Polarography

The ppt describes its procedure

1. POLAROGRAPHY By: N. S ai sree

2. CONTENTS Voltammetry Polarography Polarographic analysis Electrodes and its examples Principle Three electrode cell Polarographic data & Polarogram Half wave potential & polarographic wave equation Polarographic maxima Oxygen wave Applications

3. VOLTAMMETRY V oltammetry is the general name given to a group of electro analytical methods in which the current is measured as a function of applied potential where in the polarization of the indicator or working electrode is enhanced. The field has been developed from polarography. The word polarography first recorded in 1935 – 1940 Polaro (graph ) + graphy (field of study)

4. POLAROGRAPHY It is an electrochemical technique of analysing solutions that measure the current flowing between two electrodes in the solution as well as the gradually increasing applied voltage to determine respectively the concentration of solute and its nature. Created by: Jaroslav Heyrovsky for that he awarded Nobel Prize in 1959 Figure 1

5. POLAROGRAPHY Is a method of analysis based on the measurement of current electrolysis of an electroactive species at a given electrode potential under controlled conditions. It is the branch of voltammetry where the working electrode is a dropping mercury electrode (DME) or a static mercury drop electrode (SMDE ) Figure 2

6. Contd... Reference electrode Indicator electrode In this method, a reference electrode and an indicator electrode are required. Reference electrode- it is larger in size and non polarized(depolarized) Indicator electrode- it is smaller in size and polarized Figure 3 Figure 4

7. Why reference electrode with larger area and indicator electrode with smaller area??? Indicator electrode: It is smaller in size and polarized i.e it adopts the potential externally imposed on it. Reference electrode : It is larger in size and non polarized i.e it retains to a constant potential throughout the measurement.

8. Polarographic analysis process and the conditions for polarographic wave formation. Polarographic analysis : Electrolytic analysis carried out under special conditions. specific characteristics： A、A polarized electrode and a depolarized electrode are used as working electrode & referenceelectrode B. No stirring Incomplete electrolysis (only a small amount of analyte is consumed) Figure 5

9. Polarized electrode and depolarized electrode If the electrode potential has great changes when infinite small current flow through the electrode, such electrode is referred to as polarized electrode. eg. DME If the electrode potential does not change with current , such electrode is called ideal depolarized electrode. eg. SCE Figure 6

10. EXAMPLES OF MERCURY ELECTRODES In polarography, mercury is used as a working electrode, mercury it is a liquid. The working electrode is often a drop suspended from the end of a capillary tube. Examples: HMDE (Hanging mercury drop electrode) DME (dropping mercury electrode) SMDE (static mercury drop electrode) Figure 7

11. WHY MERCURY? Mercury as working electrode is useful because: It displays a wide negative potential range Its surface is readily regenerated by producing a new drop or film Many metal ions can be reversibly reduced into it. Figure 8

12. PRINCIPLE: Study of solutions or of electrode processes by means of electrolysis with two electrodes, one polarizable and one unpolarizable, the former formed by mercury regularly dropping from capillary tube. POLARIZED ELECTRODE: Dropping Mercury Electrode (DME) DEPOLARIZED ELECTRODE: Saturated Calomel Electrode

13. Contd.. Mercury continuously drops from reservoir through a capillary tube into the solution. The optimum interval between drops for most analyses is between 2 and 5 seconds. Figure 9

14. Contd... Figure 10 Figure 11

15. Three electrode cell Three electrode cell: Working, Reference, Counter/ auxilliary current flows between working and counter electrodes. Potential controlled by potentiostat between working and reference electrodes. Figure 12

16. Contd... Two special electrodes Supporting electrolyte : Usually relatively higher concentration of strong electrolytes (alkali metal salts) serves as supporting electrolyte Dissolved oxygen is usually removed by bubbling nitrogen through the solution Voltage scanning Under unstirred state, recording current-voltage curve.

17. POLAROGRAPHIC DATA Obtained from an automatic recording instrument is called a polarogram , and the trace is called a polarographic wave. POLAROGRAM It is a graph of current versus potential in a polarographic analysis. 3 categories: collectively referred to as residual current referred to as diffusion current resulting from the reduction of the sample called the limiting current The diffusion current of a known concentration of reference standard are first determined followed by the determination of the diffusion current of the unknown concentration

18. POLAROGRAM i r ( residual current ) which is the current obtained when no electrochemical change takes place. i av ( average current/limiting current )is the current obtained by averaging current values throughout the life time of the drop while i d ( diffusion current ) which is the current resulting from the diffusion of electroactive species to the drop surface. Figure 13

19. Limiting diffusion current -- A basis of polarographically quantitative analysis When the applied voltage exceeds the decomposition voltage, diffusion-controlled current is expressed as: i = K(C-C ) When the applied voltage gets more negative, C →0, current becomes only diffusion limited, then id = KC I d reaches a limiting value proportional to ion concentration C in bulk solution, and do not changes with applied voltage longer

20. Half-wave potential —polarographic qualitative analysis E½ at ½ i The potential at which current is equal to one half the limiting current is called half wave potential. Figure 14

21. How it works?? The applied voltage is gradually increased, typically by going to a more positive( more negative decomposing potential) A small residual current is observed. When the voltage becomes great enough, reduction occurs at the analytical electrode causing a current. The electrode is rapidly saturated so current production is limited – based on diffusion of the analyte to the small electrode.

