It contains what is amperometry and where it will be derived and what is the principle behind the amperometry. Instrumentation of amperometry and the purpose of dipping mercury electrode and rotating platinum electrode. The advantage over rotating platinum electrodes. Amperometric titration curves for reducible ions and non-reducible ions. What tells the Ilkovic equation and how it relates to the amperometry is also included. Applications, advantages, and disadvantages of amperometric titration are also included. Questions related to amperometry and amperometric titration are given for practice. The contents taken from the websites are also given.
Amperometry refers to the measurement of current under a constant applied voltage and under these conditions it is the concentration of analyte which determine the magnitude of current.
In Amperometric titrations, the potential applied between the indicator electrode (dropping mercury electrode) and the appropriate depolarizing reference electrode (saturated calomel electrode) is kept constant and current through the electrolytic cell is then measured on the addition of each increment of titrating solution. It is a form of quantitative analysis.
Otherwise called as Polarographic or polarometric titrations.
It contains what is amperometry and where it will be derived and what is the principle behind the amperometry. Instrumentation of amperometry and the purpose of dipping mercury electrode and rotating platinum electrode. The advantage over rotating platinum electrodes. Amperometric titration curves for reducible ions and non-reducible ions. What tells the Ilkovic equation and how it relates to the amperometry is also included. Applications, advantages, and disadvantages of amperometric titration are also included. Questions related to amperometry and amperometric titration are given for practice. The contents taken from the websites are also given.
Amperometry refers to the measurement of current under a constant applied voltage and under these conditions it is the concentration of analyte which determine the magnitude of current.
In Amperometric titrations, the potential applied between the indicator electrode (dropping mercury electrode) and the appropriate depolarizing reference electrode (saturated calomel electrode) is kept constant and current through the electrolytic cell is then measured on the addition of each increment of titrating solution. It is a form of quantitative analysis.
Otherwise called as Polarographic or polarometric titrations.
These are chemical shift reagents and solvent induced shifts have their application in resolving the NMR Spectra of complex structures by inducing shift with respect to reference compound. Thus useful in interpretation of structures of complex organic compounds.
The Detailed Theory and instrumentation of Both Amperometry and Biamperometric analysis is given with Titration curves and Applications.
Medha Thakur (M.Sc Chemistry)
For UG students of All Engineering Branches (Mechanical Engg., Chemical Engg., Instrumentation Engg., Food Technology) and PG students of Chemistry, Physics, Biochemistry, Pharmacy
The link of the video lecture at YouTube is
https://www.youtube.com/watch?v=t3QDG8ZIX-8
In mass spectrometry, fragmentation is the dissociation of energetically unstable molecular ions formed from passing the molecules in the ionization chamber of a mass spectrometer. The fragments of a molecule cause a unique pattern in the mass spectrum.
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
These are chemical shift reagents and solvent induced shifts have their application in resolving the NMR Spectra of complex structures by inducing shift with respect to reference compound. Thus useful in interpretation of structures of complex organic compounds.
The Detailed Theory and instrumentation of Both Amperometry and Biamperometric analysis is given with Titration curves and Applications.
Medha Thakur (M.Sc Chemistry)
For UG students of All Engineering Branches (Mechanical Engg., Chemical Engg., Instrumentation Engg., Food Technology) and PG students of Chemistry, Physics, Biochemistry, Pharmacy
The link of the video lecture at YouTube is
https://www.youtube.com/watch?v=t3QDG8ZIX-8
In mass spectrometry, fragmentation is the dissociation of energetically unstable molecular ions formed from passing the molecules in the ionization chamber of a mass spectrometer. The fragments of a molecule cause a unique pattern in the mass spectrum.
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
content- Principle
Ilkovic equation
Construction and working of dropping mercury electrode and rotating platinum electrode
Applications
Polarography is a voltammetric technique in which chemical species (ions or molecules) undergo oxidation (lose electrons) or reduction (gain electrons) at the surface of a dropping mercury electrode (DME) at an applied potential. Polarography only applies to the DME.
