Laboratory Experiment pubs.acs.org/jchemeduc
Cyclic Voltammetry Simulations with DigiSim Software: An UpperLevel Undergraduate Experiment Stephania J. Messersmith* Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, United States S Supporting Information *
ABSTRACT: An upper-division undergraduate chemistry experiment is described which utilizes DigiSim software to simulate cyclic voltammetry (CV). Four mechanisms were studied: a reversible electron transfer with no subsequent or proceeding chemical reactions, a reversible electron transfer followed by a reversible chemical reaction, a reversible chemical reaction followed by a reversible electron transfer, and a reversible electron transfer followed by a reversible chemical reaction followed by a reversible electron transfer. The experiment concludes with a student discussion of three journal articles that contain CV data. In addition to introducing students to the theory of CV, the experiment also reinforces the physical chemistry concepts of equilibrium and rate of reaction. Thus, it is suitable for a physical chemistry, instrumental analysis, or integrated/ interdisciplinary laboratory experiment. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Interdisciplinary/Multidisciplinary, Laboratory Instruction, Physical Chemistry, Computer-Based Learning, Electrochemistry, Equilibrium, Kinetics, Thermodynamics
C
reversible chemical reaction, EC mechanism, where E indicates an electrochemical reaction and C indicates a chemical reaction; eq 3, a reversible chemical reaction followed by a reversible electron transfer, CE, mechanism; and eq 4, a reversible electron transfer followed by a reversible chemical reaction followed by a reversible electron transfer, ECE mechanism. The mechanisms are shown with the following reactions:
yclic voltammetry (CV) is a technique that is often overlooked in the undergraduate curriculum. One reason is the cost of the specialized instrumentation required to conduct the experiment; the problem of cost has been addressed with in-house-built units.1−3 A second reason for the hesitation to include CV is the prioritization of time to more commonly used methods. A third reason is the difficulty level of the theory behind the shape of the voltammogram as well as the concept of reversibility. This laboratory exercise is designed to address student understanding of the theory using a simulation program, DigiSim.4 It is designed to either standalone in the curriculum or be an introduction to one or more wet lab experiments.5−10 Understanding the theory behind the shape of the voltammogram as well as reversibility requires a fundamental understanding of kinetics and equilibria, making this experiment not only appropriate for learning about cyclic voltammetry in an instrumental analysis laboratory but also valuable in a physical chemistry laboratory; this versatility should earn the experiment a place in the jam-packed curriculum. The in-class portion of the assignment typically requires four to six hours, depending upon group size and the depth of discussion. Students work in groups of two or three, with groups of three more likely to require more time for discussion and completion.
■
O+e⇌R
A⇌O O+e⇌R
R⇌A
(2)
O+e⇌R
(3)
R⇌A
A+e⇌B
(4)
where O represents an oxidized species, R is a reduced species, and A and B are additional species. Students input an initial concentration for species O for mechanism 1, run a simulation of the CV (Figure 1), and view a “movie”. The movie provides a concentration-distance profile that varies with potential (Figure 2). In the upper right-hand side of the screen, the students view the CV as a function of potential. The student handout walks the students through a correlation of the CV with the concentration-distance profile, explaining how the progression of the diffusion controlled reaction at the electrode interface results in the shape of the CV. A series of questions in the handout guides a group discussion. They then vary the concentration of species O and discuss changes that arise in the CV. After mechanism 1 and its resulting CVs are
EXPERIMENT
Four mechanisms are included in this study: eq 1, a reversible electron transfer with no subsequent nor proceeding chemical reactions; eq 2, a reversible electron transfer followed by a © XXXX American Chemical Society and Division of Chemical Education, Inc.
(1)
O+e⇌R
A
dx.doi.org/10.1021/ed300633n | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
■ ■
Laboratory Experiment
HAZARDS The experiment is a computer simulation. Thus, no chemical hazards exist. DISCUSSION Undergraduate students in an upper-level, integrated instrumental and physical chemistry laboratory have successfully completed this experiment. The quality of data obtained and associated laboratory reports varied with student and with group, though all students illustrated a fundamental knowledge of CV in their reports. The Instructor’s Guide accompanying this article should be consulted for optimum student learning (see Supporting Information). A refresher on equilibrium constants and rate constants should precede this experiment either through independent study or in the classroom. Unfortunately, it was also found that a few of the students were not comfortable with the concepts of oxidation, reduction, and reduction potentials. In the author’s course, the ferrocyanide/ferricyanide couple has been studied by CV.5 Prior to the implementation of the author’s DigiSim simulation laboratory, laboratory reports did not illustrate an understanding of reversibility nor did students place emphasis on the shape of the CV as it relates to the concentration distance profile. After implementation of this lab, the majority of the students illustrated a higher level of understanding in their laboratory reports.
Figure 1. Simulated CV for mechanism 1, such that the initial concentration of species O is 0.001 M, and the formal reduction potential is 0 V. The scan was initiated in the negative potential direction starting at 0.3 V to a limit of −0.3 V and returned the initial potential.