22. How it works?? The reduced species alters the surface of the mercury electrode. To prevent problems, the mercury surface is renewed by “ knocking off ” a drop –providing a fresh surface. This results in an oscillation of the data as it is collected.

23. The diffusion theory and polarographic wave equation We have already known: id = KC In above equations, K is called Ilkovic constant, it is expressed as follows: K = 607 n D 1/2 m 2/3 t 1/6 Thus, id = 607nD 1/2 m 2/3 t 1/6 C

24. id = 607nD 1/2 m 2/3 t 1/6 C From above, when temperature, matrix solution and capillary characteristic are kept constant, i d is proportional to C. Concentration of electroactive analyte (mmolL -1 ) Drop time(sec) Number of transferring electrons in electrode reaction(e/mol) Diffusion current ( μ A) Density of analyte in solution(Cm 2 .sec -1 ) Mercury mass flow(mg. sec -1)

25. Polarographic wave equation E = E 1/2 – RT/ nF ln( i /(i d -1)) When i = ½ id , log term in above equation is equal to zero, corresponding potential is called halfwave potential E 1/2 E 1/2 independent on the concentration basis of qualitative analysis

26. Interference current in classical DC polarograph Residual current (1) redox reactions of impurities in solution (2) charging of Hg drop (non-faradic current / non- redox current) Migration current The current produced by static attraction of the electrode to sought-for ion

27. Polarographic Maximum (or malformed peak ) Reproducible maxima often occur in CV curve unless eliminated by the addition of suitable maximum suppressor such as MC or gelatin. Curve a is unsuppressed oxygen maximum curve b is the oxygen wave in presence of gelatin. Figure 15

28. Oxygen wave Dissolved oxygen is easily reduced at many working electrodes. Thus an aqueous solution saturated with air exhibits two distinct oxygen waves. The first results from the reduction of oxygen to hydrogen peroxide: The second wave corresponds to the further reduction of hydrogen peroxide:

29. Contd... Figure 16

30. Factors that affect limiting diffusion current Characteristics of capillary& pressure of Hg viscosity Composition of solution Temperature concentration Factors that affect Half wave potential Type and concentration of supporting electrolyte Temperature Forming complex

31. ???? Why does Nitrogen gas pass through the solution before elecctrolysis??? Pure Nitrogen is passed through the solution before connecting the electrolysis so as to remove dissolved oxygen and during purification process a current of pure nitrogen is maintained over the surface of the solution.

32. Polarographically quantitative analytical methods （ i d ) = K·C Direct comparison method Calibration curve method Standard addition method

33. APPLICATIONS Polarography is used for determination of Oxygen content of fluids including whole body fluids , fermentation liquors &milk for studying the respiration rates of microorganisms Several mercury containing antiseptics and insecticides were determined polarographically Hormones like thyroxine,insulin,adrenaline and several sex hormones are estimated by polarography It is used for the determination of antibiotics such as pencillin,streptomycin and chloramphenicol Several Alkaloids can also be estimated by polarography.

34. Contd... In electrochemistry polarography allows the measurement of potentials and yields information about the rate of the electrode process, adsorption, desorption phenomena. Calculation of the rate constant is possible with polarography in this way very fast reactions of order 10 5 –10 10 litre mol -1 sec -1 can be determined. Polarography prooved useful in mechanistic studies . Elimination of Mannich bases, hydration of multiple bonds in unsaturated ketones and aldolization are example studies .

35. Contd... Inorganic applications: In inorganic analysis polarography is used predominately for trace metal analysis like copper, zinc, iron, lead, nickel, manganese etc.. Composition of alloys Purity of elements OBJECTIVE PARAMETER MEASURED Identity of element Half wave potential Quantity of element Diffusion current

36. Contd... Organic applications: Electroreducible or oxidisable functional group can be determined by polarographic technique by using dme. The functionl group can be inferred from Half wave potential and the quantity of the substance can be determined from diffusion current measurement. example functional groups like Nitro and Nitroso groups, azo and diazo compounds, aldehydes, ketone, organic peroxides lactons, activated C=C some acids and organo metalic compounds. Multi stage reduction of groups like Nitro to Nitroso to Hydroxyl amine to Amino group can also be achieved

37. Contd... The following table gives examples of E 1/2 of some compounds. Functional group E 1/2 (V) Benzaldehyde -1.51V Iso propyl phenyl ketone -1.82V Aldehydes and ketones -1.3V to -2.0V Nitro compounds -0.1V to -0.7V

38. Contd... Example of reduction:

39. Contd... One of the easiest and most frequently encountered organic reduction is that of the nitro group. In Nitro furans and nitroimidazoles, for example the reaction is

40. Examples of drugs analysed by polarography prazosin Nifedipine Felodipine Amlodipine Spiranolactone Digitoxin p- Benzoquinone Vitamin K and its derivatives Azo and diazo compounds Keto steroids

41. RFERENCES A Textbook of pharmaceutical analysis Third edition by A.connors pgno:154-172 Vogel’s Textbook of Quantitative chemical analysis by J mendham;RC denney;JD Barnes; M Thomas, B Sivasankar sixth edition pgno;361-387 Quantitative Analysis; sixth edition by R.A.DY;Jr.A.l.underwood Pgno:11-1 to 11-11 Practical pharmaceutical chemistry fourth edition-part two ; edited by: A.H Beckett T.B Stenlake Pgno209-242