Objective of polarography
Polarography is an electroanalytical technique that measures the current flowing between two electrodes in the solution (in the presence of gradually increasing applied voltage) to determine the concentration of solute and its nature respectively
Polarography is based upon the principle that gradually increasing voltage is applied between two electrodes, one of which is polarisable (dropping mercury electrode) and other is non-polarisable and current flowing between the two electrodes is recorded.
A sigmoid shape current-voltage curve is obtained from which half wave potential as well as diffusion current is calculated.
Diffusion current is used for determination of concentration of substance.
Half wave potential is characteristic of every element.
Ilkovic equation is a relation used in polarography relating the diffusion current (id) and the concentration of the non-polarisable electrode, i.e., the substance reduced or oxidised at the dropping mercury electrode (polarisable electrode).
Definitions of types of currents
1. Residual current (ir), 2. Migration current (im): , 3. Diffusion current (id) 4.Half wave potential 5. Limiting current (il)
Dropping mercury electrode- Dropping mercury electrode (DME) is a polarisable electrode and can act as both anode and cathode.
The pool of mercury acts as counter electrode,
i.e., anode if DME is cathode or
cathode if DME is anode.
The counter electrode is a non-polarisable electrode.
To the analyte solution, electrolyte like KCl is added i.e., 50-100 times of sample concentration.
Pure nitrogen or hydrogen gas is bubbled through the solution, to expel (remove) out oxygen.
Eg: If the analyte solution contains cadmium ions, then cadmium ions are discharged at cathode (-)
Cd2+ + 2e- → Cd
Then, gradually increasing voltage is applied to the polarographic cell and current is recorded.
Graph is plotted between voltage applied and current. This graph is called Polarograph and the apparatus is known as Polarogram.
The diffusion current produced is directly proportional to concentration of analyte and this is used in quantitative analysis.
The half wave potential is characteristic of every compound and this is used in qualitative analysis.
Graph is plotted between voltage applied and current. This graph is called Polarograph and the apparatus is known as Polarogram.
The diffusion current produced is directly proportional to concentration of analyte and this is used in quantitative analysis.
The half wave potential is characteristic of every compound
The earliest voltammetric technique
Heyrovsky invented the original polarographic method in 1922, conventional direct current polarography (DCP).
It employs a dropping mercury electrode (DME) to continuously renew the electrode surface.
Diffusion is the mechanism of mass transport.
When an external potential is applied to a cell
containing a reducing substance such as CdCl2,
The following reaction will occur:
Cd2+ + 2e + Hg = Cd(Hg)
The technique depends on increasing the applied
voltage at a steady rate and simultaneously
record photographically the current-voltage
curve (polarogram)
The apparatus used is called a polarograph .
When an external potential is applied to a cell
containing a reducing substance such as CdCl2,
The following reaction will occur:
Cd2+ + 2e + Hg = Cd(Hg)
The technique depends on increasing the applied
voltage at a steady rate and simultaneously
record photographically the current-voltage
curve (polarogram)
The apparatus used is called a polarograph .
Capillary tube about 10-15cm
Int. diameter of 0.05mm
A vertical distance being maintained betwwen DME and the solution
Drop time of 1-5 seconds
Drop diameter 0.5mm
The supporting electrolyte
is a solution of (KNO3, NaCl, Na3PO4) in which the sample (which must be electroactive) is dissolved.
Function of the supporting electrolyte
It raises the conductivity of the solution.
It carries the bulk of the current so prevent the
migration of electroactive materials to working
electrode.
It may control pH
It may associate with the electroactive solute as
in the complexing of the metal ions by ligands.
This Power point presentation was prepared to describe in detail about the two main voltammetry techniques of electrochemistry which is polarography and voltammetry. In these slides, I discussed the working, instrumentation, various electrodes, advantages, disadvantages..etc. Hope you understand the topic.
