Monday 31 October 2016

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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