CHEM426 WSU Solid Electrode Electrochemistry Lab Reports

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write a formal lab report for ELECTROCHEMISTRY lab


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FUNDAMENTAL EXPERIMENTS OF SOLID ELECTRODE ELECTROCHEMISTRY By Cynthia Earles (Updated by Jeremy Lessmann) Additional Note: For Part 2 of the experiment make sure you take any necessary additional scans to include the following in your report Figures of Merit Checklist: 1. Calibration plot Of DPV for several concentrations of potassium ferrocyanide 2. Equation of calibration with standard deviations of slope and intercept 3. Table of true concentrations (standards and pseudo-unknown), estimated concentrations with standard deviations (from equations not repeat trials), and RSD’s. 4. Sensitivity 5. Detection limit 6. Voltammograms, printed from Excel and in proper format for publication in the ACS Journal Analytical Chemistry (Consult the Author Guidelines under Submission & Review (http://pubs.acs.org/journal/ancham )) Introduction Electrochemical methods cover a wide range of analytical techniques. The fundamental signal measured is electrical in nature, either current (Faradaic) or voltage (potentiometric), resulting from redox reactions. Like other analytical techniques, electrochemical methods yield both quantitative and qualitative information. In addition, some electrochemical methods, will give information about non-redox chemical processes occurring before or after the redox reaction. Charge transfer can occur homogeneously in solution or heterogeneously on electrode surfaces. For a heterogeneous reaction, the electrode acts as either a source (for reduction) or a sink (for oxidation) of electrons transferred to or from species in solution. Because this is a surface phenomenon, the electroactive species in the solution needs to get to the electrode surface. Therefore, in a still solution, the concentration of species at the electrode surface depends on the mass transport of these species from bulk solution. If the kinetics of the electrode reaction are much faster than this transport the reaction is reversible. Redox reactions play a fundamental role in biochemistry and chemistry, thus it is useful to measure and tabulate the tendencies of various substances to gain or lose electrons. Standard electrode potentials, E°, are measured relative to hydrogen. The standard potential is defined for a cell in which all activities are unity. The formal potential is the reduction potential that applies under specified conditions. For example, biochemists call the formal potential at pH 7 E°'. Whenever a proton appears in a redox reaction, reduction potentials are pH-dependent. Chronoamperometry, cyclic voltammetry and differential pulse voltammetry are faradaic techniques used in these experiments. They differ with respect to applied wave form and resulting signal. Chronoamperometry is a potential step method that has a square wave form. A potential is stepped from an initial value that causes no current to flow to a potential that causes current to flow. The resulting current is measured as it decreases over time. The signal follows the Cottrell equation shown in equation 1. This equation can be used to calculate the surface area of an electrode or the concentration of analyte in solution. Cyclic voltammetry is a potential sweep method that has a triangular wave form. The potential is increased linearly with time to some specified potential value and then decreased over the same period of time back to the initial potential. The current at Ep can be calculated using the Randles-Sevcik (equation 2). Cyclic voltammetry is principally used to diagnose mechanisms of electrochemical reactions because a voltammogram gives qualitative information about nonredox processes occurring before or after the redox reaction. Cyclic voltammetry will be used in these experiments to study E, EC, ECE, EC', and adsorption mechanisms (figure 1). Differential pulse voltammetry is a differential technique so the response is similar to the first derivative of a conventional voltammogram. The peak potential, Ep is approximately Ep/2 Equation 1. Cottrell Equation iL = nFAD1/2C/(tp)1/2 i = current (A) n = number of electrons F = faradays constant (C/mol) A = area of the electrode (cm2) D = diffusion coefficient (cm2/s) C = concentration (mol/mL) t = time (s) Equation 2. Randles-Sevcik Equation iL = Kn3/2AD1/2CV1/2 K = 2.6 x 105 A = area of the electrode (cm2) D = Diffusion coefficient (cm2/s) C = concentration (mol/mL) V = scan rate (V/s) n = number of electrons Figure 1. Reaction Mechanisms: red = reduced species ox = oxidized species n = number of electrons e- = electron z = electro-inactive molecule E: red ® ox + n eEC': red ® ox + n eox + z ® red EC: red ® ox + n e- ox + z ® z' ECE: red1 ® ox1 + n1 eox1 + z ® red2 red2 ® ox + n2 eAdsorption: z ® redads redads ® oxads + n e- Experimental Instruments These instructions specifically apply to the Obbligato Objectives Farraday MP computer controlled electrochemical workstation. A) E Mechanism: A Chronoamperogram, differential pulse voltammogram, and cyclic voltammogram of ferrocyanide will illustrate characteristic signals of an E, or normal, reaction mechanism. All three of these techniques are faradaic and differ with respect to the applied wave form. The computer controls the wave form generated by the FarradayMP. Prepare a 100 mM Fe(CN)64- solution in 1.0 M KCl using a 10 mL volumetric flask. Also prepare 50 mL of 1 M KCl. The glass cell holds 50 mL. Add the 50 mL of 1.0 M KCl to the cell and connect the electrodes as follows: Black: Working electrode (glassy carbon) Red: Auxiliary (Pt wire) White: Reference (Ag/AgCl) You will need to construct this reference electrode yourself. Just like you did in Chem 222. Lower the black electrode holder onto the glass cell. The electrodes should be immersed in the solution. The 1 M KCl will be used to record the background. If you are familiar with using windows this system is easy to manipulate. Window headings are italicized. 