Polarographic technique is applied for the qualitative or quantitative analysis of electroreducible or oxidisable elements or groups.
It is an electromechanical technique of analyzing solutions that measures the current flowing between two electrodes in the solution as well as the gradually increasing applied voltage to determine respectively the concentration of a solute and its nature.
The principle in polarography is that a gradually increasing negative potential (voltage) is applied between a polarisable and non-polarisable electrode and the corresponding current is recorded.
Polarisable electrode: Dropping Mercury electrode
Non-polarisable electrode: Saturated Calomel electrode
From the current-voltage curve (Sigmoid shape), qualitative and quantitative analysis can be performed. This technique is called as polarography, the instrument used is called as polarograph and the current-voltage curve recorded is called as polarogram
1. Polarography
DR. RUPINDER KAUR
ASSOCIATE PROFESSOR
DEPT. OF PHARMACEUTICAL CHEMISTRY
ISF COLLEGE OF PHARMACY
WEBSITE: - WWW.ISFCP.ORG
EMAIL: RUPINDER.PHARMACY@GMAIL.COM
ISF College of Pharmacy, Moga
Ghal Kalan, GT Road, Moga- 142001, Punjab, INDIA
Internal Quality Assurance Cell - (IQAC)
2. Introduction
Polarographic technique is applied for the qualitative or quantitative analysis of
electroreducible or oxidisable elements or groups.
It is an electromechanical technique of analyzing solutions that measures the current
flowing between two electrodes in the solution as well as the gradually increasing
applied voltage to determine respectively the concentration of a solute and its nature.
created by: Jaroslav Heyrovsky
2
3. 3Principle
The principle in polarography is that a gradually increasing negative potential
(voltage) is applied between a polarisable and non-polarisable electrode and the
corresponding current is recorded.
• Polarisable electrode: Dropping Mercury electrode
• Non-polarisable electrode: Saturated Calomel electrode
From the current-voltage curve (Sigmoid shape), qualitative and quantitative analysis
can be performed. This technique is called as polarography, the instrument used is
called as polarograph and the current-voltage curve recorded is called as polarogram.
4. 4
The point of inflection in the current-
voltage curve is known as Half-wave
potential and is characteristic or typical for
every element or functional group –
Qualtitative aspect.
The diffusion current measured is
proportional to the concentration of that
particular compound- Quantitative aspect.
Thus, polarography can be used for both
qualitative and quantitative analysis of
compounds.
5. 5
Polarographic measurements are governed by ilkovic equation
Ilkovic Equation — The linear relationship between the diffusion current (id) and the
concentration of electroactive species is shown by the Ilkovic equation:
id = 607 nCD1/2m2/3t1/6,
id = diffusion current in microamperes;
n = number of electrons required per molecule of electroactive Substance,
D = diffusion coefficient, in square cm per second,
C = concentration, in millimoles per L,
m = mass of mercury flow from the DME, in mg per second,
t = drop time, in seconds.
Polarographic measurements
6. 6
Organic and inorganic samples can be analysed.
Even low concentrations of the samples can be detected (10-3 M to 10-4 M).
Rapidity of the technique.
Qualitative and quantitative analysis can be performed easily.
Mixture of compounds or elements could be analysed without seperation.
Advantages of Polarography
7. 7Mercury electrodes
In polarography, mercury is used as a working
electrode, because mercury is a liquid. The working
electrode is often a drop suspended from the end of
a capillary tube.
3 examples of electrodes:
• HMDE (Hanging mercury drop electrode)
• DME (dropping mercury electrode)- Most
commonly used electrode. Mercury drops form
at the end of the capillary tube as a result of
gravity. The optimum interval between drops
for most analysis is between 2 to 5 seconds.
• DSME (static mercury drop electrode)
•
8. 8
Hg yields reproducible current-potential data.
This reproducibility can be attributed to the continuous
exposure of fresh surface on the growing mercury drop.
With any other electrode (such as Pt in various forms), the
potential depends on its surface condition and therefore on
its previous treatment.
The vast majority of reactions studied with the mercury
electrode are reductions.
Why Dropping Mercury Electrode?
