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Polarography principle and instrumentation
- 1. POLAROGRAPHY PRINCIPLE AND INSTRUMENTATION Government Institute of science -KIRAN A. BARBATKAR M. SC. FIRST YEAR
- 2. INTRODUCTION When a solution is subjected to electrolysis, the electrical properties like emf, current, resistance etc depend on nature and concentration of various species present in that solution. Simple principle of polarography is the study of solutions or of electrode processes by means of electrolysis with two electrode, one polarizable and one non – polarizable.
- 3. History of polarography • Polarography was invented in 1922 by Czech chemist Jaroslav Heyrovsky for which he won NOBEL PRIZE in 1959. • An electromechanical technique of analyzing solutions that measures the current flowing between two electrodes in the solutions as well as the gradually increasing applied voltage to determine respectively the concentration of a solute and it’s nature. - Created by Heyrovsky. Jaroslav Heyrovsky (1890-1967)
- 4. Principle Study of solutions or of electrode processes by means of electrolysis with two mercury electrode, one polarizable and one non polarizable, the former formed by mercury regularly dropping from capillary tube. Polarized electrode – Dropping Mercury Electrode (DME) Depolarized electrode - Saturated Calomel Electrode (SCE)
- 5. Mercury continously drops from reservoir through capillary tube into the solutions The optimum interval between drops for most analysis is between 2 to 5 seconds.
- 6. Why mercury? Mercury as a working electrode is useful because : It displays a wide negative potential range. It’s surface is readily regenerated by producing a new drop or film. Many metals ions can be reverisbly reduced into it.
- 7. Polarogram It is a graph of current versus potential in a polarograpic analysis. 3 categories : Collectively referred as residual Current Referred to as diffusion current resulting from reduction of sample Called as limiting current
- 8. Types of current: Residual Current : It is a non faradic current This current increases linearly with the applied voltage, and it is observed even when the purest, air free solutions are used. Migration current : Migration of charged particles in the electric field is caused by the potential difference existing between the electrode surface and the solution . The migration current can be practically removed if an indifferent electrolyte is added to the solution in a concentration so large that it’s ions carry essentially all the current. The concentration of the added electrolyte must be at least 100-fold that of the electro active material.
- 9. Diffusion Current : It is calculated by following equation known as Ilkovic equation Where, id = the average diffusion Current in microamperes during the life of drop n= the number of Faraday of electricity required per mole of the electrode reaction D=the diffusion coefficient of the reducible or oxidisable substance expressed as Cm^2/sec C= concentration in mmol per litre M=the rate of flow of mercury from the DME expressed in mg per second an T = drop time in seconds
- 10. CONSTRUCTION : DME A small polarizable electrode is Produced by Passing a steam of mercury Through a resistance glass capillary. The assembly consists of – 1. a mercury reservoir. 2. A connecting tube between the mercury reservoir and Capillary tube. 3. A capillary tube. 4. A glass electrolyte cell. A polarography instrumentation
- 11. Instrumentation : The basic component of the experimental set up are the polarograpic cell and the DME. The polarograpic cell is a H – shaped vessel. The two tubes are connected through an agar agar salt bridge and a sintered glass tube. One half of the cell act as anode which is usually a reference electrode such as saturated Calomel Electrode SCE. The other half cell contains the solution of the sample analyte and the DME as the cathode.
- 12. The DME consists of a 5-20 cm long capillary tube with an inner diameter 0.06-0.08mm through which mercury is forced under pressure exerted by approximately 50 cm column or mercury connected to a mercury reservoir. The electrical Circuit consists of a DC source, potentiometer, a voltmeter and microammeter usually a galvanometer. The DME is connected to the negative terminal while SCE is connected to the positive terminal of DC source. Using the potentiometer an emf upto 3 volt may be gradually applied to the cell. Since the DME is connected to the negative terminal of DC source the applied voltage is given a negative sign by convention where as Current read on the ammeter is taken to be positive.
- 13. Reference : https://www.slideshare.net/mobile/chelsisoliva1/polarography-61782546 Pinteresthttps:///in.pinterest.com http://www.ceb.cam.ac.uk/data/images/groups/CREST/Teaching/hydro/fig1.gif https://chem.libretexts.org/Textbook_Maps/Analytical_Chemistry_Textbook_Maps/Map%3A_Analytical_Chemi stry_2.0_(Harvey)/11_Electrochemical_Methods/11.4%3A_Voltammetric_Methods http://web2.uconn.edu/rusling/DC%20Polarography.pptx http://www.searchmap.eu/blog/jaroslav-heyrovsky/ Vogels textbook of quantitative analysis ( page no 591-611) Basic concepts of analytical chemistry by S.M.Khopkar (page no 505-513).