Voltammetry and Polarography

short notes

1. Chapter-Seven VOLTAMMETRY Prepared By:-Melaku M. 1

2. 7.1 INTRODUCTORY VOLTAMMETRY In voltammetry 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 . A plot of current as a function of applied potential is called a voltammogram 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. The earliest voltammetric technique to be introduced was polarography, which was developed by Jaroslav Heyrovsky (1890–1967) in the early 1920s, for which he was awarded the Nobel Prize in chemistry in 1959. Since then, many different forms of voltammetry have been developed, a few of which are hydrodynamic and stripping voltammetry. 2

3. 7.2 Voltammetric Excitation Measurements Although early voltammetric methods relied on the use of only 2 electrodes, modern voltammetry makes use of a 3 -electrode potentiostat. A time-dependent potential excitation signal is applied to the WE, changing its potential relative to the fixed potential of the RE. The resulting current between the working and auxiliary electrodes is measured. The auxiliary electrode is generally a platinum wire , and the SCE and Ag/AgCl electrode are common reference electrodes. Several different materials have been used as working electrodes, including mercury , platinum, gold, silver, and carbon. The earliest voltammetric techniques, including polarography, used mercury for the working electrode . Since Hg is a liquid, the WE often consists of a drop suspended from the end of a capillary tube. 3

4. 4  Normal (DC) Voltammetry  Pulse Voltammetry Normal-Pulse Voltammetry Differential-Pulse Voltammetry Staircase Voltammetry Square-Wave Voltammetry 7.3. Types of Voltammetric Techniques

5. Voltammetric Techniques Normal Polarography (Voltammetry) The earliest voltammetric experiment was normal polarography at a dropping mercury electrode. In normal polarography (voltammetry) the potential is linearly scanned , producing voltammograms ( polarograms ) such as that shown in Figure below. This technique is usually called Direct Current (DC) polarography

6. SHAPE OF THE POLAROGRAM A graph of current versus potential in a polarographic experiment is called a polarogram. Cd 2+ + 2e Cd 6 Linear Sweep/Normal (DC) Polarography

7. 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. 7 Normal Polarography

8. PULSE VOLTAMMETRY - Voltammetric techniques which make use of potential pulse - A sequence of potential steps, each with a duration of about 50 ms , is applied to the working electrode After each potential step - charging current decays rapidly and exponentially to a negligible value - faradaic current decays slowly - Hence observed current late in the pulse life excludes charging current Have better sensitivity and low detection limits (10 -8 M levels ) - There are different pulse voltammetric techques based on the excitation waveform and the current sampling regime Normal-Pulse Voltammetry Differential-Pulse Voltammetry Staircase Voltammetry Square-Wave Voltammetry 8

9. 9 Pulse Voltammetric Techniques Fig.. Potential-excitation signals and voltammograms for (a) normal pulse polarography , (b) differential pulse polarography , (c) staircase polarography, and (d ) square-wave polarography. See text for an explanation of the symbols. Current is sampled at the time intervals indicated by the solid circles (  ).

10. PULSE VOLTAMMETRY - One potential pulse is applied for each drop of mercury when the DME is used - Consists of a series of pulses of increasing amplitude applied to successive drops at a preselected time near the end of each drop lifetime - Electrode is kept at a base potential between pulses at which no reaction occurs - Base potential is kept constant 10 Normal-Pulse Voltammetry

11. Normal-Pulse Voltammetry Pulse width ~50 ms Potential Base potential Time Quiet time Pulse period Step ∆E Pulse amplitude Sample period 11 PULSE VOLTAMMETRY

12. PULSE VOLTAMMETRY Normal-Pulse Voltammetry - Pulse amplitude increases linearly with each drop - Current is measured about 40 ms after each pulse is applied (at which time charging current is negligible ) - Diffusion layer is thinner than that of DC polarography due to short pulse duration Higher faradaic current than DC polarography 12

13. PULSE VOLTAMMETRY Normal-Pulse Voltammetry - Voltammogram has a sigmoidal shape - Limiting current ( i l ) is given by t m = time after application of pulse when the current is measured 13

