a line!
If we prepare a plot with V
add
(xaxis) versus
[H
+
]V
add
(yaxis) we should get a straight line with slope = K
a
.
At the extrapolated xintercept of this plot, where [H
+
]V
add
= zero, V
add
= V
eq
.
So we can find
the amount of NaOH needed to reach the endpoint without detecting the endpoint precisely with
an indicator.
Part I. Standardization of NaOH
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 Spring '08
 Thummel
 pH, Weak acid, Gran Plot, CHM 280L

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