Satish Pradhan Dnyanasadhana College, Thane .

1. SatishPradhan Dnyanasadhana College, Thane. • Department of ChemistryT.Y.B.Sc. Analytical ChemistryPaper-IV Sem-VIpolorography • 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)

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. 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.

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

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

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

25. Quantitative AnalysisQuantitative 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 .

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 -0.05 to -0.15 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 + 2e- ---------------------OH-. • H2o2 +2H+ +2e_ ……………………> 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