1. Cyclic Voltammetry (CV): CV should be the default experiment that opens. If not choose Cyclic Voltammetry from New Experiment. On the control panel set the parameters to the following: Init E = -0.200 V Switching E =-0.200, 0.600 V Current Range = 100 µA Scan Speed = 50 mV/s # Scans = 1 Click the mouse on the “Play” button. After the run is finished go to File and select Export Data. The output on the screen should be saved and exported as an excel file and subtracted from the subsequent cyclic voltammogram in excel. Add 0.5 mL of 100 mM ferrocyanide solution to the 50 mL of 1 M KCl. Run a CV of this solution, export and subtract the new voltammogram from the saved background in excel. Print the resulting voltammogram. It is not necessary to change the ferrocyanide solution when changing techniques. Remember that oxidation only occurs at the surface of the electrode. Results and Questions: On the printed background corrected voltammogram indicate what part of the curve follows the Nernst equation and what part is diffusion controlled. Record the EP and EP/2. 2. Differential Pulse voltammetry (DPV): Stir the above solution for 10 seconds and let it stand for 30 seconds. Choose New Experiment and select Differential Pulse Voltammetry and set the following parameters: Init E = 200 mV Final E = 900 mV Step Voltage = 2 mV Pulse height = 25 ms Pulse width = 25 ms Wait time = 55 ms Run the DPV and export the data as before. Results and Questions: Print the resulting signal. Find the E° from the voltammogram. How does the DPV waveform and output relate to the CV waveform and output? 3. Chronoamperometry (CA): Stir or agitate the solution for 10 seconds and allow it to stand for 30 seconds. Choose New Experiment, select Double-step Chronoamperometry and adjust parameters to: Init E = 0 V High E = 0.800 V Step E = 2.00 V Initial Time = 11 ms Step Time = 25 ms Final Time = 55 ms The sensitivity also needs to be changed by switching the Current Range from 100 µA to 100 mA. Run and export a single plot. Results and Questions: Print the chronoamperogram and, using the Cottrell equation (it is a linear equation), create a plot to calculate the electrochemical area of the electrode. Show that the result follows the Cottrell equation. The diffusion coefficient for Fe(CN)64- is 0.63 x 104 cm2/s. Why did the sensitivity need to be changed? B) Formal Potentials: The oxidation of 4-methyl catechol involves 2 H+. Whenever an H+ appears in a redox reaction, the redox potential is pH-dependent. Remember that standard reduction potentials are defined at pH 0. Prepare 500 mL of McIlvaine buffer by dissolving 11.68 g Na2HPO4, 1.85 g citrate and 2.72 g KCl in ddH2O. Transfer to a 500 mL volumetric flask and dilute to the mark. Adjust to pH 7. Make a stock of 4-methyl catechol (10 mM is good) in water. Change the Current Range settings to 10 µA. Now run (using parameters below) and export a voltammogram of the buffer alone. At this time take two more background scans. The first of the two background voltammogram can be used for the EC and EC’ experiments but you need to extend the voltage range to 1000 mV. The last background scan needs to be run with 2 scans for the ECE experiment. Add 4-methyl catechol to the 50 mL of buffer (100 µM final concentration) and record the pH. Run and export a single plot, and subtract the background. Take the first derivative of the voltammogram. Determine Eo’ from the peak of this first derivative plot. Init E = -0.400 V Switching E =-0.400, 0.500 V Current Range = 100 µA Scan Speed = 50 mV/s # Scans = 1 Determine Eo’ at pH 6 and 5 by adjusting the pH of the solution in the cell. Print the voltammograms for each pH and the first derivative plots. Results and Questions: Read pages 366 - 369 in Harris (4th Ed., pg. 359-362 or pg. 303-305, 6th ed). Plot the formal potential (V) vs. pH and calculate the slope of the resulting line. How can Eo be determined from the plot? The pK’s for 4-methyl catechol are approximately 9 and 10. How would the slope change over this pH range? Calculate the formal potential, Eo’, at pH 7 (follow example in Harris). C) Other Reaction Mechanisms: 1. EC, ECE, and EC’: Prepare a stock solution, in McIlvaine buffer, of 250 µM dopamine (DA) (made fresh daily) and a stock solution of 500 µM ascorbic acid (AA) in two 50 mL volumetric flasks. a. EC mechanism: Make a dilution of the 500 µM AA stock solution to 100 µM by transferring 10 mL of stock solution to a 50 mL volumetric flask. Dilute to the mark with buffer. Run a CV using the same parameters and background as the formal potential experiment except extend the voltage range to 1000 mV. Print the resulting background corrected voltammogram and qualitatively relate its shape to the EC mechanism. How