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

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)

Basic Terms 1) Polarization of electrode 2) Polarisable electrode 3) Non polarisable electrode 4) Electrode potential 5) Decomposition potential 6) Over voltage

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

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: Such electrode has its own potential. This electrode can not take up any potential applied to it. Surface area of this electrode is very large. Current density at this electrode is very large.. Ex. Large pool of Mercury electrode, Saturated Calomel Electrode.

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.

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

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.

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 ---------->

D C B A

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.

Factors Affecting on Limiting Current Residual Current Diffusion Current Migration Current

Instrumentation

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

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

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

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.

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

Diffusion current depends on following factors Temperature of solution Viscosity of the medium Composition of base electrolyte Dimension of the capillary The molecular or ionic state of electroactive species Pressure o the dropping mercury electrode

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

E1/2 Values for Some Metal Ions Cd 2+= -0.6 Volt Cu2+= +0.04Volt Pb2+= -0.40 Volt Ni2+= -1.42 Volts. Co2+= -1.42 Volts

(id)max = maximum current (µA) D = diffusion coefficient (cm2/s) 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

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

Method of Standard addition id1x Cs X Vs Cx=-------------------------------------- id2 X Vs+ (id2-id1) x Vx where Cx= Concentration of Unknown Cs= Concentration of Standard solution Vs= Volume of standard solution Vx= Volume of solution Unknown concentration id1= diffusion current of solution Unknown concentration. Id2 = diffusion current of standard solution+ solution Unknown concentration. id1= diffusion current of solution Unknown concentration. Id2 = diffusion current of standard solution+ solution Unknown concentration.

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 .

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

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.

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

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.

Current ( micro ampere) Voltage Polarographic maxima and its suppressor

Thank You