14. PULSE VOLTAMMETRY Normal-Pulse Voltammetry - Compared to current measured in DC polarography - Normal pulse is about 5-10 times more sensitive - Advantageous when using solid electrodes 14

15. PULSE VOLTAMMETRY - One potential pulse is applied for each drop of mercury when the DME is used - Small pulses of constant amplitude are superimposed on a linear potential ramp applied to the working electrode - Potentials are applied just before the end of each drop - Useful for measuring trace levels of organic and inorganic species 15 Differential-Pulse Voltammetry

16. The drop is then mechanically dislodged. The current is not measured continuously. Rather, it is measured once before the pulse and again for the last 17 ms of the pulse. The polarograph subtracts the first current from the second and plots this difference versus the applied potential (measured just before the voltage pulse). The resulting differential pulse polarogram is nearly the derivative of a direct current polarogram, as shown in the Figure below Differential-Pulse Voltammetry

17. PULSE VOLTAMMETRY Differential-Pulse Voltammetry Potential Quiet time Time Pulse amplitude Sample period 17

18. PULSE VOLTAMMETRY Current is sampled twice - Just before the pulse application (i 1 ) and late in the pulse life (after ~ 40 ms ) when the charging current has decayed (i 2 ) ∆i (= i 2 – i 1 ) is plotted against the applied potential and displayed (instrument does these) - The charging current contribution to the differential current is negligible - Detection limit is as low as 10 -8 M (~ 1 μ g/L) 18 Differential-Pulse Voltammetry

19. PULSE VOLTAMMETRY The Voltammogram - Consists of current peaks - The height of peaks is directly proportional the concentration of analyte - The peak shaped response exhibits higher resolution than DC polarography - The peak potential ( E p ) occurs near the polarographic half-wave potential and can be used to identify the species 19 Differential-Pulse Voltammetry

20. PULSE VOLTAMMETRY ∆ E = pulse amplitude The width at half-height of the peak (W 1/2 ) If n = 1, W 1/2 ≈ 90.4 mV at 25 o C 20 Differential-Pulse Voltammetry

21. PULSE VOLTAMMETRY - Useful for analysis of mixtures - Larger pulse amplitudes result in larger and broader peaks - Pulse amplitudes of 25-50 mV with scan rate of 5 mV/s is commonly employed - Irreversible redox systems produce lower and broader peaks than reversible systems - Used to provide information about chemical form of analyte 21 Differential-Pulse Voltammetry

22. PULSE VOLTAMMETRY - Useful for rejecting background charging current - Potential-time waveform involves successive potential steps of ~10 mV in height and ~50 ms duration - Current is measured at the end of each step where the charging current is negligible (has decayed) - Peak-shaped current response is similar to that of linear scan experiments (considered as digital version of linear scan) 22 Staircase Voltammetry

23. PULSE VOLTAMMETRY Staircase Voltammetry Step height (∆E) Potential Time Step Width Sample period 23

24. PULSE VOLTAMMETRY - 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 24 Square-Wave Voltammetry

25. PULSE VOLTAMMETRY Square-Wave Voltammetry ∆E Potential Time Amplitude (E) i 1 i 2 Sample period 25

26. 7.4. Shape of the voltammetric Wave E electrode is related to the current during the scan of a voltammogram by the equation E electrode = E appl = E 1/2 - ( 0.059/n)log ( i /i d -i ) where i is the value of the current at any applied potential. This equation holds for reversible systems. Thus, the value of n can be calculated if E appl is plotted versus log ( i /i d - i ) derived from the polarogram during the rising portion. The relationship is a straight line with a slope of ( -0.059/n) V. E 1/2 in most cases is the same as the rxn’s standard state potential Diffusion Current When the potential of the WE is sufficiently -ve, the rate of red n of Cd 2+ ions is governed by the rate at which Cd 2+ can reach the electrode. Cd 2+ + 2e Cd In the Fig above, this occurs at potentials more -ve than ‑0.7 V. In an unstirred so/n, the rate of red n is controlled by the rate of diffusion of analyte to the electrode. 26

