polarography introduction, principle, instrumentation

1. Dr. Y. S. THAKARE
M.Sc. (CHE) Ph D, NET, SET
Assistant Professor in Chemistry,
Shri Shivaji Science College, Amravati
Email: yogitathakare_2007@rediffmail.com
SEM-III
PAPER-X
ANALYTICAL CHEMISTRY –I
THERMAL AND ELECTROANALYTICAL METHOD
UNIT- IV
Electroanalytical Technique
Polarography
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2. Unit IV Electroanalytical Techniques
Polarography: Theory, Basic principle of polarography, apparatus.
Dropping mercury electrode. Supporting electrolyte, effect of supporting
electrolyte on limiting electrode. Diffusion coefficient and its evolution.
Ilkovic equation, its derivation and its applications, Ilkovic equation-
diffusion current constant and capillary characteristics determination,
Half wave potential. Polarographic maxima. Interpretation of
polarographic curve. Role of temperature on diffusion current. Reversible,
quasi reversible and irreversible electrode reaction and evaluation of
parameter using various reaction derivative polarography, modified
polarographic techniques, AC polarography, limitations of polarography,
pulse polarography. Method of quantitative analysis: Absolute,
comparative. The PILOT ION and kinetic methods.
Voltammetry: Basic, Principles, Instrumentation, Cyclic voltammetry-
Principle, Instrumentation and applications, Voltammogram, Stripping
technique, Anodic and Cathodic voltammetry, and their applications in the
determination of metal ions and biologically important compounds.
Enzyme catalyzed reaction and applications of voltammetry in monitoring
such reaction.
Related Techniques: Amperometric titration and Chronopotentiometry,
Principle methodology, and their application in qualitative and
quantitative analysis.
Electrode – The metal rod dipped in its salt solution 2
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6. Electrochemistry: The Branch of physical chemistry which
deals with the study of interconversion of electrical energy
in to chemical energy or vice-versa
Electrolytic Cell: Conversion of electrical energy in to
chemical energy
Electrochemical Cell: Conversion of chemical energy into
electrical energy
Electrode – The metal rod dipped in its salt solution
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7. Oxidation half cell Reduction half cell
Left hand side electrode Right hand side electrode
Nernst Equation
𝐸 = 𝐸0
−
2.303𝑅𝑇
𝑛𝐹
𝑙𝑜𝑔
[𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛 ℎ𝑎𝑙𝑓 𝑐𝑒𝑙𝑙]
[𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 ℎ𝑎𝑙𝑓 𝑐𝑒𝑙𝑙]
𝐸 = 𝐸0
−
0.0591
𝑛
𝑙𝑜𝑔
[𝑃𝑟𝑜𝑑𝑢𝑐𝑡]
[𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡]
Ecu= +0.34V
EZn= -0.76V
Electrode| Electrolyte || Electrolyte | Electrode
Ecell = ER – EL= 0.34-(-0.76)=0.34+0.76=1.10V
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8. Electrode potential: Due to difference in rate of dissolution and deposition of
ions the potential develops across the metal solution interface is known as electrode
potential. It depends upon the concentration (activity) of the ions in the solution.
Cathode Anode
Denoted by a positive sign since electrons
are consumed here
Denoted by a negative sign since
electrons are liberated here
A reduction reaction occurs in the
cathode of an electrochemical cell
An oxidation reaction occurs here
Electrons move into the cathode Electrons move out of the anode
The electrode whose potential is known or arbitrarily fixed is known as reference
electrode.
E.g. Hydrogen electrode, Sat Calomel Electrode
The electrode whose potential is to be determining by combined with another
electrode of known potential is known as indicator electrode. E.g. Glass
electrode, quinhydrone electrode.
Ecell = ER – EL= 0.34- (-0.76) = 0.34 + 0.76 = 1.10V
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9. Electrolytic cell
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11. POLAROGRAPHY
INTRODUCTION
Polarography is a method of analysis in which the solution to be
analyzed is electrolyzed in such a way that the graph of current
against voltage shows what is in the solution and how much is
present the method was developed in 1922 by Czech chemist Jaroslav
Heyrovsky who won the Nobel prize for his discovery.
