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EXPERIMENT 5
ACID/BASE TITRATIONS: TITRATION OF A DIPROTIC WEAK ACID WITH STRONG BASE
This is a group experiment. To save time, there will be only one unknown per group.
I. INTRODUCTION
Purposes of the experiment are:
•
to illustrate partial mechanization of the titration process,
•
to understand the titration curve for a diprotic weak acid,
•
to quantify the diprotic acid,
•
to quantify and compare dissociation constants calculated using exact and
approximate equations,
•
to compare hydrogen ion concentrations calculated using approximate and exact
equations.
Of particular interest is to understand errors associated with approximate calculations of
dissociation constants and hydrogen ion concentration.
II. BACKGROUND INFORMATION
There is a table of symbols at the end of this experiment. This experimental description
should be accompanied by a description of the constantflow titrant delivery system to be used in
this experiment.
A. Reactions and volume/concentration relationships
This experiment involves the titration of a diprotic weak acid alone or a diprotic weak acid
plus some of the conjugate base of the acid with a strong base (e.g. NaOH).
1. Diprotic acid alone
. The reaction of a diprotic weak acid with strong base (e.g. NaOH)
takes place in two steps as follows:
O
H
BH
Na
NaOH
BH
2
2
+
+
→
+
−
+
(Complete at 1
st
end point)
(1a)
O
H
B
Na
NaOH
BH
2
2
+
+
→
+
−
+
−
(Conj. base from 1
st
reaction)
(1b)
The relationship between the initial concentration of the weak acid, C
BH2
0
, the volume of the
sample, V
s
, the concentration of the base, C
b
, and volume of base required to reach the first end
point, V
ep1
, is given by
0
BH2
s
b
ep1
C
V
C
V
=
(
2
a
)
where the symbols are defined before the equation. In the absence of any conjugate base in the
sample initially, the same volume of titrant will be required to titrate the sample from the first to
the second end point as was required to titrate the sample to the first end point. In other words,
the volume required to reach the second end point, V
ep2
, will be twice that required to reach the
first end point.
ep1
ep2
2V
V
=
(
2
b
)
The concentration of weak acid
in the sample titrated
will be given by:
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s
b
ep1
0
BH2
V
C
V
C
=
(
2
c
)
If the original sample was diluted before the titration step, it will be necessary to take the dilution
step into account.
2. Diprotic acid with conjugate base
. If the sample contains initial concentrations of both the
diprotic acid, C
BH2
0
, and the conjugate base, C
BH

0
, then, in addition to Reactions 1a and 1b, the
conjugate base contained in the sample initially, C
BH

0
, will also react with titrant as follows:
O
H
B
Na
NaOH
BH
2
2
+
+
→
+
−
+
−
(Conj. base in sample initially)
(3a)
In this case, a larger volume of titrant will be needed to titrate the sample from the first end point
to the second than from the start to the first end point. The volume needed to reach the first end
point will be the same as in Eq. 2a. The volume needed to reach the second end point, V
ep2
, is
related to the other volumes and concentrations as follows:
)
C
C
2
(
V
)
C
C
(
V
C
V
C
V
0
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 Fall '11
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