27. Diffusion Current In this case, the limiting current is called the diffusion current . The so/n must be perfectly quiet to reach the diffusion limit in polarography. Thus, the diffusion current is the limiting current when the rate of electrolysis is controlled by the rate of diffusion of species to the electrode. Current  rate of diffusion  [C] o - [C] s The [C] o & [C] s are the concs in the bulk so/n & at the electrode surface. The greater the d/ nce in concs the more rapid will be the diffusion. At a sufficiently -ve potential, the red n is so fast that the [C] s << [C] o and equation above reduces to the form Limiting current = diffusion current  [C] o The ratio of the diffusion current to the bulk solute conc is the basis for the use of voltammetry in analytical chemistry 27

28. Cont… The magnitude of the diffusion current, is given by the Ilkovic equation: l d = (7.08 x 10 4 )nCD 1/2 m 2/3 t 1/6 where I d = diffusion current, measured at the top of the oscillations in the Figure above with the units µA n = # of es per molecule involved in the oxid n or red n of the electro active species. C = conc of electro active species, with the units mmol/L D = diffusion coefficient of electro active species, with the units M 2 /s m =rate of flow of Hg, in mg/s t = drop interval, in s The number 7.08 x 10 4 is a combination of several constants whose dimensions are such that l d will be given in , µA Thus, i d is proportional to the conc of a certain species under specific conditions & the above equation may be expressed as follows: i d = kc where k is constant under the specific conditions. 28

29. Cont… If k is constant for a series of standard so/ns of various concs and an unknown, a calibration plot can be constructed & the unknown conc can be determined. Clearly, the magnitude of the diffusion current depends on several factors in addition to analyte conc. In quantitative polarography, it is important to control the temp within a few tenths of a degree. The transport of solute to the electrode should be made to occur only by diffusion ( no stirring ). 29

30. 7.5. Hydrodynamic Voltammetry In hydrodynamic voltammetry the so/n is stirred by rotating the electrode. Current is measured as a function of the potential applied to a solid WE. The same potential profiles used for polarography, such as a linear scan or a differential pulse, are used in hydrodynamic voltammetry. The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations resulting from the growth of the Hg drops. Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for the analysis of analytes that are reduced or oxidized at more positive potentials. 30

31. 7.6. Amperometry and Polarography 7.6.1. Amperometry A constant potential is applied to the WE, and current is measured as a function of time. Since the potential is not scanned, amperometry does not lead to a voltammogram. One important application of amperometry is in the construction of chemical sensors. One of the first amperometric sensors to be developed was for dissolved O 2 in blood The design of the amperometric sensor is shown below and is similar to potentiometric membrane electrodes. A gas‑ permeable membrane is stretched across the end of the sensor & is separated from the working and counter electrodes by a thin so/n of KCI. The WE is a Pt disk cathode, & an Ag ring anode is the counter electrode =>Although several gases can diffuse across the membrane (O 2 , N 2 , CO 2 ), only O 2 is reduced at the cathode 31

32. Cont… Fig:- Clark amperometric sensor for the determination of dissolved O 2 32

33. 7.6.2. Polarography In polarography, the current flowing through the cell is measured as a function of the potential of the WE. Usually this current is proportional to the conc of the analyte. The WE is a dropping Hg electrode or a Hg 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 Hg drop is measured with respect to the RE. In the hanging mercury drop electrode( HMDE) a drop of the desired size is formed by the action of a micrometer screw that pushes the mercury through a narrow capillary tube. 33

34. Cont… In the dropping mercury electrode(DME) mercury drops form at the end of the capillary tube as a result of gravity . Unlike the HMDE, the mercury drop of a DME grows continuously and has a finite lifetime of several seconds . At the end of its lifetime the Hg drop is dislodged, either manually or by gravity, and replaced by a new drop . The static mercury drop electrode(SMDE) uses a solenoid-driven plunger to control the flow of mercury. The SMDE can be used as either a hanging mercury drop electrode or as a dropping mercury electrode . A single activation of the solenoid momentarily lifts the plunger, allowing enough Hg to flow through the capillary to form a single drop. To obtain a DME the solenoid is activated repeatedly. A Hg film electrode consists of a thin layer of mercury deposited on the surface of a solid carbon, platinum, or gold electrode 34