The basic idea is to pass the current between two electrodes one
large in area and other very small. Normally both electrodes are of
Mercury the large electrode is a pool of Mercury at the bottom of the
cell the small electrode is a drop of Mercury coming out of a very fine
capillary tube.
Thus if a study increasing voltage is applied to such a cell it is possible
to construct a reproducible current-voltage curve
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12. The electrolyte is an electroactive dilute solution of material to be
analyzed in a suitable medium containing an excess of different
electrolyte called base or supporting electrolyte. The purpose of the
latter electrolyte is to carry the bulk of the current and to raise the
conductivity of the solution. From the current voltage curve
information about the nature and concentration of material may be
obtained
Thus “polarography is that method of instrumental analysis which
consists of the measurement of potential difference as a current
flowing in the solution and the result obtained can does be interpreted
in terms of nature and concentration of many substances.”
The value of current flowing through the cell at any applied voltage is a
measured with the help of an instrument known as polarograph
(because the curve obtained are graphical representation of the
polarization of dropping Mercury electrode) and the curves obtained
with it are known as polarogram. This technique is known as
polarography Dr. Yogita Sahebrao Thakare
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14. PRINCIPLE
/ Pool of Mercury (Anode)
(Cathode)
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15. Polarization refers to an effect reducing the performance of batteries. This effect is
a displacement of electrode potential from the equilibrium value. There are two
types of polarization: activation and concentration. All electrochemical reactions
occur in a series of steps at the interface between electrode and electrolyte.
Activation polarization refers to the condition wherein the reaction rate is
determined by the slowest step. The term “
activation”is used because an activation
energy barrier is associated with this slowest, rate-limiting step. The second type,
concentration polarization, occurs when the reaction rate is limited by diffusion in
the solution (in conventional batteries the electrolyte is a solution).
OR
The extent of potential change caused by the current is known as polarization.
Such changed is caused by the various physical and chemical factors at the
electrode.
OR
The shift in electrode potential which result from the effect of current flow with
respect to the zero current flow potential

16. Jaroslav Heyrovsky was the inventor of polarographic method and the father of
electroanalytical chemistry. His contribution to electroanalytical chemistry cannot be
overestimated . All modern voltammetric methods used now in electroanalytical chemistry
originated from polarography.
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17. (Pool of mercury)
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18. Cathode ( - )
Anode (+)
G
DC source
P
Pool of mercury
V
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WHY = ?

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22. In polarography electrode potential is applied in the form of voltage due to that
voltage analyte is diffused and the current flow takes place. That current we
measured in polarography and that is a known as a diffusion current.
 Potential is applied in the form of voltage
 Analyte present will diffused due to applied voltage
 Current flow takes place due to diffusion of the ions in the electrolyte
 Current is known as diffusion current
Working
Consider a polarographic cell containing a solution of cadmium chloride to which an
external E.M.F. is applied. The positively charged ions present in the solution will be
attracted to the dropping Mercury electrode by an external force and by a diffusive
force resulting from the concentration gradient formed at the surface of the
electrode. Thus the total current flowing through the cell may be regarded as a sum
of electrical and diffusion force.
When the applied voltage is increased and the current is recorded a graph will be
obtained which is shown below
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23. Types of current
i) Residual current(Ir)
ii) Migration current
iii) Diffusion current(Id)
iv) Limiting current
D
C
A B
It can be seen from the graph that from A to B, a small current flows. This is known as
residual current and is carried by the supporting electrolyte and impurity present in
the sample. At the point B, the potential of the electrode becomes equal to the
decomposition potential of the Cd2+ ion. The current then increase along the curve
BC. At point C current no longer increases linearly with applied voltage but reaches a
study limiting value at point D. After this no increase in current is observed at higher
cathode potential. Thus, the current corresponding to the curve CD is known as
limiting current. The difference between the residual current and the limiting current
is called diffusion current and is denoted by Id
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24. i) Residual current (Curve AB): The current is not zero when no reducible ions are present.
The residual current is because of impurity present in the analyte which get reduced on the
cathode (faradic current ) or current is produced by helmholtz double layer produce by
cation of supporting electrolyte (Condenser current).
