Advanced Pharmaceutical Analysis

1. Amperometry
Nuzhat Tasnim Amin
Lecturer
Dept. of Pharmacy
Varendra University

2. Definition
• Amperometry is an electroanalytical technique in which amperemeter is used to
measure current.
• Amperometric titration refers to a class of titration in which the equivalent point
is determined through measurement of the electric current produced by the
titration reaction. It is a form of quantitative analysis.
• In amperometric titration the voltage applied across the indicators electrode and
the reference electrode is kept constant and diffusion current passing through
the cell is measured and plotted against the volume of the reagent added.
• The principle is that, the potential applied between polarizable and non-
polarizable electrode is kept constant and the diffusion current is measured
during the titration. During the titration the concentration of electro reducible
ion change and hence the diffusion current also change.

3. Diffusion Current
• The region in which more number of electrons is present is called higher
concentration region and the region in which less number of electrons is present
is called lower concentration region. Current produced due to motion of charge
carriers from a region of higher concentration to a region of lower concentration
is called diffusion current.

4. Application
• This method is widely used for the determination of sulphate which could not be
determined accurately by any other electro analytical method.
• Successive determination of chloride, bromine, iodide.
• In this technique is widely used in Karl-fisher moisture titration.
• Isolation of:
a) Phosphate with uranyl acetate
b) Lead with dichromate ions
c) Sulphate with lead nitrate
d) Iodine with mercuric nitrate

5. Application
The method is specially used for precipitation titration. The following
are some of the examples of precipitation titration which can be
accurately performed.
I. Pb2+ V/S K2Cr2O4 Or, K2SO4
II. Ba2+ V/S K2Cr2O4
III. Mg2+ V/S 8 hydroxy quinoline
IV. Ni2+ V/S Dimethyl glyoxime
V. Cu2+ V/S Cupferon

6. Advantages
• The accuracy is higher than other electro analytical methods.
• The temperature remains constant throughout experiment.
• In amperometric titration traces of elements can be detected with
good precision. The range of sensitivity is higher than conductometric
and potentiometric titration.
• It is not necessary for the reaction which occurs during the titration is
reversible or irreversible.

7. Advantages
• As the general principle of amperometric titration and polarographic
method quite similar, but still amperometric titration method is more
accurate and sensitive as compared to polarography.
• Several non reducible substance like magnesium, phosphate, sulphate
ion can be determined by titrating against the reducible reagent
which gives a diffusion current.
• The apparatus used is simple and easy to set up.
• The titration can usually be carried out rapidly.

8. Advantages
• The depolarizing substances which cannot be determined very accurately by
polarography, can be successfully determined by amperometric titration.
• The presence of foreign salts does not interfere, instead these act as supporting
electrolyte in order to eliminate the migration current.
Migration current:
Additional current produced by electrostatic attraction of cations to the surface of a dropping electrode; an unpredictable and undesirable effect to be avoided during analytical voltammetry.
• Titrations can be carried out in cases in which the solubility relations are such that
potentiometric or visual indicator methods are unsatisfactory.
• A number of amperometric titrations can be carried out at dilution at which
many visual or potentiometric titrations no longer yield accurate results.

9. Disadvantages
1. Inaccurate results are sometimes obtained because of coprecipitation.
2. The titration cannot be carried out at potential more negative than -2 volt because
hydrogen will be evolved.
3. More time is required to remove dissolved oxygen.
4. The presence of foreign salts which does not interfere in the amperometric titration
must not be present in larger concentration.
5. The reducible metal ions concentration decreases during the complex formation
but the complex ion formed is not reducible. Hence, we get a curve in which
diffusion current decreases till the end point, that is when the complexation is
complete, further addition of complex forming agent has no effect on diffusion
current. That means, diffusion current is constant.

10. Titrand is reducible but titrant and product not
When solution containing Pb2+ (reducible titrand) ion is titrated against SO4
2- (non
reducible titrant) ion, a precipitate of PbSO4 is formed. The titration can be
performed at fixed potential 0.8 Volt v/s saturated calomel electrode. As titration
proceeds, concentration of Pb2+ ion decreases due to its reducible nature and
diffusion current also decreases till it becomes minimum at equivalence point.
Further addition of titrant will not change the diffusion current and it will remain
constant beyond end point. The values of diffusion current are plotted against the
volume of titrant added. The intersection of two extra plotted portions of the
curves gives the endpoint.

11. Titrant is reducible but titrand and product not
When solution containing Cl- (non reducible titrand) ion is titrated against with the
reducible species such as Ag+ (reducible titrant), a precipitation of AgCl is formed.
The titration can be performed at fixed potential 0.8 Volt v/s saturated calomel
electrode. As titration proceeds, concentration of Cl- ion will not produce any
diffusion current It will only convert into AgCl, so the diffusion current will remain
constant. Further addition of titrant will increase the diffusion current beyond the
end point as the titrant undergoes reduction. As its concentration increases diffusion
current also increases. The values of diffusion current are plotted against the
volume of titrant added. The intersection of two extra plotted portions of the
curves gives the endpoint.

12. Titrand and titrant both are reducible but
product not
The titration of solution containing Pb2+ (reducible titrand) ion is titrated against
Cr2O7
2- (reducible titrant) is performed at potential of -0.8 Volt v/s SCE. As titration
proceeds, concentration of Pb2+ ion decreases due to its reducible nature and
diffusion current also decreases till it becomes minimum at equivalence point.
Further addition of titrant the diffusion current once again increases. The values of
diffusion current are plotted against the volume of titrant added. V shaped curve is
obtained. The intersection of two extra plotted portions of the curves gives the
endpoint.

13. Oxygen Removal
• Initially, A slow stream of pure analytical grade N2 – gas is bubbled through the
solution for 15 minutes to get rid of dissolved O2 completely.
• A known volume of the reagent is introduced from the semi micro burette (B),
while N2 is again bubbled through the solution for about 2 minutes to ensure
thorough mixing as well as complete elimination of traces of O2 from the added
liquid.
• The flow of N2 gas through the solution is stopped, but is continued to be passed
over the surface of the solution gently so as to maintain an O2 free inert
atmosphere in the reaction vessel.

14. Amperometry V/S Polarography
• Principle of amperometric titration is same as polarographic method,
in both case diffusion current is measured. But there are some
differences:
• In polarography electrode potential will be increased and electrode
potential will be responsible for diffusion current. But in
amperometry electrode potential will be fixed.
• In polarography only electro reducible substance can be analyzed. But
in amperometry, electro reducible substance as well as electro non
reducible substance can be analyzed.

15. Thank You….