Polarography and Voltammetry

VOLTAMMETRY
2
 Discovery of Polarography - Jaroslav Heyrovsky, 1922
 Noble Prize, 1959
 Electro analytical technique
 Application of a Potential (E) to an electrode and the monitoring of the resulting
Current (I) flowing through the electrochemical cell.
 Applied potential is varied or the current is monitored over a period of time (t).
Thus, all voltammetric techniques can be described as some function of E, i, and t.
 Oxidation and Reduction

WORKING ELECTRODE
 Indicator Electrode
 Reaction such as Oxidation or Reduction takes
place.
 Geometries & Materials
Dropping Mercury
Electrode (DME)
Platinum Disk
Electrode
Gold Electrode
Glassy carbon
Electrode
3

REFERENCE ELECTRODE
 Standard Electrode
 Potential remains Constant
 Calomel Electrode
 Silver/Silver Chloride Electrode
AgCl(s) + e– → Ag(s) + Cl–
Hg2Cl2(s) + 2e– → 2Hg(l) + 2Cl–
Calomel Electrode
Silver Electrode
4

COUNTER ELECTRODE
 Auxiliary Electrode
 serves to Carry the current through the cell
 thin Pt wire, Au, Graphite
Platinum wire
Gold wire
Graphite
5

Dropping Mercury Electrode
(DME) – Polarography
Platinum Electrode – Cyclic
Voltammetry
6

POLAROGRAPHY
 Branch of Voltammetry
 Unique electrode – Dropping Mercury Electrode (DME)
PRINCIPLE:
By gradually increasing voltage is applied between two electrodes, one of which is
polarizable and the other is non-polarizable and the current flowing between the two
electrodes is recorded.
7
Instrument - Polarograph
Curve–Voltage Curve - Polarogram

INSTRUMENTATION
Working Electrode –
Dropping Mercury
Electrode (DME)
Reference Electrode –
Mercury Pool
Mercury
Pool
8
Potentiostat - Control and Measuring Device for electrochemical cell
Computer – Result
Electrochemical cell
o Reaction takes place
o Sample dissolved in a solvent, an ionic electrolyte, and three (or
sometimes two) electrodes.

WORKING
 Flask A contains an experimental solution that was
saturated by H2 or N2 gas through tube B.
 Mercury from reservoir C falls to the solution at the end of
the capillary tube D at the rate – 20-30 drops per minute.
 Drops act as cathode and continuously renewed.
 Mercury Pool E – Anode (Potential constant)
 Cathode & Anode are connected to the Battery F.
 Applied Potential can be varied by moving contact G
through the Potentiometer wire HI
 Current Strength – Galvanometer J
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 E.M.F increased between C and E by moving contact G from I to H
 Potential increases which increase the Current.
 Curve occurs can be seen in automatic registering apparatus – Polarograph
 Current-Voltage curve – Polarogram/Polarographic waves
Polarograph
Polarogram
10

POLAROGRAM
Voltage
Current
Residual current:
Due to supporting electrolyte
It occurs due to condenser or capacitance current and small
faradic current.
ir = ic + if
ic → condenser current, due to formation of Helmholtz
double layer at mercury surface
if → faradic current due to small impurities
Migration Current (im) :
Due to migration of cations from the bulk of the solution towards
cathode due to diffusive force
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 electroreducible
ions from the bulk of the sample to the surface of the mercury
droplet due to concentration gradient.
11

ILKOVIC EQUATION
𝒊𝒅 = 𝟕𝟎𝟔𝒏𝑫𝟏 𝟐𝒎𝟐 𝟑𝒕𝟏 𝟔
id – Diffusion current in microamperes
n - is the number of electrons exchanged in the electrode reaction
D - is the diffusion coefficient of the depolarizer (cm2 s-1)
m - is the rate of mass flow of the mercury (mg s-1 ),
t - is the drop time (s)
C - is the depolarizer concentration
The current for the polarographic plateau can be predicted by the Ilkovic equation,
If mean currents are measured, the equation becomes
𝒊𝒅 = 𝟔𝟎𝟕𝒏𝑫𝟏 𝟐𝒎𝟐 𝟑𝒕𝟏 𝟔
12

Half-wave Potential:
 Potential at mid-point wave (i = id/2) is known as Half-wave potential (E1/2)
 E1/2 diffusion current is half.
 E1/2 – characteristic feature of the element
 It gives the qualitative analysis, knowing the value of E1/2 one can predict the element
present in the solution.
 If the substance not identified, it is possible to identify by means of the polarographic
curve. The reducible material characterised Half-wave potential, this is the potential at the
point of inflection of its current – potential curve (half way up its polarographic wave).
 At the mid point of the polarographic wave the concentration of the ions which is
discharged is half the value in the bulk solution which depends on the magnitude of the
diffusion current and on the concentration of electrolyte.
𝑬 = 𝒄𝒐𝒏𝒔𝒕. −
𝑹𝑻
𝒏𝑭
ln
𝑎𝑀
+
𝑎𝑀
𝑎𝑀 - Activity of metal
𝑎𝑀
+
- Activity of ion
13

Limiting Current:
The current reaches a steady state value called the limiting current. At this point, the rate of
the diffusion of ions is equal to the rate of reduction of ions.
𝑬 = 𝒄𝒐𝒏𝒔𝒕. −
𝑹𝑻
𝒏𝑭
ln
𝑐𝑀
+
𝑐𝑀
𝑐𝑀
+
& 𝑐𝑀 are the corresponding concentrations
Suppose a solution of M+ ions of concentration is reduced at the DME cathode, and the maximum
concentration of the metal M amalgam formed in the drops is cM; at the half-wave point the respective
concentration at the drop surface will be ½ cM
+ & ½ cM
14

