polarography

polarography

1. welcome

2. PRESENTED BY FATHIMA SHIREEN C.P MSc CHEMISTRY ROLL.NO: 6 MALABAR CHRISTIAN COLLEGE POLAROGRAPHY

3. A plot of current as a function of applied potential is called a polarogram and is the electrochemical equivalent of a spectrum in spectroscopy, providing quantitative and qualitative information about the species involved in the oxidation or reduction reaction. What is polarography? A time ‑ dependent potential is applied to an electrochemical cell, and the current flowing through the cell is measured as a function of that potential.

4. History Jaroslav Heyrovsky was the inventor of the polarographic method, and the father of electroanalytical chemistry, for which he won the Nobel Prize in 1959. His contribution to electroanalytical chemistry can not be overestimated. All modern voltammetric methods used now in electroanalytical chemistry originate from polarography. On February 10, 1922, the "polarograph" was born as Heyrovský recorded the current-voltage curve for a solution of 1 M NaOH. Heyrovsky correctly interpreted the current increase between -1.9 and -2.0 V as being due to deposition of Na + ions, forming an amalgam .

5. PRINCIPLE Polarography consists of electrolysis of the analyte solution by applying potential difference between two electrodes: (a) one that has a fixed and known potential called the reference electrode , and (b) the other that has a variable potential, called the polarizable electrode or an indicator electrode (also called the working electrode ). As voltage is applied to the polarizable electrode, the resulting change in the current through the solution is monitored and the “ current-voltage” curve is traced.

6. Instrumentation Working electrode saturated calomel electrode ( SCE ) Dropping Mercury Electrode ( DME ), a microelectrode Reference electrode Counter electrode / current –carrier auxiliary electrode Pt wire Supporting electrolyte KCl

7. DME It consists of a fine bore capillary glass tube connected to a levelling bulb filled with mercury The diameter of the capillary and the height of the levelling bulb are adjusted so that the mercury falls from the capillary in small uniform drops at the rate of about 20 drops per minute The lower end of the capillary is dipped into the analyte solution taken in a beaker Each drop kept at the capillary end for 2 to 3 seconds Acts as cathode Forms amalgam with reducible species and discharged at the bottom of the vessel

8. Why Dropping Mercury Electrode? The surface area is reproducible with any capillary The constant removal of electrode surface eliminates the poisoning effect Mercury forms amalgams with any metals and hence makes many metal ions easily reducible Surface area can be calculated from the weight of the drop The diffusion current assumes steady value immediately and thus is reproducible There is a large Hydrogen over voltage on mercury. Therefore reduction of even alkali metal ions can be observed The small dimensions of the DME enable one to carry out electrolysis even in small volumes of solution. For this reasons, polarography offers the advantages of micron analytical methods, capable of estimating even traces of analytes in solutions

9. Disadvantages of DMG Mercury may be easily oxidized. Thus limits the feasible range of electrode The area of the microelectrode is constantly changing as the size of the drop changes The capillary may be easily plugged, the care must be taken to avoid touching the tip of the capillary with any foreign material

10. sce metal-insoluble metal salt electrodes Hg, Hg 2 Cl 2 (s)| KCl (aq) Concentration of KCl : saturated Electrode Potential : 0.2415 Pure mercury is placed at the bottom of a glass tube It is covered with a paste of mercurous chloride (calomel) in pure mercury The tube is then filled with saturated solution of KCI added through the side tube S 1 This solution also fills the curved side tube S 2 ending in a jet It is through this side tube that the calomel electrode is coupled with any other electrode A platinum wire sealed into a thin glass tube containing a little mercury at the bottom serves to make electrical contact with the external circuit

11. Schematic representation of polarographic circuit

12. The electroactive species ( analyte ) is taken in a highly conducting electrolytic medium such as KCl solution The concentration of this electrolyte is about 100 times higher than that of the analyte And is electroinactive in the sense that it will not undergo oxidation or reduction on the electrode surface in the range of the required applied potential Hence, it is called a supporting (or indifferent) electrolyte The need for such a medium is to practically eliminate the migration current. Elimination of migration current The use of supporting electrolyte

13. In such a set up, the migration current is practically carried by the ions of the supporting electrolyte. This will allow the analyte species to follow diffusional transport towards the electrode surface and thereby make the current almost fully diffusion-controlled. It is the majority current due to the migration of the charged particles in the electric field, caused by the potential difference existing between the electrode surface and the solution Migration current

14. • Limiting current ( il ) region Diffusion current (id) region The residual current ( ir ) region The polarogram has three current regions: The advantages of the dropping mercury electrode (DME) Ilkovic diffusion current equation Diffusion current equation Diffusion current equation id = k •n •D 1/2 •m 2/3 •t 1/2 •Cox The factors determining the diffusion current • The analyte concentration The characteristics of the capillary The effect of dropping mercury potential Temperature effect

15. Factors affecting the shape of the polargram The residual current The current maxima The presence of oxygen

16. In polarography, the current flowing through the cell is measured as a function of the potential of the working electrode. Usually this current is proportional to the concentration of the analyte. Apparatus for carrying out polarography is shown below. The working electrode is a dropping mercury electrode or a mercury droplet suspended from a bottom of a glass capillary tube. Analyte is either reduced (most of the cases) or oxidized at the surface of the mercury drop. The current –carrier auxiliary electrode is a platinum wire. SCE or Ag/ AgCl reference electrode is used. The potential of the mercury drop is measured with respect to the reference electrode.

17. SHAPE OF THE POLAROGRAM A graph of current versus potential in a polarographic experiment is called a polarogram.

18. When the potential is only slightly negative with respect to the calomel electrode, essentially no reduction of Cd 2+ occurs. Only a small residual current flows. At a sufficiently negative potential, reduction of Cd 2+ commences and the current increases. The reduced Cd dissolves in the Hg to form an amalgam. After a steep increase in current, concentration polarization sets in: The rate of electron transfer becomes limited by the rate at which Cd 2+ can diffuse from bulk solution to the surface of the electrode. The magnitude of this diffusion current I d is proportional to Cd 2+ concentration and is used for quantitative analysis. The upper trace in the Figure above is called a polarographic wave.

19. – Three electrodes in solution containing analyte : microelectrode whose potential is varied with time Reference electrode: potential remains constant (or calomel) Supporting electrolyte: excess of nonreactive electrolyte (alkali metal) to conduct current

20. When the potential is sufficiently negativ around ‑1.2 V, reduction of H + begins and the curve rises steeply. At positive potentials (near the left side of the polarogram), oxidation of the Hg electrode produces a negative current. By convention, a negative current means that the working electrode is behaving as the anode with respect to the auxiliary electrode. A positive current means that the working electrode is behaving as the cathode. The oscillating current in the Figure above is due to the growth and fall of the Hg drops. As the drop grows, its area increases, more solute can reach the surface in a given time, and more current flows. The current increases as the drop grows until, finally, the drop falls off and the current decreases sharply.