University of Baghdad
College of Science for Women
Department of Chemistry
Instrumental Analysis Chemistry
Second Semester
Analytical Electrochemistry
Polarography
Lecturer
Asst. Prof. Dr. Maha Al-Tameemi
Lecture Time: Group A&B on Tuesday at (8:30 AM -11:30 PM )
Texts:
❖ Principle
th
of Instrumental Analysis, by Douglas A. Skoog, F.James Holler, Stanley R. Crouch, Chapters 9, 10, 11, and
12(6 Edition)
❖ Spectrochemical Analysis, by James D. Ingle,Jr and Stanley R. Crouch, Chapters 7, 8, 9, 10, and 11.
❖ Most Analytical Chemistry Textbooks.
❖ Handout
Polarography
Polarography: is one of the Voltametric methods of analysis in which chemical species(ions or
molecules)undergo oxidation (lose electrons)or reduction(gains electrons)at the surface of a dropping
mercury electrode(DME) at an applied potential, polarography only applies to the DME
Objective of photography
• 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
• Usually, this current is proportional to the concentration of the analyte.
• The apparatus for carrying out polarography contains three electrodes:
• The working electrode is a dropping mercury electrode, or a mercury droplet(DME) suspended from
the bottom of a glass capillary tube(microelectrode whose potential is varied with time)
• The analyte is either reduced (in most of the cases) or oxidized at the surface of the mercury drop.
• The counter or auxiliary electrode is a platinum wire)completes the circuit, conducts electrons
from the signal source through the solution to the working electrode
• The reference electrode (potential remains constant) calomel SCE or Ag/AgCl
• The potential of the mercury drop is measured by the reference electrode. 2
The Polarographic Cell
• The mercury drop, which is normally a cathode of the polarographic
cell flows from a glass capillary tube, which is attached to a reservoir
of mercury.
• The counter electrode, which is a pool of mercury into which the
drop falls, acts as an anode.
• In the original polarograph, the cell is connected in series with a
galvanometer (for measuring the flow of current) in an electrical
circuit that also contains a battery or other source of direct current
and a device for varying the voltage.
• The electroactive species which is being studied is present in a highly
conducting electrolytic medium.
• The concentration of this electrolyte is about 100 times higher and is
electroactive in the sense that it will not undergo a charge transfer
reaction on the electrode surface.
• Hence it is called a supporting or indifferent electrolyte.
• The need for such a medium is to let the majority current due to an
applied electric field called migration current be carried by the ions
of the supporting electrolyte.
• This will allow the species of interest to follow diffusional transport
towards the electrode surface
3
Typical electrochemical cell used in polarography
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Dropping Mercury Electrode
Construction
❖ A dropping Mercury Electrode (DME) consists of a mercury
reservoir from which mercury drops come down as small drops
through a capillary it generally acts as a cathode(-) it is known as
an indicator electrode or microelectrode
❖ The height of the mercury reservoir is adjusted to set the drop
time (1 to 5 seconds)
❖ Drop time is the time required to form every fresh drop of
mercury from a capillary
❖ Inside the tubing wire contact is made where mercury flows
through
❖ The anode consists of a mercury pool at the bottom of the
reservoir which acts as the reference electrode and its area is
large so that is not polarized
❖ Both anode and cathode are connected across the appropriate end
of the battery
❖ The applied voltage can changed by adjusting the sliding contact
along the potentiometer wire EF
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Working
▪ DME is a polarizable electrode {a very large change in potential(the change in potential energy equals the gain in
kinetic energy, which can then be used to find the speed)upon passages of a small current}
▪ Can be act as both anode(+) and cathode(-) but generally used as cathode
▪ The pool of mercury act as counter electrode (anode(+) if DME is cathode(-)
▪ Note: a pool of mercury due large surface area has low current density and is non-polarizable therefore has
constant
▪ Example: Consider the polygraphy cell containing a solution of cadmium chloride to which external EMF is
applied
▪ To the analyte, (i.e. cadmium) supporting electrolyte like KCL is added 50-100 times of sample concentration to
eliminate the migration current(additional current produced by electrostatic attraction of cations to the surface of a
dropping electrode an unpredictable voltammeter)
▪ Pure nitrogen or hydrogen gas is bubbled to expel out the dissolved oxygen
▪ The positively charged ions(Cd+2) present in the solution will attract to the drooping mercury electrode(-)by an
electrical force and by diffusion force resulting from the concentration gradient formed at the surface of the
electrode
▪ Thus total current flowing through the cell may be regarded as the sum of electrical and diffusive forces
▪ When the applied voltage is increased and the current is recorded a graph will be obtained
▪ The graph is plotted between the voltage applied and the current; this graph is called a polarograph, and the
apparatus is known as a polarogram
Why Dropping Mercury Electrode?