A.) Comparison of Voltammetry to Other Electrochemical Methods
1.) Voltammetry: electrochemical method in which information about an analyte is
obtained by measuring current (i) as a function of applied potential
- only a small amount of sample (analyte) is used
Instrumentation – Three electrodes in solution containing analyte
Working electrode: microelectrode whose potential is varied with time
Reference electrode: potential remains constant (Ag/AgCl electrode or calomel)
Counter electrode: Hg or Pt that completes circuit, conducts e- from signal source through solution to the working electrode
Supporting electrolyte: excess of nonreactive electrolyte (alkali metal) to conduct current
B.) Theory of Voltammetry
1.) Excitation Source: potential set by instrument (working electrode)
- establishes concentration of Reduced and Oxidized Species at electrode based on Nernst Equation:
- reaction at the surface of the electrode
Analyte selectivity is provided by the applied potential on the working electrode.
Electroactive species in the sample solution are drawn towards the working electrode where a half-cell redox reaction takes place.
Another corresponding half-cell redox reaction will also take place at the counter electrode to complete the electron flow.
The resultant current flowing through the electrochemical cell reflects the activity (i.e. concentration) of the electroactive species involved
2.) Current generated at electrode by this process is proportional to concentration at
surface, which in turn is equal to the bulk concentration
For a planar electrode:
measured current (i) = nFADA( )
where:
n = number of electrons in ½ cell reaction
F = Faraday’s constant
A = electrode area (cm2)
D = diffusion coefficient (cm2/s) of A (oxidant)
= slope of curve between CMox,bulk and CMox,s
In electronics, short-channel effects occur in MOSFETs in which the channel length is comparable to the depletion layer widths of the source and drain junctions. These effects include, in particular, drain-induced barrier lowering, velocity saturation, Quantum confinement and hot carrier degradation
Similar to Lect. 5 polarographic maxima and its interpretation (20)
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This presentation explain most important property of liquid viscosity. This presentation includes definition, its experimental determination, effect of temperature and Application which is very useful to the B Sc-II (Sem-III) student of SGBAU, Amravati
This presentation includes the most important property of liquid that is surface tension. This Presentation is useful for the B Sc II students of SGBAU, Amravati
This presentation includes the most important cell used in polarography Dropping Mercury Electrode. Its structure, uses/Advantages and limitations are explain here
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Lect. 5 polarographic maxima and its interpretation
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
Polarographic Maxima and Its
Interpretation
24- Feb--21 1
2. -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
i (A)
Potential applied on the working electrode is usually swept over (i.e. scan)
a pre-defined range of applied potential
0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte
V vs SCE
Working electrode is
no yet capable of
reducing Cd2+
only small residual
current flow through
the electrode
Electrode become more and more
reducing and capable of reducing Cd2+
Cd2+ + 2e- Cd
Current starts to be registered at the
electrode
Current at the working
electrode continue to rise as
the electrode become more
reducing and more Cd2+
around the electrode are being
reduced. Diffusion of Cd2+
does not limit the current yet
All Cd2+ around the electrode has
already been reduced. Current at
the electrode becomes limited by
the diffusion rate of Cd2+ from the
bulk solution to the electrode.
Thus, current stops rising and
levels off at a plateau
id
E½
Base line
of residual
current
A
D
C
B
24- Feb--21
24- Feb--21
24- Feb--21
3. 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
Dr. Yogita Sahebrao Thakare
24- Feb--21
7. DEPARTURE FROM DIFFUSION -LIMITED CURRENTS
Ilkovic equations are applicable to currents that are limited by the
diffusion of the electroactive species to the surface of the drop.
Conditions may arise when the cell current differs in magnitude from
the diffusion limited current. It has frequently been observed that the
ideal current-voltage curve obtained with the dropping mercury
cathode is distorted by the occurrence of peaks of maxima. These
maxima are reproducible and vary in shape from sharp peaks to round
hump. With the increase in applied voltage, the current rises
abnormally reaching a critically value and then rapidly decreasing to a
limiting value corresponding to normal diffusion current plateau.
