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. 26.12.20016

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). 26.12.20016

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) 26.12.20016

Basic Terms Polarization of electrode Polarisable electrode Non polarisable electrode Electrode potential Decomposition potential Over voltage Back emf 26.12.20016

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. 26.12.20016

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. 26.12.20016

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. 26.12.20016

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 26.12.20016

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 1890-1967 26.12.20016

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. 26.12.20016

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 ----------> 26.12.20016

D C B A 26.12.20016

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. 26.12.20016

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Factors Affecting on Limiting Current Residual Current Diffusion Current Migration Current 26.12.20016

Instrumentation 26.12.20016

Classical Polarography Cell: SCE // ox (x M) / Hg (DME) Dropping Mercury Electrode 26.12.20016

Basic Assembly for DC Polorography - G polorogram. Mercury Reservoir V P - Solution + N2 Solution + 26.12.20016

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 26.12.20016

Limitations of Dropping mercury electrode 1 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. 26.12.20016

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 26.12.20016

Diffusion current depends on following factors 1 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 26.12.20016

APPLICATION OF POLOGRAPHY Qualitative Analysis: The half Wave potential is characteristic of reducible species which is used for qualitative analysis. 26.12.20016

E1/2 Values of Some Metal Ions Cd 2+=-0.06 Volt Cu2+= +0.04Volt Pb2+= -0.40 Volt Ni2+= -1.42 Volts. Co2+= -1.42 Volts E1/2 Values of Some Metal Ions 26.12.20016

(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 26.12.20016

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 26.12.20016

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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 . 26.12.20016

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 26.12.20016

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 + 2 e- ---------------------OH-. H2o2 +2 H+ +2 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. 26.12.20016

Diffusion current in uA +0.2 -0.4 -0.8 -1.2 -1.6 -2.0 +0.2 -0.4 -0.8 -1.2 -1.6 -2.0 Applied potential against SCE (Volts) Oxygen Wave 26.12.20016

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. 26.12.20016

Current ( micro ampere) Voltage Polarographic maxima and its suppressor 26.12.20016

Thank You 26.12.20016