Class notes MSc chemistry Electroanalytical Methods, ISBN: 9783662047576
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MSc chemistry
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MSc Chemistry
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Electroanalytical Methods
This notes are class notes of advanced physical chemistry for the course of post graduation and further doctoral studies. It explains about electroanalytical methods in chemistry.
Class notes MSc chemistry Electroanalytical Methods, ISBN: 9783662047576
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ELECTROANALYTICAL TECHNIQUE
VOLTAMETRY
Voltametry comprises a group of electroanalytical methods that are based upon the
potential current behaviour of a polarisable electrode in the solution being analysed.
In voltametry, a measured small potential is impressed across a pair of electrodes, one
of which is a non-polarisable reference electrode(SCE) and the other a polarisable
inert electrode. The current which flows depends upon the composition of the solution
Voltametry and voltametric analysis are concerned with the study of current - voltage
relation at a micro electrode, called working electrode (Pt or Au). The third elecrode,
auxiliary electrode can be a simple wire of Pt or Ag or a mercury pool. The term
polarography is used when the polarisable electrode is dropping mercury electrode.
Mercury can not be made more positive than about +0.25V aganist SCE. In
amperometry, both electrodes are polarisable
Principle of polarography
When solution containing various ions is electrolysed with a steadily increasing
voltage, ions with lower discharge potentials will discharge first in preference to those
whose discharge potentials are higher. As the electrode becomes smaller than their
concns in the bulk of the solution and hence a concn gradient would be set up, if the
electrolysis is carried out without stirring the electrolyte. If a steady increasing
voltage is applied to a cell containing dropping mercury cathode and a pool of
mercury at the bottom of the cell as anode, it is possible to construct a reproducible
current voltage curve. The electrolyte is an electroactive dilute solution of material
under examination in a suitable medium containing an excess of indifferent
electrolyte, called base or ground solution or supporting electrolyte. Supporting
electrolyte is useful in carrying the bulk of the current as well as raising the
conductivity of the solution. From the current- voltage curve, information as to the
nature and concentration of the material may be obtained
Polarography is an instrumental technique which consists in the
measurement of potential difference as current flows in solution and the result so
obtained can thus be interpreted in terms of the nature and behaviour of many
, substances and systems. The value of current flowing through the cell at any given
applied voltage is measured with the help of an instrument called polarograph
Apparatus
The dropping mercury electrode, which consists of mercury reservoir from which
mercury trickels down as small drops through a capillay acts as cathode called an
indicator or micro electrode. The anode consists of a pool of mercury (reference
electrode) and its area is correspondingly large so that incapable of becoming
polarised. Dissolved oxygen removed from the solution by passing an inert gas like
nitrogen or hydrogen just before the determination. P is a potentiometer by which any
EMF upto three volts may be readily applied to the cell. S is a shunt for adjusting the
sensitivity of the galvanometer G.
Working
An external EMF is applied to the cell charged CdCl2. All the positively charged ions
present in the solution will be attracted to the indicator or micro electrode by an
electrical force and by a diffusive force resulting from the concn gradient formed at
the surface of the electrode. From A to B, practically no current will flow through the
cell. The small current which does flow is called the residual current, which can be
explained as the sum total of current due to the reduction of traces of impurities like
iron,copper or oxygen. Charging current which results from the fact that the mercury
solution interface acts like a capacitor of continuously increasing area. The magnitude
of the residual current is small and reproducible, must eliminate reducible impurities
B is a point at which the potential of the indicator electrode becomes equal to the
decomposition potential of the cadmium ions wrt metallic cadmium electrode. The
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