At a Pt surface, reduction of solvent is expected to compete with reduction of
many analyte species, especially in acidic solutions.
The high overpotential for H+ reduction at the mercury surface. Therefore, H+
reduction does not interfere with many reductions.
9. 9
A mercury electrode is not very useful for performing oxidations, because Hg is
too easily oxidized.
The electrode cannot be used over +0.4 V (vs S.C.E.), because Hg dissolves and
anodic wave is recorded. Lesser than -1.8 V, hydrogen is liberated.
The capillary is difficult to maintain since dust or other articulate matter can block
the capillary.
Problems with mercury electrode
10. 10
The polarographic apparatus consists of a
polarisable electrode (DME) and a reference
electrode (SCE).
Between these electrodes, the required potential
range (0 to -3 V) can be applied.
It consists of a sample cell made up of glass, in
which the sample solution t be analysed is kept.
The capillary is dipped into the solution to be
analysed and the height of the mercury
reservoir is adusted so that droptime of about 2-
7 sec is set.
Method of Analysis
11. 11
Supporting electrolytes like potassium chloride is added to the sample solution to
eliminate migration current.
The oxygen present in the sample solution is rremoved by passing Nitrogen or using
alkaline pyrogallol solution.
Maximum suppressors are added in the required concentration range.
When all these things are done, the initial and final potential is set in the instrument
and the current-voltage curve is recorded.
From the polarogram, half wave potential and diffusion current is determined and
thus qualitative and quantitative analysis is performed.
12. 12Polarographic Maxima
An increase of current above the limiting
value in the form of a maximum is often
noticed on polarographic waves known as
polarographic maxima.
The origin of the maximum is sometimes
connected with the mechanism of the
electrode process as in the case of catalytic
discharge of ions, while in many cases, it is
caused by the increased transport of the
depolarizer towards the electrode by a
streaming motion of the solution.
The presence of this reproducible polarographic maxima leads to error in
determining half wave potential and diffusion current.
13. 13Elimination of Polarographic maxima
Maximum suppressors which increases the viscosity eliminate the polarographic
maxima such as:
• Gelatin (0.002% to 0.01%)
• Dyes (methyl red)
• Surfactants (triton)
14. 14
Residual current (ir): It is the sum of the relatively larger condensor current and a very
small Faradic current. Condensor current is due to the formation of Helmholtz double
layer at the mercury surface and Faradic current is due to the traces of impurities.
Migration current (im): It is due to migration of cations from the bulk of the solution
towards cathode due to diffusive force, irrespective of concentration gradient.
Diffusion current (id): Diffusion current is due to the actual diffusion of
electroreducible ion from the bulk of the sample to the surface os the mercury droplet
due to concentration gradient.
Limiting current (id): Beyind a certain potential, the current reaches a steady value
called as the limiting current. At this point, the rate of diffusion of ions is equal to the
rate of reduction and the state of electrode is said to be concentration polarised.
Different types of current in polarography
15. 15Factors affecting Diffusion Current
Concentration: Diffusion current is directly proportional to the concentration of the
electroreducible ions.
Temperature: Diffusion current also varies with respect to temperature (directly
proportional).
Viscosity of the medium: Diffusion coefficient depends on the viscosity of the
medium. Hence, diffusion current also varies.
Capillary characteristics: The bore size of the capillary, drop time in seconds and
the pressure of the mercury will all affect the diffusion current as they alter the flow
characteristics of mercury droplet.
Presence of maximum suppressors: Maximum suppressors like gelatin, dye stuffs
and surfactants will affect the diffusion current.
16. 16
Dissolved oxygen and peroxides
Trace metals and metal –containing drugs
Antiseptics and insecticides
Vitamins
Hormones
Antibiotics
Alkaloids
Blood serum and cancer diagnosis
Pharmaceutical Applications
17. 17
Only small volume of sample is required.
Turbid and coloured solutions can be analyzed.
It can be used for the determination of substances, which are not electrochemical
active (indirect).
Prior separation of excepients is not required.
Its sensitivity is sufficient for the determination trace elements and toxic impurities.
High speed analysis which is important for QC
Advantages of Polarography
