2. Introduction
• Polarographic methods has been adopted to detect end point of
titration
• From Ilkovic equation id is directly proportional to conc. Of
reducible species.
• Graph of diffusion current vs volume of titrant added allows the end
point of titration to be determined.
3. Titration procedure
• Known volume of experimental solution is placed in polarographic
cell.
• Suitable supporting electrolyte usually KCl is added.
• DME is inserted in cell and it act as cathode.
• SCE electrode use as reference elec.
• N2 is bubbled for 5 min. removal of dissolved O2
• Applied voltage depend upon reducible species & id is measure .
• Titrant is added through microburette.
4. Titration Curves
• Shape of titration curves depend upon the species reduce at DME
and applied potential
They are classified in three types.
1. Titrand is reducible but other species are not.
2. Titrant is reducible but other species are not.
3. Both titrand and titrant are reducible but product is not.
5. 1.Titrand is reducible but other species are not.
• Titrand containing reducible
species such as Pb2+
is titrated
with a non- reducible species
such as SO4
2+
or oxalate ions
which is also precipitation
agent.
• Diffusion current decreases
linearly with increasing vol.of
titrant added.
•current
Volume of titrant
6. 2. Titrant is reducible but other species are not.
• Titrand containing Non
reducible species such as SO4
2+
or oxalate ions with a Pb2+
or
Ba2+
reducible species. Titration
proceeds the current remain
steadily till the end point since
no species reduced.
• After equivalence point
Diffusion current increases
linearly with increasing vol.of
titrant added.
•current Volume of titrant
7. 3. Both titrand and titrant are reducible but
product is not.
• A titrand containing reducible
species such as Pb2+
is titrated
with reducible species such as
Cr2O7
2-
ions.
• In this diffusion current
decreases first since titrand
contain reducible species till
end point and than increases
with addition of titrant which is
also reducible
• V shaped graph shown in fig.
•current
Volume of titrant
8. Advantages
• Method is quite accurate & end point determined by
means of a graph.
• The electrode surface continuously renewed.
Consequently current are reproducible& this is useful in
precipitation titration.
• Hg exhibits a high over voltage for evolution of
hydrogen. This use to yield irreversible polarographic
waves.
• It give more accurate result for diluted solution as
compare to other methods.
• Reading near end point are not important in this
method.
9. Disadvantages
• Considerable time required to remove dissolved oxygen before and
during titration.
• The titration cannot carried out at potential more negative than 2
volts since H2 be evolved.
• Co-precipitation can give inaccurate results.
