Rotated disk electrode
The following experiment will be put in place once we have available the instrumentation from Pine Instruments. Description of some of the work can be found on Pine page http://www.pineinst.com/echem/expt3.htm
The following text, is modified from the original Pine Instruemnts Document
Rotated Disk Electrode
(Text from Pine Instrument Company, modified by Petr Vanýsek 14 April 1999
)The text has been converted from MS Word to html and some formatting was lost. The WP file in rich text format can be downloaded here.
Purpose
The aim of this experiment is to use one method of cyclic voltammetry, the rotated disk voltammetry, to determine the diffusion coefficient of the analyte.
 | Bipotentiostat |
| Pine Instrument Company AFMSRX Analytical Rotator |
| Platinum rotated disk electrode (mounted on arbor) |
| Three electrode cell (with large opening for rotated disk electrode) |
| Platinum auxiliary electrode |
| SCE reference electrode |
| Alumina polishing solution |
| Description | per expt |
per 20 expts |
| potassium ferricyanide, K3Fe(CN)6 (329.26 g/mol) | 250 mg | 5 g |
| potassium nitrate, KNO3 (101.11 g/mol) | 30 g | 600 g |
| ultrapure water | 300 ml | 6 l |
Description
A general definition for the term voltammetry is any electrochemical technique that involves controlling the potential of an electrode while simultaneously measuring the current flowing through that electrode. The electrode in question is usually called the working electrode in order to distinguish it from other electrodes that are present in the electrochemical cell.
Voltammetry is usually performed by connecting an electrochemical potentiostat to an electrochemical cell. The cell contains a test solution and three electrodes: working, reference, and auxiliary. Special electronic circuitry within the potentiostat permits the working electrode potential to be controlled with respect to the reference electrode without any appreciable current flowing through the reference electrode. Rather, the current is forced to flow between the working electrode and the auxiliary electrode as such a magnitude, that the desired potential is maintained between the working and the reference electrodes. This unusual arrangement has two principle benefits. First, the reference electrode is protected from internal electrochemical changes caused by current flow. Second, measurement errors related to the resistance of the test solution are kept to a minimum.
There are quite a number of voltammetric techniques. Each differs in the precise manner that the working electrode potential is changed during the experiment. In some techniques, a potential sweep is applied to the working electrode, in others; a sudden potential step or complex pulse sequence is used. Another distinguishing feature is whether or not the solution is moving with respect to the surface of the working electrode. In most cases, the solution is motionless, but there exist many hydrodynamic methods in which solution moves toward the electrode along a well-defined flow pattern. The rotated disk electrode is an example of a hydrodynamic method.
The analyte used in this experiment is the ferricyanide anion, Fe(CN)63–, which contains an iron atom in the +3 oxidation state. At the surface of a working electrode, a single electron can be added to the ferricyanide anion. This causes it to be reduced to the ferrocyanide anion, Fe(CN)64–, which contains an iron atom in the +2 oxidation state. This simple one electron exchange between the analyte and the electrode is very well behaved, and it is reversible. This means that the analyte can be easily reduced to Fe(CN)64– and then easily oxidized back to Fe(CN)63– again.
A pair of analytes differing only in oxidation state is known as a redox couple. The electrochemical half-reaction for the Fe(CN)63– / Fe(CN)64– redox couple can be written as follows:
| Fe(CN)63– + e– Fe(CN)64– (1) |
The formal potential associated with this half-reaction is near +400 mV vs. the normal hydrogen electrode (NHE). If the working electrode is held at a potential more positive than +400 mV, then the analyte tends to be oxidized to the Fe(CN)63– form. This oxidation at the working electrode causes an anodic current to flow (i.e., electrons go into the electrode from the solution). At potentials more negative than +400 mV, the analyte will be reduced to Fe(CN)64–. This reduction at the working electrode causes a cathodic current to flow (i.e., electrons flow out of the electrode into the solution).
Rotated Disk Voltammetry
The working electrode potential is slowly swept back and forth across the formal potential of analyte. The working electrode itself is rotated at a very high speed. This rotational motion sets up a well-defined flow of solution towards the surface of the rotating disk electrode. The flow pattern is akin to a vortex that literally sucks the solution (and the analyte) towards the electrode.
Experimental results are generally plotted as a graph of current versus potential, and a typical rotated disk voltammogram is shown in Figure 1. The voltammogram exhibits a sigmoidal shaped wave, and the height of this wave provides the analytical signal.