The fact that Mercury is a liquid metal provides several advantages such as:
✔ Hg yields reproducible current‑potential data.
✔ Excellent renewability and reproducibility of the surface, this reproducibility can be attributed to the continuous
exposure of fresh surfaces to the growing mercury drop.
✔ With any other electrode (such as Pt in various forms), the potential depends on its surface condition and
therefore on its previous treatment.
✔ Many reactions studied with the mercury electrode are reductions.
✔ At a Pt surface, the reduction of solvent is expected to compete with the reduction of many analyte species,
especially in acidic solutions.
✔ The high overpotential for H+ reduction at the mercury surface. Therefore, H+ reduction does not interfere with
many reductions.
✔ Hg displays a wide potential range of operations in aqueous solution due to its large hydrogen overpotential.
✔ Hg surface is readily regenerated by producing a new drop or film
✔ Many metal ions be reversibly reduced into it.
✔ Mercury can be easily purified as it is a liquid with an atomically smooth surface.
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Principle
• Study of solutions or of electrode processes by means of electrolysis with two electrodes, one
polarizable and the other unpolarizable, the former formed by mercury regularly dropping
capillary tube.
• Polarized electrode dropping mercury electrode(DME)(If the electrode potential has great
changes when infinite small current flow through the electrode, such electrode is referred to
as Polarized electrode like DME)
• Depolarized electrode saturated calomel electrode(If the electrode potential dose not change
with current , such electrode is called ideal depolarized electrode like SCE)
• Hg continuously drops from reservoir through
a capillary tube into the solution.
• The optimum interval between drops for most
analytes is between 2 and 5 seconds
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Definition of Types of Currents
1-Residual Current (ir):
The current that flows in the absence of a depolarizer (i.e. due to the
supporting electrolyte, has to be taken into consideration while
interpreting the polarograms
• It is the sum of the relatively large condenser current(ic) and very
small faradic current(if)
Condenser current(ic): is due to the formation of Helmholtz
double layer at the mercury surface
Faradaic current(if)
• Due to the oxidation or reduction of trace impurities like O2,
This indicates that dissolved oxygen interferes with the
determination of most metal ions.
• Therefore, dissolved O2 has to be removed by bubbling
nitrogen for 5-10 min into the solution to expel oxygen
before recording the polarogram
• Heavy metal ions in deionized water
• Ions salt from supporting electrolyte
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2-Diffusion Current(id)
• The difference between Residual current and limiting current is called diffusion current(id)
• Diffusion current is due to the actual diffusion of elctoreducible ions from the bulk of the sample to the surface of
the mercury droplet due to the concentration gradient
• When the potential of the working electrode is sufficiently negative, the rate of reduction of Cd2+ ions is governed
by the rate at which Cd2+ can reach the electrode. Cd2+ + 2e Cd
• In the figure below it occurs at potentials more negative than ‑0.7 V.
• In an unstirred solution, the rate of reduction is controlled by the rate of diffusion of the analyte to the electrode.
• In this case, the limiting current is called the diffusion current.
• The solution must be perfectly quiet to reach the diffusion limit in polarography.
• Thus, the diffusion current is the limiting current when the rate of electrolysis is controlled by the rate of diffusion
of species to the electrode.