does it compare to the normal case? b. ECE mechanism: Make a dilution of the 250 µM DA stock solution to 100 µM DA by transferring 20 mL of stock to a 50 mL volumetric flask. Dilute to the mark with buffer. Use the parameters described for the formal potential experiment except change the number of scans to 2. Run and display a single plot CV. Subtract the background saved previously. Print the resulting voltammogram and qualitatively relate its shape to the ECE reaction mechanism. How does it compare to the normal case? c. EC’ mechanism: Using the DA and AA stock solutions prepare a solution that is 100 µM DA and 200 µM AA in a 50 mL volumetric flask. This is accomplished by adding 20 mL DA stock and 20 mL AA stock to a 50 mL volumetric flask and then diluting to the mark with buffer. Run and display the background corrected single plot using the same parameters as described for the formal potential experiment. Print the resulting voltammogram and relate its shape to the EC’ mechanism. Is this reaction chemically reversible? Is it charge transfer reversible? 2. Adsorption: Prepare a 5 µM solution of haloperidol by dissolving 0.9 mg of haloperidol in a minimal amount of ethanol (5 - 10 drops) Dissolving the haloperidol may require patience. Dilute with H2O in a 50 mL volumetric flask. Add 5 mL of this aqueous solution to another 50 mL volumetric flask and dilute to the mark with buffer. Change to the following parameters: Init E = 0 V Switching E = 0, 1000 V Current Range = 1 mA Scan Speed = 100 mV/s # Scans = 1 Run and save a buffer background. Leave the buffer in the cell. Remove the working electrode and soak it for 1 minute in the 5 µM solution of haloperidol. Reconnect the working electrode to the black lead and immerse it in the cell containing buffer. Run and display a single plot, background correct voltammogram. Print the resulting voltammogram. Integrate the peak to find the concentration of adsorbed species on the surface of the electrode. In part A the surface area of the electrode was calculated enabling the concentration of haloperidol per unit area to also be calculated. Run another voltammogram. How and why is the second voltammogram different from the first? ESSAY Whitesides' Group: Writing a Paper** By George M. Whitesides* 1. What is a Scientific Paper? A paper is an organized description of hypotheses, data and conclusions, intended to instruct the reader. Papers are a central part of research. If your research does not generate papers, it might just as well not have been done. ªInteresting and unpublishedº is equivalent to ªnon-existentº. Realize that your objective in research is to formulate and test hypotheses, to draw conclusions from these tests, and to teach these conclusions to others. Your objective is not to ªcollect dataº. A paper is not just an archival device for storing a completed research program; it is also a structure for planning your research in progress. If you clearly understand the purpose and form of a paper, it can be immensely useful to you in organizing and conducting your research. A good outline for the paper is also a good plan for the research program. You should write and rewrite these plans/outlines throughout the course of the research. At the beginning, you will have mostly plan; at the end, mostly outline. The continuous effort to understand, analyze, summarize, and reformulate hypotheses on paper will be immensely more efficient for you than a process in which you collect data and only start to organize them when their collection is ªcompleteº. 2. Outlines 2.1. The Reason for Outlines I emphasize the central place of an outline in writing papers, preparing seminars, and planning research. I especially believe that for you, and for me, it is most efficient to write papers from outlines. An outline is a written plan of the organization of a paper, including the data on which it rests. You should, in fact, think of an outline as a carefully organized and presented set of data, with attendant objectives, hypotheses, and conclusions, rather than an outline of text. An outline itself contains little text. If you and I can agree on the details of the outline (that is, on the data and organization), the supporting text can be assembled fairly easily. If we ± [*] Prof. G. M. Whitesides Department of Chemistry and Chemical Biology Harvard University Cambridge, MA 02138 (USA) E-mail: gmwhitesides@gmwgroup.harvard.edu [**] The text is based on a handout created on October 4, 1989. Adv. Mater. 2004, 16, No. 15, August 4 do not agree on the outline, any text is useless. Much of the time in writing a paper goes into the text; most of the thought goes into the organization of the data and into the analysis. It can be relatively efficient in time to go through several (even many) cycles of an outline before beginning to write text; writing many versions of the full text of a paper is slow. All writing that I doÐpapers, reports, proposals (and, of course, slides for seminars)ÐI do from outlines. I urge you to learn how to use them as well. 2.2. How Should You Construct an Outline? The classical approach is to start with a blank piece of paper, and write down, in any order, all important ideas that occur to you concerning the paper. Ask yourself the obvious questions: ªWhy did I do this work?º; ªWhat does it mean?º; ªWhat hypotheses did I mean to test?º; ªWhat ones did I actually test?