Polarography[1]

Polarography

1. Submitted To:- Diksha Thakur Ma'am Submitted By:- Nitesh Polarography

2. CONTENT What is polarography?? Principle Instrumentation Components used Types of Currents Types of Polarography Applications 02

3. Polarography It is a electro- analytical technique that measure the current flowing between two electrodes in the solution. It is a type of voltametric technique. It can be used for qualitative as well as quantitative analysis. History Polarography was discovered by Jaroslav Heyrovsky and got a Nobel prize in 1959. In the past two decades many variations of heyrovsky's classical polarographic method have evolved, principally to improve the sensitivity and resolution of this analytical method. 03

4. Principle Polarography is a voltammetric technique in which chemical species (ions or molecules) undergo oxidation (lose electrons) or reduction (gain electrons) at the surface of a dropping mercury electrode (DME) at an applied potential. - Polarizable Electrode& WE: - Dropping mercury Electrode - Non-polarizable Electrode& RE :- Calomel Electrode From the current-voltage curve qualitative and quantitative analysis can be done. The instrument is called Polarograph and the curve is called as Polarogram and the technique is called Polarography. 04

5. In Polarography, mercury is used as a working electrode , because mercury is a liquid metal and thus the electrode can be renewed after each droplet. There are three types of electrodes:- DME( Dropping Mercury Electrode) SMDE(Static Mercury Drop Electrode) HDME( Hanging Mercury Drop Electrode) Types of Mercury Electrode Used in Polarography 06

6. Why Dropping Mercury Electrode?? Each fresh drop exposes a new Hg surface to the solution. The resulting behavior is more reproducible than that with a solid surface because the liquid drop surface does not become contaminated in the way solid electrodes can be contaminated . The natural interval between drops is 2-6s. It is easier, too, to reduce most metals to their mercury amalgum . Solid electrodes have surface irregularities because of their crystalline nature.So, Liquid mercury provide smooth, reproducible surface that does not depend on any pretreatment(Polishing or Etching) . 08

7. Instrumentation

8. Components It consists of:- The Polarographic Cell 3 Electrodes N2,H2 Gas Analyte Solution Potentiostat A PC with the application software The three - Electrode system for aqueous work consists of the DME as the working Electrode. It is also known as heart of Polarography. A Pt wire or foil auxiliary Electrode, and an SCE for reference . A narrow capillary is connected to a mercury column, which has a pressure head that can be raised and lowered. 11

9. Reference Electrode Reference Electrode& Auxiliary Electrode The purpose of the auxiliary electrode (AE) is to provide a pathway for current to flow in the electrochemical cell without passing significant current through the reference electrode. Remember that if a reduction occurs at the WE, there must be an oxidation that takes place at the AE. Care should be taken that electrode products formed at the AE do not interfere with the WE reaction. Which is used to measure the potential of working electrode. This electrode is larger in size relative to working electrode and is none polarize (depolarized) i.e. it retains a constant potential. An ideal reference electrode has a stable, well-defined electrochemical potential. Common reference electrodes (SCE, Ag/AgCl, Cu/CuSO4) 02

10. Working DME is a polarizable electrode and can act as both anode and cathode. The pool of Mercury acts as counter Electrode(in some cases). Which is Non- Polarizable in nature. To the analyte solution, electrolyte like KCL is added i.e 50-100 times of the sample concentration. Then, gradually increasing voltage is applied to the polarographic cell and current is recorded. Graph is plotted between voltage and current. 13

11. Mode Of Transport Of Ions By Convection:- By Mechanical stirring or by heating, as it increases current increases. Movement of mass due to a natural or mechanical force. By Migration:- Electrostatic atrraction b etween Positive species and the Negative cathode, Movement of a charged species due to a potential gradient. The current produced here is known by Migration Current , It can be minimized by: - Adding large excess of inert electrolyte( not reducible) known by supporting electrolyte . (makes it move under diffusion only) Role of Electrolyte:- It ensure that the electroactive species reaches the electrode by diffusion - It lowers the resistance of the electrolyte. - Concentration should be o.1-1M. -KCL, HNO3 16

12. Degassing Of The Electrolyte Degassing of the electrolyte by bubbling purified nitrogen or argon. This is necessary because dissolved oxygen from the air is present in the electrolyte and, unless removed, would complicate interpretation of the polarogram. Effect of Dissolved Oxygen Oxygen is an electro reducible species its presence in solution produces a double wave in the range of 0 to -1.0V due to reduction - Thus oxygen must be removed by bubbling nitrogen through the solution for five minutes to expel oxygen. 17

13. By Diffusion 19

14. In Polarography Diffusion is the means by which an electroactive species reaches the electrode when a concentration gradient is created by the electrode reaction. The electron transfer process can decrease the concentration of an electroactive species or produce a new species that diffuses away from the electrode surface. In polarographic analysis the mode of mass transport should be only by diffusion . 20