• Id= limiting current –residual current
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3- Limiting current(il)
• This maximum current that passes
through the cell when the
concentration of the electro-active
species at the electrode surface is zero
is called ‘the limiting current’
• Beyond a certain potential, the current
reaches a steady state value
• At this point, the rate of the diffusion
of ions is equal to the rate of reduction
of ions, and the state of the electrode is
said to be concentration-polarized
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4-Migration current(im): It is due to the migration of cations from the bulk of the solution towards the
cathode due to diffusive force, irrespective of the concentration gradient
• Due to the electrostatic attraction (or repulsion) of positive species of analyte ions by the negative electrode, it
can be reduced to a negligible level by the presence of a high concentration of supporting electrolyte (1 M KCl).
• Increasing concentrations of electrolyte reduces the net current, since the rate of arrival of cationic analyte at the
negative Hg surface is decreased.
• Typically, a supporting electrolyte concentration 50‑100 times greater than the analyte
• concentration will reduce the electrostatic transport of the analyte to a negligible level.
5- Half wave potential: Is the potential at which the concentration of oxidized and reduced forms at the electrode
surface is equal, i.e., 50% of oxidized and 50% reduced forms are present.
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13
• When the potential is only slightly negative
concerning the calomel electrode, essentially no
reduction of Cd2+ occurs. Only a small residual
current flows.
• At a sufficiently negative potential, reduction of
Cd2+ commences, and the current increases. The
reduced Cd dissolves in the Hg to form an
amalgam.
• After a steep increase in current, concentration
polarization sets in: The rate of electron transfer
becomes limited by the rate at which Cd2+ can
diffuse from the bulk solution to the surface of
the electrode.
• The magnitude of this diffusion current Id is
proportional to Cd2+ concentration and is used
for quantitative analysis. The upper trace in the
Figure above is called a polarographic wave.
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Polarogram
• Polarogram: a graph of current versus potential in a polarographic analysis
• The trace is called a polarographic wave
• i (residual current) which is the current obtained when no electrochemical
change takes place
• Iav (average current) is the current obtained by averaging current values
throughout the lifetime of the drop
• Id (diffusion current), which is the current resulting from the diffusion of
electroactive species to the drop surface(reduction of the sample), which is
directly proportional to the concentration of analyte and is used in the
quantitative analysis
• The half-wave potential is characteristic of every compound and this is used in
qualitative analysis
• This maximum current that passes through the cell when the concentration of
the electro-active species at the electrode surface is zero is called ‘the limiting
current’
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•
Effect of Dissolved Oxygen
• Oxygen dissolved in the solution will be reduced at the DME leading to two well defined
waves which were attributed to the following reactions:
• O2(g) + 2H+ + 2e- < ==== > H2O2; E1/2 = - 0.1V
• H2O2 + 2H+ +2e- < ==== > 2H2O; E1/2 = - 0.9V
• E1/2 values for these reductions in acid solution correspond to -0.05V and -0.8V versus
SCE.
• This indicates that dissolved oxygen interferes in the determination of most metal ions.
• Therefore, dissolved O2 has to be removed by bubbling nitrogen into the solution before
recording the polarogram.
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Qualitative analysis
• Half wave potential, E1/2 is an important feature can be derived from the
polarogram.
• It is the potential corresponding to one half the limiting current i.e. id/2.
• El/2 is a characteristic for each element and thus used for qualitative analysis.
• El/2 depend on the nature of electrolyte solution (composition of solution), it change
with pH change.
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Quantitative analysis
•
18
•
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Quantitative Analysis
• The principal use of polarography is in quantitative analysis.
• Since the magnitude of the diffusion current is proportional to the
concentration of analyte, the height of a polarographic wave tells how
much analyte is present.
Two methods are used to find the concentration of unknown:
Standard curves method
Standard addition method
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Standard curves method
• The most reliable, but tedious, method of quantitative
analysis is to prepare a series of known
concentrations of analyte in otherwise identical
solutions.
• A polarogram of each solution is recorded, and a
graph of the diffusion current versus analyte
concentration is prepared.
• Finally, a polarogram of the unknown is recorded,
using the same conditions.
• From the measured diffusion current and the standard
curve, the concentration of analyte in the sample can
be determined.
• The figure below shows an example of the linear
relationship between diffusion current and
concentration.