º; ªWhat were the results? Did the work yield a new method of compound? What?º; ªWhat measurements did I make?º; ªWhat compounds? How were they characterized?º. Sketch possible equations, figures, and schemes. It is essential to try to get the major ideas. If you start the research to test one hypothesis, and decide, when you see what you have, that the data really seem to test some other hypothesis better, don't worry. Write them both down, and pick the best combinations of hypotheses, objectives, and data. Often the objectives of a paper when it is finished are different from those used to justify starting the work. Much of good science is opportunistic and revisionist. When you have written down what you can, start with another piece of paper and try to organize the jumble of the first one. Sort all of your ideas into three major heaps (1±3). 1. Introduction Why did I do the work? What were the central motivations and hypotheses? 2. Results and Discussion What were the results? How were compounds made and characterized? What was measured? 3. Conclusions What does it all mean? What hypotheses were proved or disproved? What did I learn? Why does it make a difference? DOI: 10.1002/adma.200400767  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1375 ESSAY G. M. Whitesides/Whitesides' Group: Writing a Paper Next, take each of these sections, and organize it on yet finer scale. Concentrate on organizing the data. Construct figures, tables, and schemes to present the data as clearly and compactly as possible. This process can be slowÐI may sketch a figure five to ten times in different ways trying to decide how it is most clear (and looks best aesthetically). Finally, put everythingÐoutline of sections, tables, sketches of figures, equationsÐin good order. When you are satisfied that you have included all the data (or that you know what additional data you intend to collect), and have a plausible organization, give the outline to me. Simply indicate where missing data will go, how you think (hypothesize) they will look, and how you will interpret them if your hypothesis is correct. I will take this outline, add my opinions, suggest changes, and return it to you. It usually takes four to five iterations (often with additional experiments) to agree on an outline. When we have agreed, the data are usually in (or close to) final form (that is, the tables, figures, etc., in the outline will be the tables, figures,... in the paper). You can then start writing, with some assurance that much of your prose will be used. The key to efficient use of your and my time is that we start exchanging outlines and proposals as early in a project as possible. Do not, under any circumstances, wait until the collection of data is ªcompleteº before starting to write an outline. No project is ever complete, and it saves enormous effort and much time to propose a plausible paper and outline as soon as you see the basic structure of a project. Even if we decide to do significant additional work before seriously organizing a paper, the effort of writing an outline will have helped to guide the research. 2.3. The Outline What an outline should contain: 1. Title 2. Authors 3. Abstract Do not write an abstract. That can be done when the paper is complete. 4. Introduction The first paragraph or two should be written out completely. Pay particular attention to the opening sentence. Ideally, it should state concisely the objective of the work, and indicate why this objective is important. In general, the Introduction should have these elements: d The objectives of the work. 1376  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim d d d d The justification for these objectives: Why is the work important? Background: Who else has done what? How? What have ...
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FUNDAMENTAL EXPERIMENTS OF SOLID ELECTRODE
ELECTROCHEMISTRY
Authors

Abstract

This study dealing with the application of Faradaic techniques (chronoamperometry, cyclic
voltammetry, and differential pulse voltammetry) for the investigation of the redox reaction.
According to the obtained results, the Faradaic techniques can be successfully used for the
characterization of redox reactions. The main difference between the mentioned techniques is
in the application of different waveforms.

1. Introduction
As many chemical and biochemical processes occur by oxidation-reduction (redox) reactions,
monitoring of this process is crucial in the understanding of many industrial and processes which
take place in living organisms. Redox reaction can be monitored as a function of current change
(Faradaic) or as a function of voltage change (potentiometric). The application of potentiometric
instrumental analysis both qualitative and quantitative analysis can be carried out. Also, some
potentiometric techniques can be successfully applied for non-redox reactions which happened
before and after oxidation-reduction reactions.
During the redox reaction, charge transfer has been realized either in solution (homogenous) or
at the surface of the electrode (heterogeneous). In heterogeneous processes, the reactants should
be transferred (mass transfer) to the electrode surface in order to enable the redox process.
Regarding the Faradaic techniques chronoamperometry, cyclic voltammetry and differential
pulse voltammetry are the techni...

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