■
Figure 2. “Movie” allows students to view the concentration-distance profile as a function of potential and correlate it with the CV.
CONCLUSIONS Enhanced student understanding of cyclic voltammetry in an undergraduate curriculum can be realized at a low cost through the use of simulation software as a standalone laboratory or as an introduction to one or more wet laboratories. Not only does this introduce students to a technique that is often used in research, but it also allows for a manipulation of equilibrium and rate constants to deepen their understanding of those physical chemistry concepts.
understood, the group moves on to mechanisms 2−4, where both the equilibrium constant, Keq, and rate constant of the forward reaction, kf, are varied. Again, group discussion is facilitated by questions in the handout that address the changes seen in the CV and the “movie”. Mechanism 4 is the most complicated mechanism and requires the students to think about the effect of relative potentials of the electrochemical reactions as well as the complication of the competing chemical reaction. Students then obtain three journal articles11 in which the authors have utilized CV to study real-world examples of electrochemical reactions. The reactions studied and results found are identified and comparisons are made to the mechanisms simulated with DigiSim. The student handout includes an introduction to CV, including a diagram and explanation of a three-electrode cell, the Nernst equation, and Randles−Sevcik equation as well as a stepwise explanation of the shape of the CV for mechanism 1 as it correlates to the concentration-distance profile throughout the potential cycle. Although the author hopes that the introduction and the explanation of the “movie” for mechanism 1 found in the student handout is a thorough yet concise explanation of the phenomena, additional resources are available to both the students and instructors both in the format of journal articles,12−16 books devoted to electrochemical methods,17−21 and instrumental analysis texts. The author currently follows this simulation experiment with a study of the ferrocyanide/ferricyanide couple by CV,5 which follows mechanism 1 and explores the effects of varying both concentration and scan rate. If additional time is available in the curriculum, one could explore wet lab experiments with additional mechanisms6−8 or CV in the context of inorganic synthesis9 or biochemistry.10
■
ASSOCIATED CONTENT
* Supporting Information S
Instructor’s notes and a student handout are provided. This material is available via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Stewart, G.; Kuntzleman, T. S. Affordable Cyclic Voltammetry. J. Chem. Educ. 2009, 86, 1080−1081. (2) Gostowski, R. Teaching Analytical Instrument Design with LabVIEW. J. Chem. Educ. 1996, 73, 1103−1107. (3) Britton, W. E.; Fleming, R. H.; Hammond, G. S.; Nichparenko, W. A Triangle Wave Generator for Electrochemical Applications. J. Chem. Educ. 1976, 53, 68. (4) BASi DigiSim® Simulation Software for Cyclic Voltammetry. http://www.basinc.com/products/ec/digisim/ (accessed Jun 2014). (5) Sawyer, D. T.; Heineman, W. R.; Beebe, J. M. Chemistry Experiments for Instrumental Methods: Experiment 4−1 Cyclic Voltammetry; John Wiley & Sons: New York, 1984; pp 79−85.
B
dx.doi.org/10.1021/ed300633n | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
(6) Sawyer, D. T.; Heineman, W. R.; Beebe, J. M. Chemistry Experiments for Instrumental Methods: Experiment 4−2 Study of Electrode Mechanism by Cyclic Voltammetry; John Wiley & Sons: New York, 1984; pp 85−89. (7) Van Benschoten, J. J.; Lewis, J. Y.; Heineman, W. R.; Roston, D. A.; Kissinger, P. T. Cyclic Voltammetry Experiment. J. Chem. Educ. 1983, 60, 772−776. (8) Baldwin, R. P.; Ravichandran, K.; Johnson, R. K. A Cyclic Voltammetry Experiment for the Instrumental Analysis Laboratory. J. Chem. Educ. 1984, 61, 820−823. (9) (a) Queiroz, S. L.; de Araujo, M. P.; Batista, A. A. An Electrochemical Experiment to Monitor the Isomerization of trans- to cis-[RuCl2(dppb)(phen)]. J. Chem. Educ. 2001, 78, 89−90. (b) Toma, H. E.; Araki, K.; Dovidauskas, S. A Cyclic Voltammetry Experiment Illustrating Redox Potentials, Equilibrium