Method for determination of concentration .

1. POLAROGRAPHY
PRINCIPLE AND INSTRUMENTATION
Government Institute of science
-KIRAN A. BARBATKAR
M. SC. FIRST YEAR

2. INTRODUCTION
 When a solution is subjected to electrolysis, the
electrical properties like emf, current, resistance etc
depend on nature and concentration of various
species present in that solution.
 Simple principle of polarography is the study of
solutions or of electrode processes by means of
electrolysis with two electrode, one polarizable and
one non – polarizable.

3. History of polarography
• Polarography was invented in 1922 by Czech chemist
Jaroslav Heyrovsky for which he won NOBEL PRIZE in
1959.
• An electromechanical technique of analyzing solutions that
measures the current flowing between two electrodes in
the solutions as well as the gradually increasing applied
voltage to determine respectively the concentration of a
solute and it’s nature. - Created by Heyrovsky.
Jaroslav Heyrovsky
(1890-1967)

4. Principle
 Study of solutions or of electrode processes by
means of electrolysis with two mercury
electrode, one polarizable and one non
polarizable, the former formed by mercury
regularly dropping from capillary tube.
 Polarized electrode – Dropping Mercury
Electrode (DME)
 Depolarized electrode - Saturated Calomel
Electrode (SCE)

5.  Mercury continously drops
from reservoir through
capillary tube into the
solutions
 The optimum interval
between drops for most
analysis is between 2 to 5
seconds.

6. Why mercury?
Mercury as a working electrode is useful because :
 It displays a wide negative potential range.
 It’s surface is readily regenerated by producing
a new drop or film.
 Many metals ions can be reverisbly reduced
into it.

7. Polarogram
 It is a graph of current versus potential in a
polarograpic analysis.
3 categories :
 Collectively referred as residual Current
 Referred to as diffusion current resulting from
reduction of sample
 Called as limiting current

8. Types of current:
Residual Current :
 It is a non faradic current
 This current increases linearly with the applied voltage, and it is observed even when the purest,
air free solutions are used.
Migration current :
 Migration of charged particles in the electric field is caused by the potential difference existing
between the electrode surface and the solution .
 The migration current can be practically removed if an indifferent electrolyte is added to the
solution in a concentration so large that it’s ions carry essentially all the current.
 The concentration of the added electrolyte must be at least 100-fold that of the electro active
material.

9. Diffusion Current :
 It is calculated by following equation known as Ilkovic equation
Where, id = the average diffusion Current in microamperes during the life of drop
n= the number of Faraday of electricity required per mole of the electrode reaction
D=the diffusion coefficient of the reducible or oxidisable substance expressed as Cm^2/sec
C= concentration in mmol per litre
M=the rate of flow of mercury from the DME expressed in mg per second an
T = drop time in seconds

10. CONSTRUCTION : DME
 A small polarizable electrode is Produced by Passing
a steam of mercury Through a resistance glass
capillary.
 The assembly consists of –
1. a mercury reservoir.
2. A connecting tube between the mercury reservoir
and Capillary tube.
3. A capillary tube.
4. A glass electrolyte cell.
A polarography instrumentation

11. Instrumentation :
 The basic component of the experimental set
up are the polarograpic cell and the DME.
 The polarograpic cell is a H – shaped vessel. The
two tubes are connected through an agar agar
salt bridge and a sintered glass tube.
 One half of the cell act as anode which is
usually a reference electrode such as saturated
Calomel Electrode SCE.
 The other half cell contains the solution of the
sample analyte and the DME as the cathode.

12.  The DME consists of a 5-20 cm long capillary tube with an inner diameter 0.06-0.08mm
through which mercury is forced under pressure exerted by approximately 50 cm column or
mercury connected to a mercury reservoir.
 The electrical Circuit consists of a DC source, potentiometer, a voltmeter and microammeter
usually a galvanometer.
 The DME is connected to the negative terminal while SCE is connected to the positive
terminal of DC source.
 Using the potentiometer an emf upto 3 volt may be gradually applied to the cell.
 Since the DME is connected to the negative terminal of DC source the applied voltage is
given a negative sign by convention where as Current read on the ammeter is taken to be
positive.

13. Reference :
 https://www.slideshare.net/mobile/chelsisoliva1/polarography-61782546
 Pinteresthttps:///in.pinterest.com
 http://www.ceb.cam.ac.uk/data/images/groups/CREST/Teaching/hydro/fig1.gif
 https://chem.libretexts.org/Textbook_Maps/Analytical_Chemistry_Textbook_Maps/Map%3A_Analytical_Chemi
stry_2.0_(Harvey)/11_Electrochemical_Methods/11.4%3A_Voltammetric_Methods
 http://web2.uconn.edu/rusling/DC%20Polarography.pptx
 http://www.searchmap.eu/blog/jaroslav-heyrovsky/
 Vogels textbook of quantitative analysis ( page no 591-611)
 Basic concepts of analytical chemistry by S.M.Khopkar (page no 505-513).