15. Types of Currents Residual Current Diffusion current Limiting Current Residual Current:- Even in the absence of analyte, a small currents flows through an electrochemical cell. This current, which is called the residual current , consists of two components: - A Faradaic Current due to the oxidation or reduction of trace impurities, - A Charging Current , which is the current that required for the charging of the so called electrical double layer at the surface of mercuric electrode. Diffusion Current:- Diffusion current is directly proportional to the concentration of the electro active material. It is the difference between limiting and residual currents. A concentration gradient is therefore established and more of that species diffused from the bulk solution to the electrode surface (Fick's law of diffusion). The resulting current flow is proportional to the rates of diffusion. I α (C-Co) or i=K(C—Co) 22

16. By increasing the applied voltage, reduction occurs more rapidly and (Co) becomes zero and the concentration gradient reaches a maximum. At this point, the rate of the diffusion and the current flowing in the cell reaches a limiting value. Ilkovic Equation It is the equation that relates the parameters that determine the magnitude of the diffusion current obtained with a dropping Mercuric electrode. id = is the average diffusion current in microampere. n = is the number of electrons consumed in the reduction of one molecule of the electroactive species. D = is the diffusion coefficient of the reducible or oxidizible substance express as Cm2/s. M = is the rate of flow of mercury from the dropping mercuric electrode express in mg/s. T = is the drop time in s. C = is the concentration in m mol/1 23

17. Limiting Current The limiting current arises essentially because the diffusion process has now attained a steady state. At this point, the rate of diffusion ions is equal to the rate of reduction. The state of electrode said to be concentration polarized. 26

18. Half Wave & Decomposition Potential Half wave potential (E½): is the potential at which the current is equal to one half the diffusion current. It is a characteristic of the nature of the reactive material, independent on concentration, depend on pH of the medium, type of supporting electrolyte and type of electrode. -It does not depend on the concentration of the electroactive species. - Through this Qualitative Analysis take place. -At this potential the [oxidized species] = [reduced species] Decomposition potential: is the potential at which the substance begins to oxidize or reduce. ` 27

19. Polarographic Maxima An increase of current above the limiting value in the form of a maximum. Abnormal peak appears on the polarogram usually in concentrated solution. Maxima may occur on waves due to anions, cations, or neutral molecules. Also this maxima distorts the waveforms and complicate the measurement of the diffusion currents. 28

20. Maxima Suppressors A maximum suppressor is a substance, usually of moderately high formula weight, that is adsorbed onto the surface of a dropping electrode and then retards or prevents motion of the solution. They include such materials as proteins and related substances (gelatin, glue, gum arabic , agar, peptone, and gum ghatti ), dyes (methyl red, fuchsin , methylene blue, bromophenol blue), cationic and anionic soaps non-ionic detergents (Triton X-100). 29

21. Types Of Polarography Differential Pulse Polarography Normal Pulse Polarography AC Polarography Square Wave Polarography 30

22. Normal Pulse Polarography In normal pulse polarography (NPP) each step begins at the same value and the amplitude of each step increases in small increments. When Hg drop is dislodged from the capillary the potential is returned to initial value in preparation for a new step. During the first 1.94s of the life of each drop no current flows,the pulse is applied in the last 0.060s of a 2s drop life. 33

23. Differential Pulse Polarography:- Differential pulse polarography is a polarographic technique that use a series of discrete potential steps rather than a linear potential ramp to obtained the experimental polarogram. The differential current is plotted vs. the average potential to obtain the differential pulse polarogram, 30

24. Application Of Polarography INORGANIC COMPOUNDS: Polarography can be used for estimation of cation and anions. DETERMINATION OF TRACE ELEMENTS: Polarography can be used for Determination of trace elements e.g CO, Al, Cu, Ni, etc 34

25. APPLICATION TO ORGANIC COMPOUND : For quantitative identification of compounds and for quantitative analysis of mixtures. DETERMINATION OF DISSOLVED OXYGEN: The determination of dissolved oxygen in aqueous solution or organic solvents can be carried out successfully with the help of polarography. 35

26. STUDIES OF COMPLEXES: Polarography is powerful tool for study of composition of complexes if the sample metal ion and complex of that metal ion in the same oxidation state. DETERMINATION OF PLANT CONTENTS: A polarographic analysis of content of essential oils. 36

27. APPLICATION TO PARMACEUTICALS : Oxidation process of medicines like epinephrine and nor-epinephrine. The use of A.C. polarography has proved advantageous in the analysis of tetracycline. 37

28. DETERMINTION OF PESTICIDE AND HERBICIDE : Polarography is also used for the determination of pesticides or herbicides residues in foods. ANALYSIS OF BIOLOGICAL SYSTEMS: The possibility of being able to determine vitamins, alkaloids, hormones, terpenoid substances, and natural coloring substances has made polarography useful in analysis of biological systems 38

29. References https://chem.libretexts.org/Courses/BethuneCookman_University/B-CU%3A_CH-345_Quantitative_Analysis/Book%3A_Analytical_Chemistry_2.1_(Harvey)/11%3A_Electrochemical_Methods/11.04%3A_Voltammetric_and_Amperometric_Methods. http://hishamezzat.weebly.com/uploads/9/0/6/0/9060375/chapter3.pdf. James W. Robinson Undergraduate Book Instrumental Techniques for Analytical Chemistry By Frank Settle. http://chemistry.du.ac.in/study_material/4103-A/MSc_Polarography.pdf. http://www.umich.edu/~chem241/lecture13final.pdf. 39

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