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Example 1
Using a Standard Curve
• Suppose that 5.00 mL of an unknown sample of Al(III) was placed in a 100mL
volumetric flask containing 25.00 mL of 0.8 M sodium acetate (pH 4.7) and 2.4 mM
pontachrome violet SW (a maximum suppressor). After dilution to 100 mL, an aliquot
of the solution was analyzed by polarography. The height of the polarographic wave
was 1.53 µA, and the residual current measured at the same potential with a similar
solution containing no Al(III) was 0.12 µA. Find the concentration of Al(III) in the
unknown.
Id= limiting current –residual current
• The corrected diffusion current is 1.53 ‑ 0.12 = 1.41 µA.
• In the figure below, 1.41 µA corresponds to [AI(III)] = 0.126 mm.
• Since the unknown was diluted by a factor of 20.0
(from 5.00 mL to 100 mL) for analysis,
the original concentration of unknown must have been
(20.0)(0.126) = 2.46 mM.
Standard addition method
• The standard addition method is most useful when the sample matrix is unknown or difficult
to duplicate in synthetic standard solutions.
• This method is faster but usually not as reliable as the method employing a standard curve.
• First, a polarogram of the unknown is recorded. Then, a small volume of concentrated solution
containing a known quantity of the analyte is added to the sample.
• With the assumption that the response is linear, the increase in diffusion current of this new
solution can be used to estimate the amount of unknown in the original solution.
• For the greatest accuracy, several standard additions are made.
C1 Concentration of unknown
C2 Concentration of standard solution
V2 mL of standard solution is added to V1 mL of unknown
I1 Diffusion current of unknown
I2 Diffusion current of standard solution
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Example 2:
Standard Addition Calculation
• A 25mL sample of Ni2+ gave a wave height of 2.36 µA (corrected for residual current) in a polarographic
analysis.
• When 0.5mL of solution containing 28.7 mM Ni2+ was added, the wave height increased to 3.79 µA. Find
the concentration of Ni2+ in the unknown.
• The increase in wave height is proportional to the increase in Ni2+ concentration.
• First, calculate the change in wave height (3.79 - 2.36 = 1.43 µA).
• Then, we can use the following formula:
• After doing the math, we obtain a concentration approximately equivalent to 12.33µM of Ni2+ in the
original unknown sample.
Example 1
Example 2
Advantages of DME
• Surface area is reproducible with a given capillary
• It provides a smooth, fresh surface for the reaction
• The high overpotential for H+ reduction at the mercury surface, i.e., H2 does not evolve on the Hg surface unless
a very high negative potential(-1.2V) is reached and thus H+ will not interfere in the determination of many ions
• Each drop remains unaffected and does not become contaminated by the deposited metal
• Difusion equilibrium is readily established at the mercury-solution interface
• The surface area of a drop can be calculated from the weight of a drop
• Mercury forms amalgam (solid solution ) with most of the metals
Limitation
• It is highly toxic and has a measurable vapor pressure so care should be taken in the handling
• The surface area of a drop of mercury is never constant
• Applied voltage produces a change in surface tension and hence change in drop size
• Mercury has limited application in analysis
• Capillary is very small and thus can be easily blocked
• It can not be used at higher positive potential due to the oxidation of mercury prevents the use of more
positive(oxidizing)potential
• Hg can not used for the determination of species that can be oxidized above +0.4V
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Application of Polarography:
Qualitative analysis: helps in the characterization of organic matter and various metal
interactions from the half-wave potential of the current v/s voltage graph
Quantitative analysis: polarography is used in the determination of the concentration of drugs,
metal ions, etc in the given sample
Determination of inorganic compounds: polarography is used in the determination of cations
and anions in the presence of interfering ions
Determination of organic compounds: polarography is used in the determination of structure,
quantitative analysis of the mixture of organic compounds
estimation of dissolved oxygen: the amount of oxygen dissolved in an aqueous solution or
organic solvent can be calculated with the help of polarography
Pharmaceutical applications: tetracycline antibiotics, and sulphonamides can be analyzed by
polarography
Polarography has been used extensively to determine trace metals in Pharmaceutical products
and to estimate drugs that contain metals as a constituent, the metals examined include
antimony, arsenic, cadmium, copper, iron, lead, magnesium, mercury, vanadium, and zinc
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