Constants and Substitution Reactions in Coordination Chemistry. J. Chem. Educ. 2000, 77, 1351− 1353. (c) Boyd, D. C.; Johnson, B. J.; Mann, K. R. Photochemical and Electrochemical Studies of an Organometallic Sandwich Compound. J. Chem. Educ. 1992, 77, A315−A316. (10) (a) Kulczynska, A.; Johnson, R.; Frost, T.; Margerum, L. D. How do Structure and Charge Affect Metal-Complex Binding to DNA? An Upper-Division Integrated Laboratory Project Using Cyclic Voltammetry. J. Chem. Educ. 2011, 88, 801−805. (b) Smith, E. T. Voltammetric Simulations of Two-Electron/Two Proton Coupled Mechanisms: An Indirect Method for Obtaining Reduction Potentials and Acid Dissociation Constants? Curr. Sep. 2004, 21, 11−13. (11) Students select three of the following, with one being reversible, one a CE mechanism, and one an EC mechanism: (a) Claus, R.; Lewtak, J.. P.; Muller, T. J.; Swarts, J. C. J. Structural Influences on the Electrochemistry of 1,1′-di(hydroxyalkyl)ferrocenes. Structure of [Fe{η5-C5H4−CH(OH)−(CH2)3OH}2]. Organomet. Chem. 2013, 740, 61−69. (b) Tahsini, L.; Specht, S. E.; Lum, J. S.; Nelson, J. J. M.; Long, A. F.; Golen, J. A.; Rheingold, A. L.; Doerrer, L. H. Structural and Electronic Properties of Old and New A2[M(pinF)2] Complexes. Inorg. Chem. 2013, 52, 14050−14063. (c) Rafiee, M.; Shoaei, S. M.; Khalafi, L. Kinetic Study of Electrochemically Induced C-P Bond Formation of Catechols with Trialkylphosphites. Electrochim. Acta 2012, 80, 56−59. (d) Atkins, S.; Sevilla, J. M.; Blazquez, M.; Pineda, T.; Gonzalez-Rodriguez, J. Electrochemical Behaviour of Carbamazepine in Acetonitrile and Dimethylformamide Using Glassy Carbon Electrodes and Microelectrodes. Electroanalysis 2010, 22, 2961−2966. (e) Messersmith, S. J.; Kirchbaum, K.; Kirchhoff, J. R. Luminescent Low-Valent Rhenium Complexes with 1, 2-Bis(dialkylphosphino)ethane Ligands. Synthesis and X-ray Crystallographic, Electrochemical, and Spectroscopic Characterization. Inorg. Chem. 2010, 49, 3857−3865. (f) Rafiee, M.; Nematollahi, D. Electrochemical Study of Catechol-Boric Acid Complexes. Electrochim. Acta 2008, 53, 2751−2756. (g) Nematollahi, D.; Workentin, M. S.; Tammari, E. Electrochemical Oxidation of Catechol in the Presence of Cyclopentadiene. Investigation of Electrochemically Induced DielsAlder Reactions. Chem. Commun. 2006, 1631−1633. (h) Nematollahi, D.; Rafiee, M. Diversity in Electrochemical Oxidation of Dihydroxybenzoic Acids in the Presence of Acetylacetone. A Green Method for Synthesis of New Benzofuran Derivatives. Green Chem. 2005, 7, 638− 644. (i) Orsini, J.; Geiger, W. E. Electrochemical Characterization of Equilibria Involving Catalyst Precursors: Cyclic Voltammetry of Labile Organorhodium Complexes in Coordinating Solvents. Organometallics 1999, 18, 1854−1861. (12) Kissinger, P. T.; Heineman, W. R. Cyclic Voltammetry. J. Chem. Educ. 1983, 60, 702−706. (13) Mabbott, G. A. An Introduction to Cyclic Voltammetry. J. Chem. Educ. 1983, 60, 697−702. (14) Evans, D. H.; O’Connell, K. M.; Petersen, R. A.; Kelly, M. J. Cyclic Voltammetry. J. Chem. Educ. 1983, 60, 290−293. (15) Faulkner, L. R. Understanding Electrochemistry: Some Distinctive Concepts. J. Chem. Educ. 1983, 60, 262−264. (16) Bott, A. W. Characterization of Chemical Reactions Coupled to Electron Transfer Reactions Using Cyclic Voltammetry. Curr. Sep. 1999, 18, 9−16.
(17) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2000; pp 1−10, 19−28, 226−255, 471−484, 488−498, 512− 515, 652−656. (18) Noel, M.; Vasu, K. I. Cyclic Voltammetry and the Frontiers of Electrochemistry; Oxford & IBH Publishing: New Delhi, 1990; pp 62− 156, http://michael-noel-electrochemistry.tripod.com/cvm.htm (accessed Apr 2014). (19) Sawyer, D. T.; Sobkowiak, A. J.; Roberts, J. L. Electrochemistry for Chemists, 2nd ed.; John Wiley & Sons: New York, 1995; pp 1−16, 53− 56, 68−77, 184−198, 206−215, 219, 271−273, 313−316. (20) Laboratory Techniques in Electroanalytical Chemistry, 2nd ed.; Kissinger, P., Heineman, W. R., Eds.; CRC Press: Boca Raton, FL, 1996; pp 5−8, 31−43, 76−95. (21) Handbook of Electrochemistry; Zoski, C. G., Ed.; Elsevier B. V.: Amsterdam, 2007; pp 3−14, 33−36, 438−444.
C
dx.doi.org/10.1021/ed300633n | J. Chem. Educ. XXXX, XXX, XXX−XXX
