1. Dnyanasadhana College, Thane.
Satish Pradhan
Dnyanasadhana College, Thane.
Department of Chemistry T.Y.B.Sc. Analytical Chemistry Paper-IV Sem-VI polorography
By Dr.Bhagure G.R.

2. Contents 1.2 Polarography (08L)
1.2.1 Difference between potentiometry and voltammetry, Basic principle of polarography, Polarizable and Nonpolarizable electrodes.
1.2.2 Polarizable and Nonpolarizable electrodes, Dropping Mercury Electrode (DME) construction, working advantages and limitations, Three electrode system.
1.2.3 Supporting electrolyte, its role, selection of supporting electrolyte.
1.2.4 DC polarogram: understanding of the terms Residual current, diffusion current, limiting current, Half wave potential, Polarographic maxima and maxima suppressors, Interference of
oxygen and its removal.
1.2.5 Ilkovic Equation and various terms involved in it (Derivation not expected).
1.2.6 Applications and advantages of polarography.
(Numerical Problems Expected).

3. Introduction:
Polorography is voltametric technique In which electrolysis of an electrolyte is carried out using dropping mercury electrode (cathode) and SCE or large pool of mercury (anode)

4. Basic Terms Polarization of electrode Polarisable electrode
Non polarisable electrode
Electrode potential
Decomposition potential
Over voltage
Back emf

5. 1. Polarization of electrode:-
It is behavior of an electrode which makes the potential of the electrode to differ from the theoretical value as calculated by Nernst’s equation.

6. 2. Polarisable electrode::
The electrode which undergoes polarization is called as polarisable electrode.
Characteristics of Polarisable electrode
Such electrode does not have its own potential.
This electrode can take up any potential applied to it.
Surface area of this electrode is very small
Current density at this electrode is also very small.
Ex. Dropping mercury electrode.

7. 3) Non polarisable electrode: The electrode which has its own potential and which can undergoes polarization is called as non polarisable electrode.
Characteristics of Non polarisable electrode:
a) Such electrode has its own potential.
b) This electrode can not take up any potential applied to it.
c) Surface area of this electrode is very large.
d) Current density at this electrode is very large..
Ex. Large pool of Mercury electrode, Saturated Calomel Electrode.

8. 4
Electrode potential: Electrical potential difference arise at an interface of an electrode and solution is called as electrode potential. 5
Decomposition potential :-The minimum potential that must be applied to an electrolytic solution to bring about continuous electrolysis. 6
Over voltage:- The difference between the observed decomposition potential and theoretical potential is called Overvoltage.
Overvoltage is due to polarization of electrode

9. Introduction:
Polarography: voltammetry at the dropping mercury electrode (DME)
DA: Hg (poison), apparatus (cumbersome), better techniques
Application:
Oxidation and reduction process
Adsorption processes on surfaces
Electron transfer mechanism
Jaroslav Heyrovsky

10. Principle of polography:-
Electrolyte + Supporting electrolyte (concentration 100 times higher than the analyte).
N2 gas has been passed in analyte solution to remove dissolved oxygen.
Electrolysis is carried out using DME and SCE
DME is made as cathode and SCE is made as anode.
During electrolysis metal ion goes to cathode, discharged, and reacts with Hg to form amalgam.

11. Cd+ Hg----------- CdHg. Current voltage curve obtained is called
Example.
Cd2+ +2e  Cd
Cd+ Hg  CdHg.
Current voltage curve obtained is called
as polorogram.and the instrument is called as polography.
POLOROGRAM
>

12. D C B A

13. Current A—B is called as residual current
Current B—C is called as diffusion current
Current C---D is called as Limiting current.
Diffusion current is directly proportional to the concentration of analyte.
Half wave potential is characteristics of every electro active material and it is use in qualitative analysis.

15. Factors Affecting on Limiting Current
Residual Current
Diffusion Current
Migration Current

16. Instrumentation

17. Classical Polarography
Cell: SCE // ox (x M) / Hg (DME)
Dropping Mercury Electrode

18. Basic Assembly for DC Polorography- G
polorogram.
Mercury Reservoir V P -
Solution + N2
Solution +

19. Advantages of Dropping mercury electrode
Surface is reproducible, smooth and continuously renewed.
Hg forms soluble amalgam with many metals hence lowers their reduction potentials.
High over voltage of hydrogen on mercury makes possible the deposition of ions difficult to reduce in aqueous solution
eg.alkali metal ions .
The surface area can be calculated from the weight of the drops.
5) Diffusion current assumes steady value . 1 2 3 4
Advantages of Dropping mercury electrode

20. Limitations of Dropping mercury electrode1
Hg is costly and poisons. 2
Current passing through the cell increases as drop grows and decreases as drops breaks 3
DME electrode generates some current like residual current, migration current, kinetic current which add error in current measurement. 4
Hg is oxidized ,it restricts the use of electrode as anode.

