Because mercury is a liquid, the working electrode is often a drop suspended from the end of a capillary tube. In the hanging mercury drop electrode , or HMDE, we extrude the drop of Hg by rotating a micrometer screw that pushes the mercury from a reservoir through a narrow capillary tube Figure In the dropping mercury electrode , or DME, mercury drops form at the end of the capillary tube as a result of gravity Figure Unlike the HMDE, the mercury drop of a DME grows continuously—as mercury flows from the reservoir under the influence of gravity—and has a finite lifetime of several seconds.
At the end of its lifetime the mercury drop is dislodged, either manually or on its own, and replaced by a new drop. The static mercury drop electrode , or SMDE, uses a solenoid driven plunger to control the flow of mercury Figure Activation of the solenoid momentarily lifts the plunger, allowing mercury to flow through the capillary and forming a single, hanging Hg drop.
Repeatedly activating the solenoid produces a series of Hg drops. There is one additional type of mercury electrode: the mercury film electrode. Mercury has several advantages as a working electrode. Other advantages include the ability of metals to dissolve in mercury—resulting in the formation of an amalgam —and the ability to easily renew the surface of the electrode by extruding a new drop. One limitation to using mercury as a working electrode is the ease with which it is oxidized.
Depending on the solvent, a mercury electrode can not be used at potentials more positive than approximately —0. The useful potential windows are shown in green ; potentials in red result in the oxidation or reduction of the solvent or the electrode. Complied from Adams, R. Solid electrodes constructed using platinum, gold, silver, or carbon may be used over a range of potentials, including potentials that are negative and positive with respect to the SCE Figure A solid electrode can replace a mercury electrode for many voltammetric analyses that require negative potentials, and is the electrode of choice at more positive potentials.
Except for the carbon paste electrode, a solid electrode is fashioned into a disk and sealed into the end of an inert support with an electrical lead Figure The carbon paste electrode is made by filling the cavity at the end of the inert support with a paste consisting of carbon particles and a viscous oil. For this reason a solid electrode needs frequent reconditioning, either by applying an appropriate potential or by polishing. The electrode is fashioned into a disk and sealed in the end of an inert polymer support along with an electrical lead.
A typical arrangement for a voltammetric electrochemical cell is shown in Figure In addition to the working electrode, the reference electrode, and the auxiliary electrode, the cell also includes a N 2 -purge line for removing dissolved O 2 , and an optional stir bar. When we oxidize an analyte at the working electrode, the resulting electrons pass through the potentiostat to the auxiliary electrode, reducing the solvent or some other component of the solution matrix. If we reduce the analyte at the working electrode, the current flows from the auxiliary electrode to the cathode.
In either case, the current from redox reactions at the working electrode and the auxiliary electrodes is called a faradaic current. In this section we consider the factors affecting the magnitude of the faradaic current, as well as the sources of any non-faradaic currents.
Because the reaction of interest occurs at the working electrode, we describe the faradaic current using this reaction. An anodic current is due to an oxidation reaction at the working electrode, and its sign is negative.
The relationship between the concentrations of Fe CN 6 3— , the concentration of Fe CN 6 4— , and the potential is given by the Nernst equation. We use surface concentrations instead of bulk concentrations because the equilibrium position for the redox reaction.
This is the first of the five important principles of electrochemistry outlined in Section If this is all that happens after we apply the potential, then there would be a brief surge of faradaic current that quickly returns to zero—not the most interesting of results. This concentration gradient creates a driving force that transports Fe CN 6 4— away from the electrode and that transports Fe CN 6 3— to the electrode Figure A faradaic current continues to flow until there is no difference between the concentrations of Fe CN 6 3— and Fe CN 6 4— at the electrode and their concentrations in bulk solution.
This is the second of the five important principles of electrochemistry outlined in Section Although the potential at the working electrode determines if a faradaic current flows, the magnitude of the current is determined by the rate of the resulting oxidation or reduction reaction.
Two factors contribute to the rate of the electrochemical reaction: the rate at which the reactants and products are transported to and from the electrode—what we call mass transport —and the rate at which electrons pass between the electrode and the reactants and products in solution. This is the fourth of the five important principles of electrochemistry outlined in Section There are three modes of mass transport that affect the rate at which reactants and products move toward or away from the electrode surface: diffusion, migration, and convection.
