Dc,pulse,ac and square wave polarographic techniques new

polarographic techniques

1. DC,PULSE,AC AND SQUARE WAVE POLAROGRAPHIC TECHNIQUES Presented By : Biji Saro Varghese M.Sc. I Sem-II Guided by : Prof. R. M. Jugade 1 RASHTRASANT TUKADOJI MAHARAJ NAGPUR UNIVERSITY P. G. T. DEPARTMENT OF CHEMISTRY Date-6/4/18

2. CONTENTS Introduction Polarography DC polarography Pulse polarography NPP DPP AC polarography Square wave polarography 2

3. INTRODUCTION 3 POLAROGRAPHY is an electromechanical technique of analyzing solutions that measures the current flowing between two electrodes in the solution as well as the gradually increasing applied voltage to determine respectively the concentration of a solute and its nature . C reated by: Jaroslav Heyrovsky in 1922 Received Nobel prize in 1959

4. POLAROGRAPHY 4 Is a method of analysis based on the measurement of current of 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), which are useful for their wide cathodic ranges and renewable surfaces .

5. In this method, a reference electrode and an indicator electrode are required. 5 Reference electrode- acts to maintain a constant potential throughout the measurement. It is non polarizable electrode. eg: mercury pool or saturated calomel electrode Indicator electrode- assumes the potential impressed upon it from an external source . It is polarizable electrode. eg:Dropping mercury electrode(DME) Reference electrode Indicator electrode

6. PRIN C IPLE 6 Change in electrical properties of solution when subjected to electrolysis depends on nature and concentration of electroactive species present in solution. When e.m.f is applied between these two electrodes some current flows through the circuit and the change in current is monitored w.r.t scanning e.m.f

7. POLAROGRAPHIC DATA 7 The current –potential characteristics is called as POLAROGRAM. This curve is used to determine the concentration of electroactive species. Types of currents represented on polarogram are R esidual current D iffusion current L imiting current

8. Why MERCURY? 8 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.

9. DC POLAROGRAPHY 9 Typical DC Polarogram

10. In DC polarography ,a continuously increasing DC potential is applied to DME and corresponding current is recorded in a continuous manner. The drop time of mercury is usually between 2-8 seconds. Detection limit is up to 10 -3 to 10 -4 M. 10

11. Advantages This technique was useful to determine the concentration of almost all organic and inorganic analyte. Detection limit is up to 10 -4 M. It is sensitive as compared to earlier classical techniques. 11

12. PULSE POLAROGRAPHY 12

13. In pulse polarography the potential is applied in the form of pulses and current is recorded in discrete manner. Two different pulse techniques are used Normal Pulse Polarography(NPP) Differential Pulse Polarography(DPP) 13

14. NORMAL PULSE POLAROGRAPHY(NPP) 14

15. It is also called as large amplitude pulse polarography. In this technique a constant base line potential is applied to DME and pulse of increasing amplitude are superimposed on it. The pulse is applied only for 50mS at the end of drop time while current is recorded just for last 17mS at the end of pulse. After each pulse the capillary is mechanically tapped and mercury drop falls down. The resultant polarogram is sigmoidal in nature but it is of stair case type. 15

16. DIFFERENTIAL PULSE POLAROGRAPHY(DPP) 16

17. In this technique, a continuously increasing DC potential is superimposed with pulse of constant amplitude. The pulse is applied only at the end of drop time for 50mS. The current is recorded twice during each drop .The current is recorded just before application of pulse and at the end of the pulse. The difference between these two currents is plotted as a function of baseline potential(E). The polarogram obtained consists of a peak instead of a sigmoidal curve. The peak current or the peak height is directly proportional to concentration of electroactive species. 17

18. Advantages Non faradaic condenser current is completely eliminated, residual current becomes almost zero. The total current is recorded at the end of the pulse hence, the only contributor is faradaic current. Detection limit goes down to 10 -8 M. 18

19. time E AC POLAROGRAPHY 19 E ac E dc i p i p α c i dc E p E dc

20. In this technique, a continuously increasing D.C. potential (E dc ) is superimposed with a constant amplitude A.C. potential. When such combination is applied to DME, two types of currents are generated at DME. The total current is the resultant of D.C. and A.C. current. The A.C. component is electronically rectified and plotted as a function of applied D.C. potential(E dc ). 20

21. Advantages Detection limit of the order of 10 -6 to 10 -8 M can be achieved. There is no need of purging nitrogen gas to remove oxygen because a.c. polarography is insensitive to irreversible process like oxygen reduction. 21

22. ∆E Potential Time Amplitude (E) i 1 i 2 Sample period SQUARE WAVE POLAROGRAPHY

23. Large amplitude differential technique. The wave form applied to the working electrode is a symmetric square wave superimposed on a base staircase potential. Current is sampled twice during each square-wave cycle. One at the end of the forward pulse (i 1 ) and one at the end of the reverse pulse (i 2 ).This results in square-wave modulation. Modulation amplitude is very large. Reverse pulses cause the reverse reaction of any product formed from the forward pulse. The net current (i 1 – i 2 ) is then plotted versus the base staircase potential. This gives the peak-shaped polarogram Peak current is proportional to the concentration of the analyte . The net current is larger than the forward or reverse currents.

24. Advantages Higher sensitivity than differential-pulse in which reverse current is not used. Detection limit is up to 10 -8 M. Reduced analysis time due to higher scan rates. 24

25. REFERENCES Quantitative analysis, fifth edition by R.A.Day,Jr . and A.L.Underwood (Prentice-hall Of India) Electroanalytical chemistry, theory and applications, by R.T.Sane and A.P.Joshi (Quest Publications) Basic concepts of analytical chemistry, third edition by S.M.Khopkar (New Age International Publishers)

26. 26 THANKYOU