As the Mercury drop grows ions (cations) from supporting electrolyte gather
around it if the drop is negatively charged these ions are positively charged. Considered
potassium chloride solution, the potassium ions in it will be attracted to the drop. They are
not reduced to potassium atoms unless the negative potential is a very high but remain close
to the Mercury surface forming the electrical double layer the effect is like charging up a
condenser. When the drop falls off a new drop forms and a new condenser is charged up.
This cause a continuous flow of electric current which increases as the potential of drops
increases. It is observed that the charging current is zero at the point at which the surface
tension is maximum this happen at about 0.52 V more negative than the saturated calomel
saturated calomel electrode in the case of electrolyte containing trace of impurities a small
impurities a small faradic current is also superimposed upon the condenser current it is a
current it is a practice to include this in the residue all current. Thus we can write rest your
write rest your current is equal to faradic current + condenser current.
Ir = If + Ic
The concentration of supporting electrolyte is 50 to 100 times more than that of an
analyte that is supposed positive the value of residual current is a very very low.
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25. ii) Migration current : The electrode active material reaches the surface of electrode
(Cd2+) by two process-
a) The first involves the migration of charged particle in the electric field caused by
the potential difference existing between the electrode surface and the solution.
b) The second involves the diffusion of particle from bulk of solution towards the
cathode.
The current required for the about to process is called migration current. Jaroslav
Heyrovsky prove that the migration current can be almost eliminated if our
independent electrolyte is added to the solution in a concentration so large that it
carry almost all current the flowing.
Now we are applying a voltage is in the increasing order in the form of electrode
potential. Under the influence of applied voltage there is the migration of ions. We
apply the voltage in increasing order negative potential of cathode that is dropping
Mercury electrode is increased because it is a polarizable and smaller in area on the
other hand the same positive potential applied to the anode that is pool of Mercury
which is non polarizable electrode, its potential does not increased because of large
area.
Hence because of more negative potential the migration of k+ ions takes place
towards the cathode that is dropping Mercury electrode the current is due to
supporting electrolyte only as its concentration is more than analyte and for analyte
migration current is negligible by increasing the concentration of
electrolyte KCL or NaCl we can decrease the migration current.
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Example will make the concept more clear suppose the solution contains 0.1
M KCL and 0.01 M cadmium ions. The current is carried through the cell by
all ions present. The fraction of total current carried by each ion is depend
upon the relative concentration compared with other ions and transport
number. In present case about 90% of the current will be transported to the
cathode by the potassium ions present. If the concentration of potassium
ions is increased to more than 99% of the total cation present, the relative
current carried by the other cations are reduced practically to zero. Thus all
the current through the cell will be transported by the potassium ions only.

27. Diffusion current (Curve BC): Now applied potential is that much sufficient to
decomposed the analyte present. Here Cd2+ will get reduced due to concentration
gradient and potential difference. The Cd2+ cross to diffuse through the Helmholtz
double layer and it will go through dropping mercury electrode. When Cd2+ get
reduced or discharge at the cathode then it forms metallic Cd0 which reacts with
mercury to form amalgam then this amalgam get diffused in to the electrode. Due to
this diffusion, rapid increased in the current flowing through the cell is observed and
that is known as diffusion current. This is the actual current we measured in
polarography for analyte.
This current is directly proportional to the concentration of the substance being
reduced or oxidized at the dropping Mercury electrode. The diffusion current is given
by Ilkovic equation as follows
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28. iv) Limiting current (Curve CD): At limiting current (flat region or plateau ) the rate
of supply of Cd2+ ions from the bulk of the solution to the indicator electrode
surface becomes equal to the rate of their deposits that is no more diffusive
force operative of ions of Cd2+. At point C the rate of supply of Cd2+ from the
bulk of the solution to the indicator electrode surface becomes equal to the rate
of their deposition. Hence at potential greater than C the concentration of
undischarged Cd2+ at the microelectrode surface is a negligibly small as
compared to the cell ions in the solution. Therefore no further increase in the
current can be expected after C but a small steady increase in current will be
results between C and D. Since is the limiting current is now formed by rate at
which Cd2+ reach the surface.
Here
the rate of deposition = to rate of travel
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