Diffusion occurs whenever the concentration of an ion or molecule at the surface of the electrode is different from that in bulk solution. If we apply a potential sufficient to completely reduce Fe CN 6 3— at the electrode surface, the result is a concentration gradient similar to that shown in Figure The region of solution over which diffusion occurs is the diffusion layer.
The longer we apply the potential, the greater the distance over which diffusion occurs. The dashed red line shows the extent of the diffusion layer at time t 3.
These profiles assume that convection and migration do not significantly contribute to the mass transport of Fe CN 6 3—. Convection occurs when we mechanically mix the solution, carrying reactants toward the electrode and removing products from the electrode. The most common form of convection is stirring the solution with a stir bar.
Other methods that have been used include rotating the electrode and incorporating the electrode into a flow-cell. The final mode of mass transport is migration , which occurs when a charged particle in solution is attracted to or repelled from an electrode that carries a surface charge. If the electrode carries a positive charge, for example, an anion will move toward the electrode and a cation will move toward the bulk solution.
Unlike diffusion and convection, migration only affects the mass transport of charged particles. The movement of material to and from the electrode surface is a complex function of all three modes of mass transport.
In the limit where diffusion is the only significant form of mass transport, the current in a voltammetric cell is equal to. For equation We can eliminate migration by adding a high concentration of an inert supporting electrolyte. Because ions of similar charge are equally attracted to or repelled from the surface of the electrode, each has an equal probability of undergoing migration.
A large excess of an inert electrolyte ensures that few reactants or products experience migration. Although it is easy to eliminate convection by not stirring the solution, there are experimental designs where we cannot avoid convection, either because we must stir the solution or because we are using electrochemical flow cell.
Fortunately, as shown in Figure The rate of mass transport is one factor influencing the current in voltammetry. The ease with which electrons move between the electrode and the species reacting at the electrode also affects the current. When electron transfer kinetics are fast, the redox reaction is at equilibrium. Under these conditions the redox reaction is electrochemically reversible and the Nernst equation applies.
If the electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface—and thus the magnitude of the faradaic current—are not what is predicted by the Nernst equation. In this case the system is electrochemically irreversible. In addition to current resulting from redox reactions—what we call faradaic current—the current in an electrochemical cell includes other, nonfaradaic sources. Because the movement of ions and the movement of electrons are indistinguishable, the result is a small, short-lived nonfaradaic current that we call the charging current.
Even in the absence of analyte, a small, measurable current flows through an electrochemical cell. This residual current has two components: a faradaic current due to the oxidation or reduction of trace impurities and the charging current. The shape of a voltammogram is determined by several experimental factors, the most important of which are how we measure the current and whether convection is included as a means of mass transport.
As shown in Figure For the voltammogram in Figure In the absence of convection the diffusion layer increases with time see Figure For the voltammograms in Figures The resulting voltammogram, shown in Figure The dashed red line shows the residual current.
Earlier we described a voltammogram as the electrochemical equivalent of a spectrum in spectroscopy. In this section we consider how we can extract quantitative and qualitative information from a voltammogram. For simplicity we will limit our treatment to voltammograms similar to Figure When we apply a potential causing the reduction of O to R, the current depends on the rate at which O diffuses through the fixed diffusion layer shown in Figure Using equation When we reach the limiting current, i l , the concentration of O at the electrode surface is zero and equation Equation To determine the value of K O we can use any of the standardization methods covered in Chapter 5.
Equations similar to equation To extract the standard-state potential from a voltammogram, we need to rewrite the Nernst equation for reaction We will do this in several steps. First, we substitute equation Because the concentration of [R] bulk is zero—remember our assumption that the initial solution contains only O—we can simplify this equation.
Now we are ready to finish our derivation. Substituting equation If K O is approximately equal to K R , which is often the case, then the half-wave potential is equal to the standard-state potential. Note that equation In voltammetry there are three important experimental parameters under our control: how we change the potential we apply to the working electrode, when we choose to measure the current, and whether we choose to stir the solution.