DROPPING MERCURY ELECTRODE
 Working micro electrode
 Act as a cathode
 Pure Hg (purified by dil HNO3)
 Glass capillary of 10-15 cm length
 internal diameter - 0.5 mm
 Drop is formed between 1-5 seconds
 Negative terminal of battery
15

ADVANTAGES OF DROPPING MERCURY ELECTRODE
Surface - Conductive,
Smooth & Reproducible
No contamination at
the surface of
dropping mercury
electrode
16
Mercury drop weights can
be used for the calculation
of surface area.
No poisoning of
dropping mercury
electrode occurs.
Many metal ions form
amalgams with mercury.
Hydrogen has
overvoltage with respect
to mercury (SCE) a large
number of metallic ions
can be reduced.

Limitations of DME
 Due to the formation of mercury drop gradually surface area increases and
consequently a little fluctuation in current may occur like an oscillation
which may interfere with an estimation as an average current is considered.
 DME can act as a good electrode between 0.4 to -2.66 V.
17

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APPLICATIONS
PHARMACEUTICAL
SCIENCE - Analysis of
Drugs, tablets & injection
solutions.
FOOD INDUSTRY – Analysis
of copper, lead, iron, vitamin,
etc. in various food stuff.
ENVIRONMENTAL SCIENCE -
Analysis of water samples for
the presence of contaminants
& air for the presence of
pollutants
CLINICAL ANALYSIS -
Analysis of blood samples,
hair samples & detection of
poisons in the samples
ORGANIC COMPOUNDS -
Qualitative and quantitative
determination of organic
compounds. Structure
validation and evaluation
TRACE METAL ALLOYS -
Analysis of trace metal alloys,
Minerals & their ores

CYCLIC VOLTAMMETRY
 Potentiodynamic Electrochemical technique
 Used for Quantitative determinations
 Redox Process
 This technique is based on varying the applied potential at a working electrode in
both forward and reverse directions (at some scan rate) while monitoring the
current.
 Working electrode potential is ramped linearly versus time. After the set potential is
reached in a CV experiment, the working electrode's potential is ramped in the
opposite direction to return to the initial potential. These cycles of ramps in
potential may be repeated as many times as needed.
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Three Electrode Setup
Reference electrode Standard Calomel Electrode
Working electrode – - Glassy carbon, Platinum, & Gold
Counter electrode - Platinum and Graphite.
Working
 In the forward scan, the potential first scans negatively,
starting from a greater potential (a) and ending at a lower
potential (d)
 (d) - Switching potential, the point where the voltage is
sufficient enough to cause oxidation or reduction of an
analyte
 Reverse scan occurs from (d) to (g) - potential scans
positively
 This cycle can be repeated, and the scan rate can be
varied
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 Cyclic voltammogram resulting from a single electron
reduction and oxidation.
 Reduction process occurs from (a) the initial potential to
(d) the switching potential. In this potential scanned
negatively to cause a reduction. The resulting current is
called cathodic current (ipc)
 Peak potential (c) – Cathodic Peak potential Epc – when all
the substrate get reduced.
 After the switching potential has been reached (d), the
potential scans positively from (d) to (g).
 This results in anodic current (Ipa) and oxidation to occur.
The peak potential at (f) is called the anodic peak potential
(Epa) and is reached when all of the substrates at the surface
of the electrode has been oxidized.
21

Important parameters - peak potentials (Epc, Epa) and peak currents (ipc, ipa)
If the electron transfer process is fast the reaction is said to be electrochemically reversible,
Peak separation is
ΔEp = Epa – Epc = 2.303 RT / nF
The formal reduction potential (Eo) for a reversible couple is given by
𝐸0 =
𝐸𝑝𝑐 + 𝐸𝑝𝑎
2
For a reversible reaction, the concentration is related to peak current by the Randles–Sevcik
expression,
ip = 2.686 × 105  n3 2 A c0D1 2n1 2
where ip - peak current in amps
A is the electrode area (cm2),
D is the diffusion coefficient (cm2 s–1), c0 is the concentration in mol cm–3
υ is the scan rate in V s–1
22

Applications of Cyclic Voltammetry
 Used to study qualitative information about electrochemical processes under
various conditions, such as the presence of intermediates in oxidation-reduction
reactions, and the reversibility of a reaction.
 To determine the electron stoichiometry of a system, the diffusion coefficient of
an analyte, and the formal reduction potential, which can be used as an
identification tool.
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Determination of organic and inorganic compounds in aqueous
and nonaqueous solutions
Measurement of kinetic rates and constants
Adsorption processes on surfaces
Electron transfer and reaction mechanisms
Thermodynamic properties of solvated species
Complexation and coordination values.
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Quantitative determination of pharmaceutical
compounds
Determination of metal ion concentrations in water
to sub–parts-per-billion levels
Determination of redox potentials
Determination of number of electrons in redox
reactions
Kinetic studies of reactions
25

Limitations of Voltammetry
 Substance must be oxidizable or reducible in the range were the
solvent and electrode are electrochemically inert.
 Provides very little or no information on species identity.
 Sample must be dissolved
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REFERENCES:
 Crow, Principles and applications of electrochemistry, Chapman and Hall, 1988.
 Glasstone, Introduction to Electrochemistry, Von Nostrand
 Christopher m. A. Brett and Ana brett, Electrochemistry: principles, methods, and applications, 1993.
 Frank A Settle, Instrumental techniques for Analytical chemistry
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Polarography and Voltammetry

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