POLAROGRAPHY

Definition: Polarography is an electrochemical method of analysis based on the measurement

of current flow resulting from the electrolysis of a solution at a polarisable microelectrode as a
function of applied voltage.
Principle:
 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:
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).
id = 607 nD1/2 m2/3 t1/6 C
Where,
id = Diffusion current in microamperes
607 = Constant of various numerical factors including: Faraday constant (П),
density of Hg, etc.,
n = Number of electrons duly involved in the electrode reaction,
D = Diffusion coefficient in cm2.sec-1,
m = Weight of Hg flowing via the capillary in mg.sec-1,
t = Drop time in seconds,
C = Concentration in mmol/L.
The Ilkovic Equation holds good for the ‘drop-time’ to vary between 2 to 8 seconds. In order
to accomplish this aim and objective the following two critical adjustments may be done
carefully:
 Length of capillary
 Manouvering the Hg-pressure to bring the drop time very much within the range
(i.e., 2-8 sec)
There are four major governing factors that influence the Ilkovic equation:
1. Both ‘m’ and ‘t’ shall change with the dimensions of the capillary (its length) and the
applied pressure of Hg reservoir to form the ‘drop’.
2. Height of Hg column must be maintained constantly as the ‘drop time’ solely depends
upon the applied pressure by the column of Hg at the tip of DME and ‘analyte’
solution interface.
3. Applied voltage in a DME-assembly is responsible for causing possible changes
occurring in the prevailing ‘surface tension’ of a drop at the tip of electrode.
4. Evidently, the variations in temperature and viscosity must be at bare minimum level
because it disturbs the ‘diffusion coefficient’ most significantly.

Definition:
1. Residual current (ir):
The current that flows in the absence of the depolarizer (i.e. due to the supporting
electrolyte) is called residual current. This has to be taken into consideration while
interpreting the polarograms.
It is the sum of the relatively larger condenser current (ic) and a very small faradic
current (if).
ir = if + ic
ic (condenser current) – is due to the formation of Helmholtz double layer at the
mercury surface.
if (faradic current) – is due to the traces of impurities.
2. Migration current (im):
It is due to migration of cations
from the bulk of the solution
towards cathode due to diffusive
force, irrespective of concentration
gradient.
3. 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
ion from the bulk of the sample to
the surface of the mercury droplet
due to concentration gradient.
4. Half wave potential
Half wave potential is the potential at which the concentration of oxidised and reduced
forms at electrode surface is equal. i.e., 50% of oxidised and 50% of reduced forms are
present.
5. Limiting current (il):
Beyond a certain potential, the current reaches a steady state value called as the limiting
current. At this point, the rate of the diffusion of ions is equal to the rate of reduction
of ions, and the state of electrode is said to be concentration polarised.
Dropping Mercury Electrode (DME):
Dropping mercury electrode (DME) is a working electrode arrangement for polarography in
which mercury continuously drops from a reservoir through a capillary tube (internal diameter
0.03 - 0.05 mm) into the solution. The optimum interval between drops for most analyses is
between 1 and 5 s. The unique advantage
to the use of the DME is that the constant
renewal of the electrode surface, exposed
to the test solution, eliminates the effects
of electrode poisoning.
Construction:
 The assembly consists of a
mercury reservoir.
 It consists of fine capillary having
bore size ranged from 20-50 µ and 10-15
cm long.
 The capillary is connected to
mercury reservoir by rubber tubing.
 A small glass electrolysis cell in
which the unknown solution is placed.
 The height of the mercury
reservoir is adjusted such that drop time is
1-5 seconds.
Working:
 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.
Advantages:
1. Surface area is reproducible.
 2. Electrode can be renewed and thus eliminates poisoning effect.
3. Mercury forms amalgam (solid solutions) with many metals.
4. The surface area can be calculated from the weight of the drop.
Disadvantages:
1. Capillary is very small and thus can be easily blocked.
2. Mercury is very toxic.
3. Surface area of each drop of mercury is never constant.
4. It cannot be used at higher positive potential due to oxidation of mercury.

Rotating Platinum Electrode:

DME has disadvantage that it cannot be used at high potential due to oxidation of mercury.
Therefore, platinum electrode is used in such cases.
Why the platinum electrode is rotated?
If the platinum electrode is stationary then diffusion current will be slowly attained, so to
overcome this problem platinum electrode rotated at constant speed, which results in increasing
sensitivity and rate of attaining steady diffusion current.
Construction:
The construction of the rotating platinum electrode is
evident from the figure.
The electrode is constructed from a standard ‘mercury
seal’.
It consists of about 5mm platinum wire having 0.5mm
diameter below standard mercury seal by passing
through small hole.
A wire from mercury seal is connected to the source
that applies voltage.
The tubing forms the stem of the electrode which is
rotated at a constant speed of 600 rpm.
Working:
Rotating platinum electrode is used as an indicator
electrode. To the analyte solution supporting
electrolyte like KCl is added i.e., 50-100 times of
sample concentration.
Pure nitrogen gas is bubbled through the solution to
expel out the dissolved oxygen.
Potential is applied across the electrodes and titration
is started.
A graph is plotted between the volume of solution
added v/s diffusion current and end point is detected.
Applications of Polarography:
1. Qualitative analysis: It helps in characterization of organic matter and various metal
interactions from half wave potential of the current v/s voltage graph.
2. Qualitative analysis: Polarography is used in the determination of concentration of
drugs, metal ions etc. in the given sample.
3. Determination of inorganic compounds: Polarography is used in determination of
cations and anions in the presence of interfering ions.
4. Determination of organic compounds: Polarography is used in determination of
structure, quantitative analysis of mixture of organic compounds.
5. Estimation of dissolved oxygen: Amount of oxygen dissolved in aqueous solution or
organic solvent can be calculated with the help of Polarography.
6. Pharmaceutical applications: Tetracycline antibiotics, sulphonamides can be
analysed by Polarography.

Long essay: 10 marks

1. Explain the construction and working of dropping mercury electrode with a neat
labelled diagram.
2. Explain the construction and working of rotating platinum electrode with a neat
labelled diagram.

Short essay: 5 marks

1. Explain the principle of polarography.
2. Explain principle of polarography with ILKOVIC equation.

Short answer: 2 marks

1. Enumerate the recent advance methodologies in polarography.
2. Give the applications of Polarography.
3. Define: (i) Residual current (ii) Migration current
4. Define: (i) Limiting current (ii) Diffusion current
5. Define: (i) Polarogaphy (ii) Half wave potential.