Not surprisingly, there are many different voltammetric techniques. In this section we consider several important examples. The first important voltammetric technique to be developed— polarography —uses the dropping mercury electrode shown in Figure Although polarography takes place in an unstirred solution, we obtain a limiting current instead of a peak current. When a Hg drop separates from the glass capillary and falls to the bottom of the electrochemical cell, it mixes the solution.
Each new Hg drop, therefore, grows into a solution whose composition is identical to the bulk solution. The limiting current—which is also called the diffusion current—is measured using either the maximum current, i max , or from the average current, i avg. See Appendix 15 for a list of selected polarographic half-wave potentials. Normal polarography has been replaced by various forms of pulse polarography , several examples of which are shown in Figure The current is sampled at the end of each potential pulse for approximately 17 ms before returning the potential to its initial value.
The shape of the resulting voltammogram is similar to Figure As a result, the faradaic current in normal pulse polarography is greater than in the polarography, resulting in better sensitivity and smaller detection limits. In differential pulse polarography Figure The difference in the two currents gives rise to the peak-shaped voltammogram. The voltammogram for differential pulse polarography is approximately the first derivative of the voltammogram for normal pulse polarography.
You may recall that the first derivative of a function returns the slope of the function at each point. The first derivative of a sigmoidal function is a peak-shaped function. Other forms of pulse polarography include staircase polarography Figure For example, suppose we need to scan a potential range of mV. At this rate, we can acquire a complete voltammogram using a single drop of Hg!
Polarography is used extensively for the analysis of metal ions and inorganic anions, such as IO 3 — and NO 3 —. We also can use polarography to study organic compounds with easily reducible or oxidizable functional groups, such as carbonyls, carboxylic acids, and carbon-carbon double bonds.
The current is sampled at the time intervals shown by the black rectangles. In polarography we obtain a limiting current because as each drop of mercury mixes the solution as it falls to the bottom of the electrochemical cell. If we replace the DME with a solid electrode see Figure We call this approach hydrodynamic voltammetry.
Hydrodynamic voltammetry uses the same potential profiles as in polarography, such as a linear scan Figure The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations from the growth of the mercury drops.
Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for analytes that undergo oxidation or reduction at more positive potentials. Another important voltammetric technique is stripping voltammetry , which consists of three related techniques: anodic stripping voltammetry, cathodic stripping voltammetry, and adsorptive stripping voltammetry. Because anodic stripping voltammetry is the more widely used of these techniques, we will consider it in greatest detail.
Anodic stripping voltammetry consists of two steps Figure The first step is a controlled potential electrolysis in which we hold the working electrode—usually a hanging mercury drop or a mercury film electrode—at a cathodic potential sufficient to deposit the metal ion on the electrode.
Moreover, it is the study of current as a function of applied potential. The curve that we get from the voltammetric analysis is named voltammogram. It shows the variation of potential with time. Here, the potential varies arbitrarily either step by step or as a continuous process. And, we can measure the actual current value as the dependent variable. Furthermore, the process opposite to voltammetry is amperometry. To conduct an experiment in voltammetry, we need at least two electrodes.
Of the two, one electrode is called the working electrode. It makes contact with the analyte. The working electrode must apply the desired potential in a controlled manner to facilitate the transfer of charge to and from the analyte. The second electrode, on the other hand, should have a known potential which can gauge the potential of the working electrode. Polarography is a subclass of voltammetry.
Coulometry and Electrogravimetry. What to Upload to SlideShare. A few thoughts on work life-balance. Is vc still a thing final. Related Books Free with a 30 day trial from Scribd. Related Audiobooks Free with a 30 day trial from Scribd.
Empath Up! Sandhya Rani Nayak. Hemant Thakur. Views Total views. Actions Shares. No notes for slide. Voltammetry and Polarography 1. Types of Voltammetric Techniques 5. Only a small residual current flows. The reduced Cd dissolves in the Hg to form an amalgam. The upper trace in the Figure above is called a polarographic wave.
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. Rather, it is measured once before the pulse and again for the last 17 ms of the pulse. Current is measured as a function of the potential applied to a solid WE. Amperometry and Polarography 7.
0コメント