1. S.M.Joshi College, Hadapsar, Pune
Polarography
Dr. S.B. Misal
S.M.Joshi College, Hadapsar, Pune

2. The theory and technique of Polarography have been developed only long time after Palmaer used the dropping mercury electrode in 1898.
It was only in 1922, that Jaroslav Herovsky studied the characteristics of voltage-current curves obtained in the electrolysis of substances at the dropping mercury electrode under conditions of concentration polarization.
Later, he was awarded a Nobel Prize in 1959 in recognition of its importance.

3. Voltammetry comprises a group of electro analytical methods in which information about the analyte is derived from the measurement of current as a function of applied potential obtained under conditions that encourage polarization of the indicator or working electrode. Generally, the working electrodes in Voltammetry are characterized by their small surface area [usually a few square millimeters] which enhances polarization. Such electrodes are generally referred to as micro electrodes.
Polarography is that branch of Voltammetry in which changes in current, resulting from the electrolysis of the solution under study are investigated using a renewable mercury droplet as the indicator electrode (cathode). The anode of the electrolytic cell called the reference electrode consists of either a mercury pool at the bottom of the cell or a calomel electrode.

4. Voltammetric Methods Historical Electrolysis at DME -1920’s
Usually 3-electrode cells
Measurement of current that results from the application of potential.
Different voltammetric techniques are distinguished primarily by the potential function that is applied to the working electrode and by the material used as the working electrode.

5. Types of Voltammetry
Polarography
Linear sweep and Cyclic Voltammetry
Hydrodynamic Voltammetry
Pulsed methods
AC Voltammetry (not here)
ip = 2.69 x 105 n3/2 A DO1/2 v1/2 CO
Voltammetry at a dropping mercury electrode

6. History
Jaroslav Heyrovský was the inventor of the polarographic method, and the father of electroanalytical chemistry, for which he was the recipient of the Nobel Prize (1959). His contribution to electroanalytical chemistry can not be overestimated. All modern voltammetric methods used now in electroanalytical chemistry originate from polarography.
On February 10, 1922, the "polarograph" was born as Heyrovský recorded the current-voltage curve for a solution of 1 M NaOH. Heyrovský correctly interpreted the current increase between -1.9 and V as being due to deposition of Na+ ions, forming an amalgam.

7. Swedish king Gustav Adolf VI awards the Nobel Prize to Heyrovský in Stockholm on 10.12.1959
The theory and technique of Polarography have been developed only long time after Palmaer used the dropping mercury electrode in It was only in 1922, that Jaroslav Herovsky studied the characteristics of voltage-current curves obtained in the electrolysis of substances at the dropping mercury electrode under conditions of concentration polarization. Later, he was awarded a Nobel Prize in 1959 in recognition of its importance.

8. PRINCIPLES OF POLAROGRAPHY
Polarography is an electrochemical method of analysis incorporating features of electrolysis. In electrolysis the aim is to remove completely a chosen constituent from the solution by passing an electric current through it for a sufficient length of time. The electrodes have relatively large surfaces and the solution is stirred to facilitate transport of electroactive material to the electrode. The electrolysis in Polarography is of short duration and the electrode on which the constituents are placed out is a dropping mercury electrode or other micro electrode so that the currents are very small.

9. Polarography used in analytical chemistry, involves measurements of current-voltage curves, obtained when voltage is applied to electrodes immersed in the solution being investigated. One of the electrodes is an indicator electrode. It is a dropping mercury electrode, consisting of a mercury drop hanging at the orifice of a fine bore glass capillary. The capillary is connected to a mercury reservoir so that mercury flows through the capillary at the rate of a few milligrams per second. The life time of each drop is usually 3 to 5 seconds. Each drop forms a new electrode; its surface is practically unaffected by processes taking place on the previous drop. Hence, each drop represents a well-reproducible electrode with fresh clean surface. The second electrode is a reference electrode; its potential remains constant during the measurement. The potential at the indicator electrode varies in the course of measurement of the current-voltage curve, because of the change of the applied voltage.
A polarogram can be plotted between the current flowing through the
polarographic cell against the increasing potential of the dropping electrode.

10. Polarogram
Potential applied on the working electrode is usually swept over (i.e. scan) a pre-defined range of applied potential
0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte
Electrode become more and more reducing and capable of reducing Cd2+
Cd e- Cd
Current starts to be registered at the electrode
Current at the working electrode continue to rise as the electrode become more reducing and more Cd2+ around the electrode are being reduced. Diffusion of Cd2+ does not limit the current yet
All Cd2+ around the electrode has already been reduced. Current at the electrode becomes limited by the diffusion rate of Cd2+ from the bulk solution to the electrode. Thus, current stops rising and levels off at a plateau
i (A) E½
Working electrode is no yet capable of reducing Cd2+  only small residual current flow through the electrode id
Base line of residual current
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
V vs SCE

11. POLAROGRAPHIC CURRENTS
The total current that flow in, consists of the following components:
a) Residual Current,
b) Migration Current and
c) Diffusion Current.
a) Residual Current
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.
b) Migration Current
It is the current due to migration of ions caused by the electrostatic
field between the two polarographic electrodes. The contribution of this
component is minimized under experimental conditions by taking large
concentrations of the supporting electrolyte. Under these conditions, only
ions of the supporting electrolyte migrate in electrostatic field. The ions of
the depolarizer do not take part in migration. They reach the surface of the
electrode by the process of diffusion.

12. c) Diffusion Current
Diffusion current is due to electroactive substances. When the potential of the d.m.e is in the plateau region the electroactive ions or molecules are reduced or oxidized as soon as they reach the electrode surface. As a result they are consumed and their concentration in a thin layer of solution in contact with the electrode becomes zero. Now, there is a concentration gradient of the depolarizer in the solution and the electroactive ions or molecules diffuse towards the drop from adjacent layers a little further away from the electrode resulting in the limiting current. When the mercury drop falls, the solution rushes into that space and the concentration gradient disappears when a new drop is formed, diffusion takes place again at the same rate as described earlier. The average current during the life time of each drop is the same and thus it leads to the flat plateau on the polarogram.
The limiting current of a wave is very important and is called diffusion current as it reflects the rate at which the ions or molecules of the depolarizer reach the electrode surface under the influence of a diffusion force.

13. The basis for the quantitative polarographic analysis is the existence of the linear proportionality between the diffusion current and the concentration of the depolarizer as given by Ilkovic equation.
The Ilkovich Equation:
id = 708nD1/2m2/3t1/6C
I has units of Amps when D is in cm2s-1,m is in g/s and t is in seconds. C is in mol/cm3
This expression gives the current at the end of the drop life. The average current is obtained by integrating the current over this time period
id = 607nD1/2m2/3t1/6C
where,
id = Average diffusion current, μA (10-6amps)
n = Number of Faradays per mol involved in the electrode
reaction
D = Diffusion coefficient of the electroactive material,
(cm2/sec)
C = Concentration of the electroactive material,
(millimoles/litre).
m = rate of flow of mercury through the capillary, (mg/sec).
t = time between successive drops of mercury, (in seconds).

14. Polarography uses mercury droplet electrode that is regularly renewed during analysis.
Applications:
Metal ions (especially heavy metal pollutants) - high sensitivity.
Organic species able to be oxidized or reduced at electrodes: quinones, reducing sugars and derivatives, thiol and disulphide compounds, oxidation cofactors (coenzymes etc), vitamins, pharmaceuticals.
Alternative when spectroscopic methods fail.