Polarographic Principles and Instrumentations

The basic premise of polarography is the study of solutions or electrode processes using
electrolysis with two electrodes, one polarisable and one non – polarisable. When a solution
is electrolysed, the electrical characteristics such as emf, current, resistance, and so on are
determined by the type and concentration of the different species present in the solution.
This article will learn about the polarographic principle and application of polarography.

What Is Polarography?
Polarographic sensors serve as the foundation for field microelectrodes. However, field
microelectrodes can also be Polarographic. Polarography, also known as electrochemical
Polarography, is an electroanalytical method for determining the reduction potential of
electroactive species.

Potentiometric techniques include polarographic methods such as coulometry, anodic

stripping voltammetry, and atomic emission spectrometry. The concept is based on
measuring the decrease in current or voltage that occurs in a polarisable electrolyte in
contact with a polarised electrode. The ability of the electrode to become polarised is
directly related to the activity of the material in the electrolyte.

Principle Of Polarography
 Polarography is based on applying a progressively rising voltage between two electrodes.
One of these voltages is polarisable (dropping mercury electrodes) and the other non-
polarisable, recording the current flowing between the two electrodes.
 A sigmoid-shaped current-voltage curve is used to calculate half-wave potential and
diffusion current.
 Diffusion current is used to calculate the concentration of a chemical.
 The half-wave potential is a property shared by all elements.
Instrumentation
A cell or electrochemical cell, an electrode, and a potentiostat are the three major components of a
polarographic device. In the case of the instrument’s probe, the potentiostat also functions as the
electrode. A cell is an electrochemical cell with electrodes that stores a solution. Each electrode in a
cell has one or more potentials applied to it, and each electrode has a potential used to it about
another electrode in the cell. The probe is the instrument’s electrode. An electrode can function as
both the working and counter electrode.

Ilkovic Equation

The Ilkovic equation is a polarography formula that connects the diffusion current (id) with the
concentration of the non-polarisable electrode, i.e., the chemical reduced or oxidised at the falling
mercury electrode (polarisable electrode).
(id)avg = 607 ncm2/3 t1/6 D1/2
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:

1. 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.

2. The counter electrode is a non-polarisable electrode.

3. To the analyte solution, electrolyte like KCl is added i.e., 50-100 times of sample concentration.

4. Pure nitrogen or hydrogen gas is bubbled through the solution, to expel (remove) out oxygen.

5. Eg: If the analyte solution contains cadmium ions, then cadmium ions are discharged at
cathode. Cd2+ + 2e- → Cd

6. Then, gradually increasing voltage is applied to the polarographic cell and current is recorded.

7. Graph is plotted between voltage applied and current. This graph is called Polarograph and the
apparatus is known as Polarogram.

8. The diffusion current produced is directly proportional to concentration of analyte and this is
used in quantitative analysis.

9. 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:
1. The construction of the rotating platinum electrode is
evident from the figure.

2. The electrode is constructed from a standard ‘mercury seal’.

3. It consists of about 5mm platinum wire having 0.5mm

diameter below standard mercury seal by passing through
small hole.

4. A wire from mercury seal is connected to the source that

applies voltage.

5. The tubing forms the stem of the electrode which is rotated

at a constant speed of 600 rpm.

Working:
1. 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.

2. Pure nitrogen gas is bubbled through the solution to expel

out the dissolved oxygen.

3. Potential is applied across the electrodes and titration is started.

5. 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.