Conductimetric titration

Conductometric Titrations
Conductometry
is a measurement of electrolytic conductivity to monitor a progress of
chemical reaction
.
Conductometry has notable application in analytical chemistry
,
where
conductometric titration
is a standard technique
.
conductometric titration
:
In this method we d
etermine the
point where reaction is
completed
w i t h t h e h e l p o f a c o n d u c t o m e t e r t h a t m e a s u r e s t h e
changes in
conductance of solution
produced by the ions in the solution.This point at which this transition
occurs is called
Equivalence point
.
At Equivalenc
e point we measure the volume of base used to neutralize the acid ions completely
in the solution. Putting these values in formula we can get the strength of acid
The principle of conductometric titration is based on the fact that during the titration
,one of the
ions is replaced by the other and invariably these two ions differ in the ionic conductivity with the
result that conductivity of the solution varies during the course of titration. The equivalence point
may be located graphically by plotting t
he change in conductance as a function of the volume of
titrant added.
1
-
Strong Acid with a Strong Base, e.g. HCl with NaOH:
HCl + NaOH
?
NaCl + H
2
O
H
+
+ Cl
-
+ Na
+
+ OH
-
?
Na
+
+ Cl
-
+ H
2
O
Before titration the conductance is high which is due to
HCl
?
H
+
+ Cl
-
mobility of
H
+
is 350
and that of
Cl
-
ion is 73. Upon addition of
NaOH
the
H
+
ion reacts with
OH
-
ion to form the
very weakly ionized water molecule. This means that the
H
+
ion is removed from the medium
and replaced by
Na
+
ion which has a mobility of 43; thus a continuous abrupt decrease in
conductance occurs during the titration till the end point. Beyond the end point there is
excess
Na
+
and
OH
-
ions with 43 and 198 mobility due to continuous addition of
NaOH
So there is
continuous increase in conductance and the curve will have a V shape , the end point is the
minimum of the curve.
Conductometric
titration of a strong acid (HCl) vs. a strong base(NaOH) .
2
.
Weak Acid with a Strong Base
,
e
.
g
.
acetic acid with NaOH
:
CH
3
COOH
+
Na
+
+
OH
-
?
CH
3
COO
-
+
Na
+
+
H
2
O
Before titration low initial conductance is observed due to low H
+
obtained during
dissociation of weak CH
3
COOH.During titration we can observe slight decrease of
conductance due to consumption of H
+
.
During progress of titration we can obser
ve slight
increase in conductancedue to the presence of CH
3
COO
-
&Na
+
and nearly constant H
+
due
to the
buffer action of the produced CH
3
COONa and the remaining CH
3
COOH.
After end
point excess NaOH will lead to
increase in conductance due to increasingof Na
+
and O