It is important to note that the layer of solution immediately adjacent to the surface of the electrode behaves as if it were stuck to the electrode. While the bulk of the solution is being stirred vigorously by the rotating electrode, this thin layer of solution manages to cling to the surface of the electrode and appears (from the perspective of the rotating electrode) to be motionless. This layer is called the stagnant layer in order to distinguish it from the remaining bulk of the solution.
Analyte is conveyed to the electrode surface by a combination of two types of transport. First, the vortex flow in the bulk solution continuously brings via convection fresh analyte to the outer edge of the stagnant layer. Then, the analyte moves across the stagnant layer via simple molecular diffusion. The thinner the stagnant layer, the sooner the analyte can diffuse across it and reach the electrode surface. Faster electrode rotation makes the stagnant layer thinner. Thus, faster rotation rates allow the analyte to reach the electrode faster, resulting in a higher current being measured at the electrode.
The Levic equation predicts the current observed at a rotating disk electrode. This equation takes into account both the rate of diffusion across the stagnant layer and the complex solution flow pattern. In particular, the Levic equation gives the height of the sigmoidal wave observed in rotated disk voltammetry. The sigmoid wave height is often called the Levic current, iL, and it is directly proportional to the analyte concentration, c. The Levic equation is written as
| iL = (0.620) n F A D2/3 w 1/2 n –1/6 c(2) |
where w is the angular rotation rate of the electrode (rad/s) and n is the kinematic viscosity of the solution (cm2/s). The kinematic viscosity is the ratio of the solution viscosity to its density. For pure water, n = 0.0100 cm2/s, and for the solvent used in this experiment (1.0 mol/l KNO3), n = 0.00916 cm2/s.
All glassware used for electrochemistry should be as clean as possible. The solvents and reagents used to make solutions should be as pure as possible. A supply of ultrapure water is required for proper solution preparation. The ultrapure water can be either deionized, ultrafiltered (DIUF) water or "conductivity water" or "HPLC grade" water.
A. Solution preparation
The two solutions required for this experiment should be prepared by the student. The electrolyte solution is 1.0 mol/l potassium nitrate in water. This solution provides an electrically conductive solvent suitable for use with voltammetry. The analyte solution is an 6.4 mmol/l solution of potassium ferricyanide made using the electrolyte solution as the solvent.
1) Electrolyte solution (250 ml)
Prepare a very clean 250 ml volumetric flask being sure that the last rinsing of this flask is done with ultrapure water. Weigh exactly approximately 25.30 g of KNO3 and transfer it quantitatively to the flask. Fill the flask with about 200 ml of ultrapure water and allow the potassium nitrate to dissolve. Once dissolution is complete, fill the flask "to the line" using ultrapure water and mix well. The resulting solution is about 1.0 mol/l KNO3.
2) Analyte solution (100 ml)
Prepare a very clean 100 ml volumetric flask being sure that at least the last rinsing of this flask is done with ultrapure water. Using a sensitive microbalance, weigh as exactly as possible 210.7 milligrams of potassium ferricyanide, K3Fe(CN)6, and transfer it into the flask. Fill the flask with about 75 ml of electrolyte solution and allow the K3Fe(CN)6 to dissolve. Once dissolution is complete, fill the flask "to the line" using electrolyte solution and mix well. The resulting solution should have an analyte concentration of 6.4 mmol/l, however a more accurate concentration can be computed, based on the actual mass of K3Fe(CN)6 that was used to prepare the solution.
B. Background scan
A simple background voltammogram of the pure electrolyte solution is a good way to confirm the purity of the solution, the cleanliness of the glassware, and the preparation of the polished working electrode all in a single step. Any electroactive impurities from the solvent or dirty glassware will show up as unexplained peaks in the background scan. In addition, a fouled or improperly polished electrode surface usually causes a larger background current.
3) Obtain a platinum disk working electrode suitable for mounting in the electrode rotator apparatus. Be sure to note the surface area of the disk in square centimeters in your lab notebook. Polish the electrode as needed using an alumina slurry on a polishing cloth. After polishing, wash the electrode with ultrapure water and wipe clean. The electrode surface should be mirror bright and free of defects.
4) Equip a clean electrochemical cell with an SCE reference electrode and a platinum auxiliary electrode. Carefully mount the platinum disk working electrode in the rotator and then lower it into the cell. Note that the electrode should not be rotating during the background scan.