24- Feb--21
8. 1. Polarographic Maxima: The shapes of polarogram are frequently distorted by the
polarographic maxima when dropping mercury electrode is used. These maxima are
recognized by the appearance of a sharp peak or a rounded hump at the top of
wave and hence interfere with the accurate evaluation of diffusion currents and
half wave potentials. Instead of curve levelling off at the limiting current wild
oscillations occur in the polarographic waves due to some absorption
phenomenon. During the curve maxima, a which more suppressing situation exists
in which ions or molecules reach at the electrode than can occur at the electrode
surface by diffusion through the unstirred solution. A streaming of the solution
observed past the DME has also been observed during the maxima. Thus the
motion of the solution brings more electroactive species into contact with the
electrode surface than would be the case, if the solutions were unstirred.
Polarographic maxima are also called as streaming Maxima and are categorized
into two groups
First Order Maxima: First order maxima arise because of non-uniform polarization
of a mercury drop. This type of maxima appears as a continuation of the rising part
of the wave and occurs only over a small range of applied potential. The current,
after reaching the maximum value, decreases rapidly to the normal diffusion
current value (plateau). First order maxima are usually found associated with the
reduction of inorganic species. (in dilute solution only). (40 times higher than
limiting current)
24- Feb--21
9. 2. Second Order Maxima : Second order maxima occurs over a wide
range of applied potential and are associated with organic
compounds. These maxima may be due to deformation in the Hg
drop or unusual convection phenomenon both within and in the
immediate vicinity of DME. This type of maxima takes the shape of
rounded humps on the waves and are usually found associated with
organic compounds. (In concentrated solution)
Second order maxima occur over a wider range of applied potential
than the first order maxima. No satisfactory explanation has been
given for the occurrence of these maxima. Almost all the ex
planations (such as those of J. Heyrovsky (1934). H.J. Antiweiter
(1937), and M. Von Stackelberg et al (1938) that have been put
forward suggest the cause in terms of unusual convection
phenomenon both within and in the immediate vicinity of the
dropping mercury electrode
24- Feb--21
10. Fig: 1 First order and second order Maxima
24- Feb--21
11.
12. The occurrence of such maxima interferes in the measurement of true diffusion
current, so that the removal or suppression of the maxima is very important. This is
often done easily by the addition to the working solution of a very small quantity of a
surface active substance, such as sugar dyestuffs, indicators, gums, gelatin and other
colloids. These surface active substances are called maximum suppressor.
Removal of Maxima: Polarographic maxima can be removes by using suppresser like
sugar dyestuffs, indicators, gums, gelatin and other colloids Out of these maximum
suppressors, gelatin is widely used, the concentration need not exceed 0. 1%, as the
higher concentrations can lead to distortion, lowering and shifting of the positions of
the polarographic waves. Other effective and commonly used maximum suppressors
are Triton X-100, (CH3)3 C.CH.C (CH3)2 C6H4 (OC2H4)9 OH and homologues, (0.002-0.004
per cent, and methyl cellulose (0.005 per cent solution). Maximum suppressors are
capable, even in small concentrations, to remove the interference and restore the
wave to its expected shape. Maximum suppressor is believed to form an adsorbed
layer on the aqueous side of the mercury-solution interface which resists compression
this prevents the streaming movement of the diffusion layer at the interface
responsible for the current maximum.
Too much suppressor should not be used. Its excess may lead to distortion, lowering
and shifting the positions of polarographic waves.
24- Feb--21
13. 2. Non-additive and Mixed Current: If more than one electrode
reaction occurs at DME, the products of the reaction may interfere
with the reacting species of the other reaction. For example, such an
interference is obtained when a polarogram is obtained using a
neutral, unbuffered solution, containing both cadmium and iodate
ions. When these ions are allowed to run separately, a reversible wave
at - 0.6 V is exhibited by cadmium ions and a well developed wave at -
1.1 V is shown by iodate ions. On running these ions together in the
same solution, the cadmium wave is still observed at -0.6 V but the
iodate wave at - 1.2 V does not attain a total height equal to the sum
of waves run separately. This non additivity of waves in the mixed
solution is due to the precipitation of Cd(OH)2 in the vicinity of the
mercury drop.