21. Ilkovic Equation (id)max= 607x n x D 1/2 x m 2/3 x t 1/6 x c where,
(id)max = maximum current (µA)
D = diffusion coefficient (cm2/s)
m = rate of flow of mercury (mg/s)
t = drop time (s)
c = analyte concentration (m moles/L)
n = number of electrons

22. Diffusion current depends on following factors1
Temperature of solution 2
Viscosity of the medium 3
Dimension of the capillary 4
The molecular or ionic state of electroactive species 5
Pressure o the dropping mercury

23. APPLICATION OF POLOGRAPHY
Qualitative Analysis:
The half Wave potential is characteristic of reducible species which is used for qualitative analysis.

24. E1/2 Values of Some Metal Ions
Cd 2+=-0.06 Volt
Cu2+= +0.04Volt
Pb2+= Volt
Ni2+= Volts.
Co2+= Volts
E1/2 Values of Some Metal Ions

25. (id)max= 607 n D (1/2) m (2/3) t (1/6) c
Quantitative Analysis Quantitative analysis can be performed using Ilkovic equation
(id)max= 607 n D (1/2) m (2/3) t (1/6) c
where,
(id)max = maximum current (µA)
D = diffusion coefficient (cm2/s)
m = rate of flow of mercury (mg/s)
t = drop time (s)
c = analyte concentration (m moles/L)
n = number of electrons

26. 1.Calibration Curve Method
Id is directly proportional to concentration.
Id= K X C
K= Id/C OR
(Wave height) h is directly proportional to concentration
h = K X C
K= h/ C

28. Other Applications Organic as well as inorganic compound
To estimate amount of dissolved oxygen.
Analysis of alloys.
Quantitative analysis as low as 10-6 M.
Complexes can be studied .

29. Riboflavin Milk, pharmaceuticals Antioxidants Fuels
Sr. No.
Type of sample
Element or compound 1
Foodstuffs
Cu, Pb, Sn, Zn 2
High purity aluminium
Ga, Zn, Cd, Ni 3
Steels
Cu, Pb, Ni, Co 4
Beer and soft drinks
Sn, Pb 5
High-purity salts
Transition metals 6
Petroleum fractions
Free sulphur 7
Riboflavin Milk, pharmaceuticals
Antioxidants Fuels 8
Fruit and vegetables
Vitamin C 9
Sea water, gases
Oxygen

30. Removal of Dissolved Oxygen from electrolyte solution
Dissolved oxygen in electrolytic solutions is easily reduced at the DME and produces polrogram consist of two waves of approximately equal height and extending over considerable range. Their position depends upon the pH of the solution.
The first wave is due to reduction of oxygen to hydrogen peroxide at to volt.
O2 + H2o+2e  H2O2+OH- (neutral or alkaline solutions).
O2+2H+ + 2e H2o2 (Acidic solutions).
The second wave is at -0.9 to -1.0 volts corresponds to further reduction of H2o2 to H2o or OH-
 H2o2 + 2 e OH-.
H2o2 +2 H e_ ……………………> 2H20.
Thus oxygen wave interferes the polographic wave during normal analysis making accurate determination difficult. It is therefore necessary to pass Nitrogen gas to remove dissolved oxygen from electrolytic solutions.

31. Diffusion current in uA +0.2 -0.4 -0.8 -1.2 -1.6 -2.0
Applied potential against SCE (Volts)
Oxygen Wave

32. Polarographic maxima and its suppressor
The current voltage curves obtained during polarographic analysis are not always well defined, S shaped curve as expected by the theory. But frequently the current keeps on increasing on the rising portion of the curve to much a higher value and falls on suddenly to attain steady limiting current. It is known as maxima. The exact reason of maxima not understood. The maxima can be eliminated by adding surface acting agent like gelatin or triton X-100. These substances are called as maxima suppressor. The proper amount or maxima suppressor must be determined by trial and error.

33. Current ( micro ampere)
Voltage
Polarographic maxima and its suppressor

34. Thank You