5) Fill the electrochemical cell with pure electrolyte solution. If desired, the oxygen in the cell may be purged by first bubbling nitrogen through the solution and then continuously blanketing the solution with a steady flow of nitrogen for the duration of the experiment. Oxygen is unlikely to interfere with this experiment, however.
6) Set the Pine potentiostat do the dummy mode. Then, make all necessary electrical connections between the potentiostat and the electrochemical cell.
7) Next, adjust the conditions on the front panel to +800 mV on K1 electrode and 0 mV on K2 electrode (The K2 electrode part is not even connected to the cell, it is just good practice to keep the value at zero.
8) Select the parameters for the sweep as follows: Sweep rate 200 mV/s, potential limits -100 mV and 800 mV (for electrode K1). Leave the settings for K2 alone. Set the current sensitivity (current convertor) such that the maximum current will be 250 µA. Note that these settings are only suggested starting points for performing a background scan. It may be necessary to make some adjustments in order to obtain a satisfactory voltammogram. In particular, the current conversion for the K1 may need to be altered.
9) Once the experiment settings have been adjusted to match those in part 8) start the scan and switch from the DUMMY to the WORKING option. The resulting cyclic voltammogram should be relatively featureless and without significant peaks. At negative potentials, the voltammogram should exhibit some cathodic current due to the reduction of hydronium ion. If the background current is excessive, the electrode should be polished. It is also possible that the electrolyte solution is contaminated, which would require preparing a new solution.
10) After acquiring a satisfactory background voltammogram, label the axes appropriately and save the graph.
Next, several rotated disk voltammograms are obtained using various rotation rates.
11) Turn on the electrode rotator and adjust the rotational speed of the electrode to 4000 rpm. Make certain that the flow of solution in the cell is non-chaotic and that the surface of the rotating electrode remains immersed in the solution.
12) On the front panel of the potentiostat adjust the settings so that they correspond to a slow (10 mV/s) sweep of the potential from +500 mV down to –100 mV and then back again. Note that the current sensitivity may need to be altered from the setting of the previous experiment. Assume that a maximum current of 1 mA will flow.
13) Once the experiment settings have been adjusted to match those in 12), turn the scan on ands switch from DUMMY to WORKING to initiate the experiment. A fairly prominent cathodic wave should appear during the sweep from +500 mV to –100 mV. The wave should have a sigmoidal appearance rather than the asymmetric peak shape typically observed during cyclic voltammetry. On the return sweep, the current signal should retrace the path followed during forward sweep. Figure 1 shows a typical rotated disk voltammogram for potassium ferricyanide. Note that the positive orientations for both the potential and current may be different on the actual plot.
Figure 1: A Rotated Disk Voltammogram for Ferricyanide
14) After acquiring a satisfactory voltammogram, label the axes and save the plot for further evaluation or more conveniently, use the same sheet for the additional experiments (15).
15) In addition to the voltammogram just acquired at 4000 rpm, repeat step 13 and acquire voltammograms at these other seven rotation rates: 3200, 2500, 1800, 1300, 900, 600 and 400 rpm. In each case, be sure to note the rotation rate and the Levic current in your laboratory notebook.
a) Using the Levic currents (sigmoid heights) from the series of rotated disk voltammograms acquired at various rotation rates, prepare a plot of the Levic current versus the square root of the angular rotation rate. Note that the rotation rates may have to be converted from units of rpm to radians per second using the relationship w = 2 p f / 60.
b) Perform a linear least squares analysis on the data to find the equation of the best straight line which fits the data.
c) Use the slope of the line together with the Levic equation to estimate the diffusion coefficient for the ferricyanide anion. Pay close attention to proper units and report your answer in cm2/s. Note that the kinematic viscosity for 1.0 mol/l KNO3 is 0.00916 cm2/s.
1) List the diffusion coefficient result that you obtained with appropriate unit and error.
2) Compare the result with that in available literature. If a voltammetry laboratory was performed prior to this one during the course, compare the values from RDE and cyclic voltammetry. Which of the two appears more reliable?
After the experiment
It is absolutely essential that proper care is taken of the electrodes. The working rotated electrode should not be left in any solution for prolonged period of time and NEVER OVERNIGHT when not used. Also, the level of the solution should not reach above the upper Teflon sleeve under which are located mounting screws. When the electrode is rotating, the level of the solution will decrease. Once the rotation stops, the level might rise above the safe limit. Hence the argument for not leaving the electrode that is not used in the solution.
Inception: 21 December 1998
Last revised: 27 February 2008 16:43
© Petr Vanýsek
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