IO-
3+3H20+ 6e- →I- + 60H-
3Cd2+ + 60H- →3 Cd(OH)2
24- Feb--21
14. 3. Anodic Waves and Mixed Anodic-Cathodic Waves: Anodic waves are
less common because of relatively small range of anodic potentials
that can be covered with the DME before Oxidation of electrode takes
place. The electrode reaction which involved the oxidation of Fe2+ to
Fe3+ in presence of citrate ion constitutes an example of anodic wave.
A diffusion current is obtained at zero volt versus SCE due to half
reaction.
Fe2+→ Fe3+ + e-.
At 0.2 V the current becomes zero because the oxidation of Fe2+ has
ceased. A cathodic wave results from the reduction of Fe3+ to Fe2+ ion .
The half wave potential is identical with that of anodic wave,
indicating that oxidation and reduction of Fe2+ and Fe3+ are reversible
at DME.
24- Feb--21
15. EVALUATION OF POLAROGRAPHIC WAVES
Wave height or diffusion current can be evaluated by the following methods
1. Extrapolation Method. The point before the initital rise of wave of residual
current is extrapolated and a line parallel to it is drawn through the diffusion
current plateau. Since the slope of the charging current curve is not linear with
changing voltage and a change in drop time affects the charging current and
faradaic current differently, estimation of residual current by extrapolation
method is not accurate for solutions of low concentrations. The method is useful
when several polarographic waves are to be analyzed on the same polarogram.
Fig: 2 Measurement of Diffusion current by Extrapolation method
24- Feb--21
16. 2. Exact Method: The measurement of wave heights ( OR Diffusion current) i.e.
Determination of E1/2 : In order to evaluate the polarogram, the wave height of each
of the wave must be measured. The measurement of diffusion current is relatively
simple with a well defined polarographic wave whose limiting current plateau is
parallels the residual current curve. Residual current can be exactly evaluated by
recording a separate polarogram for the residual current of the supporting
electrolyte alone, and is subtracted from the value of current at any particular
potential of the dropping electrode to give the diffusion current. (Both graph
measured at same applied potential
Fig: 3 Measurement of Diffusion current by Exact method
24- Feb--21
17. 3. Point Method
Point method is used for determining the diffusion current when the wave is slightly
distorted. Two lines AB and CD perpendicular to the abscissa axis are plotted. These
are divided at point F and G so that
AF= FB and CG = GD. The intersection FG with wave gives the position of half-wave
potential. A vertical line KH passing through the point E1/2 is drawn. It gives diffusion
current or wave height. Thus the wave height represents the vertical distance at the
E1/2 between the straight lines approximating the residual and diffusion current
regions. This method gives too low value of diffusion current when the polarogram is
too much distorted.
Fig: 4 Determination of wave height by point method
24- Feb--21
18. CONDITIONS FOR PERFORMING POLAROGRAPHIC DETERMINATIONS
Following conditions should be maintained in order to get precise, accurate and reproducible
polarogram.
1. Supporting electrolyte must be extremely pure, Trace levels of impurities can yield
interfering polarographic wave.
2. The mercury used in DME should be of highest purity.
3. Since nitrogen gas may contain reducible impurities, it must be purified.
4. Large concentrations of electroactive species which react prior to or near the species
being determined may have to be removed by precipitation, extraction,
chromatographic or controlled potential electrolyte techniques.
5. If the electrochemical reaction at DME involves H+ ions, the reduction (or oxidation)
depends upon the pH. The buffer that is used for controlling pH may also act as
supporting electrolyte.
6. Migration current should be suppressed by adding an excess of electrochemically inert
ions to the solution. The inert ions must have more negative potential than the
analysed ion. These inert electrolytes are called supporting or sometimes back ground
electrolytes.
7. Polarographic maxima or current maxima or the distortion of polarographic wave is
minimized by adding surface active agents such as gelatin, dye stuffs or agar- agar etc.
20. 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 20
05-August -20 Dr. Yogita Sahebrao Thakare