35. Cont… The solid electrode is placed in a so/n of Hg 2+ & held at a potential at which the red n of Hg 2+ to Hg is favorable, forming a thin Hg film. Fig 7.1 Hg electrodes: (a) HMDE (b ) DME ( c) SMDE Why Dropping Mercury Electrode ? Mercury has several advantages as a working electrode. Hg yields reproducible current potential data. This reproducibility can be attributed to the continuous exposure of fresh surface on the growing mercury drop. 35

36. Why Dropping Mercury Electrode? With any other electrode such as Pt in various forms the potential depends on its surface condition & therefore on its previous treatment. The vast majority of rxns studied with the Hg electrode are red ns . At a Pt surface, reduction of solvent is expected to compete with reduction of many analyte species, especially in acidic solutions. The high over potential for H + red n at the Hg surface, H + red n does not interfere with many reductions. A species such as Zn 2+ , which is difficult to reduce at other electrodes without simultaneously reducing H 3 O + , is easily reduced at a mercury WE Other advantages include the ability of metals to dissolve in the Hg resulting in the form n of an amalgam & the ability to easily renew the surface of the electrode by extruding a new drop. 36

37. Cont… Fig 7.2Typical electrochemical cell used in polarography 37

38. Problems with mercury electrode Hg electrode is not very useful for performing oxid ns because Hg is too easily oxidized. For most oxid ns some other WE must be employed In a noncomplexing medium Hg is oxidized near + 0.25 V Vs S.C.E. Pt electrode Vs SCE works for a range of +1.2 to –0.2 in acidic so/n +0.7 V to –1 V in basic so/n. Solid electrodes constructed using Pt, Au, Ag, or C may be used over a range of potentials including potentials that are -ve & +ve with respect to the SCE at which Hg electrodes cannot be used. Hg is toxic and slightly volatile & spills are almost inevitable. a good vacuum cleaner. To remove residual mercury, sprinkle elemental zinc powder on the surface and dampen the powder with 5% aqueous H 2 S0 4 Hg dissolves in the Zn After working the paste into contaminated areas with a sponge or brush allow the paste to dry & then sweep it up. 38

39. Problems with mercury electrode Discard the powder appropriately as contaminated mercury waste C-paste electrode is also used in voltammetry. Except for the C paste electrode, solid electrodes are fashioned into disks that are sealed into the end of an inert support and are in contact with an electrical lead (Figure 7.3a). The C-paste electrode is made by filling the cavity at the end of the inert support with a paste consisting of C particles & viscous oil. Solid electrodes are not without problems, the most important of w/h is the ease with which the electrode’s surface may be altered by the adsorption of so/n species or the form n of oxide layers. For this reason solid electrodes need frequent reconditioning, either by applying an appropriate potential or by polishing. 39

40. Cont… Electrical lead (a) (b) Solid disc electrode Fig.7.3.(a) Schematic diagram of a solid electrode (b) Typical electrochemical cell for use in voltammetry A typical arrangement for a voltammetric electrochemical cell is shown in Figure 7.3b Besides the working, reference, and auxiliary electrodes, the cell also includes a N 2 purge line for removing dissolved O 2 & an optional stir bar . Electrochemical cells are available in a variety of sizes, allowing for the analysis of so/n volumes ranging from more than 100 mL to as small as 50 mL. 40

41. Cont… Consequently, very little analyte undergoes electrolysis, and the analyte’s conc in bulk solution remains essentially unchanged Although the applied potential at the WE determines if a faradaic current flows The magnitude of the current is determined by the rate of the resulting oxidation or reduction reaction at the electrode surface The magnitude of the faradaic current is determined by the rate of the resulting oxidation or reduction rxn at the electrode surface. Two factors contribute to the rate of the electrochemical reaction: the rate at which the reactants and products are transported to and from the surface of the electrode (mass transport) and the rate at which electrons pass between the electrode and the reactants and products in solution. (kinetics of electron transfer at the electrode surface) 41

42. 42 TA YU