Polarography
On February 10, 1922, Professor Jaroslav Heyrovsky recorded
the first dependence of the current flowing through the dropping mercury
electrode on the applied potential and drew a corresponding graph in his
laboratory notebook which can be considered as the first polarogram.
He used a quite simple circuit with a mirror galvanometer.
However, he soon recognized that point-by-point measurements and plotting of
current-voltage curves were tedious and time consuming and together with M.
Shikata constructed a new instrument that recorded photographically such a
curve in several minutes.
Polarography
is a subclass of voltammetry where the working electrode is a dropping mercury
electrode (DME) or a static mercury drop electrode (SMDE),
which are useful for their wide cathodic ranges and renewable surfaces.
Mercury as working electrode is useful because:
1. It displays
a wide negative potential range
2. Its surface
is readily regenerated by producing a new drop or film
3. Many metal
ions can be reversibly reduced into it.
Advantages of DME:
1.
Its surface is
reproducible, smooth and continuously renewed, this eliminates the poisoning
effect.
2.
Mercury forms amalgams
(solid solution) with many metals.
3.
The diffusion current
assumed a steady value immediately after each change of applied potential and
is reproducible.
4.
The surface area can
be calculated from the weight of drop.
Disadvantages of DME:
1.
At potential more
positive than + 0.4 V vs SCE, mercury dissolves
producing anodic polarographic wave which masks other waves, therefore DME can
be used only for the analysis of reducible or easily oxidizable substances.
2.
The capillary is very
small so easy to be blocked→ malfunction of the electrode.
3.
Mercury is very toxic
and easily oxidized.
Principle of
polarography
The simple principle of polarography is the study of
solutions or of electrode processes by means of electrolysis with two
electrodes, one polarizable and one unpolarizable, the former formed by mercury
regularly dropping from a capillary tube.
Polarography is an electrochemical method of analysis based on the measurement of the current flow resulting from
electrolysis of a solution at a polarizable microelectrode, as a function of an applied voltage.
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= knD1/3m2/3t1/6c
id is the diffusion current in microamperes;
n is the number of electrons required per molecule of
electroactive Substance,
D is its diffusion coefficient, in square cm per
second,
C is the concentration, in millimoles per L,
m is the mass of mercury flow from the DME, in mg per second,
and
t is the drop time, in seconds.
Where k is a constant which includes Faraday constant, π
and the density of mercury, and has been evaluated at 708 for max current and
607 for average current,
Polarogram is characterised by following parameters
1.
Residual
current (ir):
Which
is the current obtained when no electrochemical change takes place.
2.
Limiting
current (il):
Which
is the current obtained by averaging current values throughout the life time of
the drop while
3.
Diffusion
current (id):
Which
is the current resulting from the diffusionof electroactive species
to the drop surface.
4.
Half
wave potential (E1/2) :
Half-wave
potential (E1/2) is a potential at which polarographic wave current is equal to
one half of diffusion current (id).
A typical polarogram
It is a graph of current versus potential in a
polarographic analysis
Applications
of Polarography: Use
in the study of
1. Dissolved oxygen and peroxides
2. Trace metals and metal
–containing drugs
3. Antiseptics and insecticides
4. Vitamins
5. Hormones
6. Antibiotics
7. Alkaloids
8. Blood serum and cancer
diagnosis
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