Lab-4-CV - Cyclic Voltammetry Analysis of a K4Fe(CN)6 Unknown via External Standard Calibration PDF

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Cyclic Voltammetry
Analysis of a K4Fe(CN)6 Unknown via External Standard Calibration...

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Brooklyn College Department of Chemistry Chem 3420 / 7420

Instrumental Analysis

Cyclic Voltammetry

Analysis of a K4Fe(CN)6 Unknown via External Standard Calibration Experiment: In this experiment you will use Cyclic Voltammetry (CV) to measure a calibration curve for K4Fe(CN)6 based on external standards and use it to determine the concentration of K4Fe(CN)6 in an unknown. CV is one of the most versatile electrochemical techniques and is routinely used to determine the midpoint reduction potentials of organic and inorganic compounds over a wide potential range. CV involves cycling the potential of a working electrode immersed in an unstirred solution of analyte and measuring the resulting current passed. The controlling potential applied across the two electrodes is the excitation signal, and in CV it is a linear potential scan with a triangular waveform. The signal response in CV is modeled with the Randles – Sevcik equation, as follows: ip = 2.686x105 n3/2AcD1/2v1/2 The peak current of the CV, ip, is determined by a combination of the number of electrons appearing in the half-reaction for the redox couple (n), electrode surface area (A), the analyte concentration (c), the analyte diffusion constant (D), the scan rate (v), and the temperature in Kelvin. The constant includes the universal gas constant (R), and the absolute temperature (T) and is understood to have units of C mol-1 V-1/2. A computer controlled CH Instruments Electrochemical Analyzer 720C is utilized in a three-electrode setup with a glassy carbon working electrode, a Ag/AgCl reference electrode, and a platinum counter electrode. Supporting electrolyte solution: Dissolve approximately 5g of KNO3 (101 g/mol) in 500 mL of deionized water to give a 0.1 M solution. K4Fe(CN)6 standards: Prepare 100 mL of an 8 mM K4Fe(CN)6•(3H2O) (422.4 g/mol) solution in the 0.1M KNO3 supporting electrolyte that you have prepared. Do this by dissolving 422.4 mg of K4Fe(CN)6 in 100 mL of the supporting electrolyte solution. Perform serial dilutions of your 8 mM K4Fe(CN)6 solution with 0.1M KNO3 to give 25

mL solutions of 2, 4, and 6 mM K4Fe(CN)6. Once prepared, combine equal portions of your 4.0 mM and 6.0 mM solutions to generate a 5.0 mM K4Fe(CN)6 solution. Unknown: The unknown is provided in a 25 mL volumetric flask. Record the number of your unknown for inclusion in your report. Dilute to the mark using the 0.1M KNO3 solution. Instrument Setup Turn on the computer, printer, and analyzer. Click on the ‘Chi720C’ icon on the computer desktop to start the software that controls the analyzer. Choose: Setup®Technique®CyclicVoltammetry®OK Choose: Setup®Parameters and set accordingly: Init E High E Low E Initial scan polarity Scan Rate Sweep Segments Sample interval Quiet time Sensitivity Check boxes OK

-0.20 V starting potential +0.80 V upper limit of potential window -0.20 V lower limit of potential window Positive 0.020 V/s 2 0.001 V frequency of data collection 0s time delay between deposition/stripping 1e-005 A/V Unmark all except ‘Scan Complete Cycles’

Cell Setup Place approximately 8 mL of the 2.0 mM K4Fe(CN)6 solution you prepared and the stirring bar into the glass cell. Be careful not to lose the stirring bar, it is quite small. Wet a small section of the polishing pad with deionized water. Polish the glassy carbon electrode on the pad using small circular movements. Place the Ag/AgCl reference electrode, the polished glassy carbon working electrode, the platinum wire counter electrode, and the gas tube through the cell top and into the solution. Connect the electrodes, as follows: Green White Red

glassy carbon working electrode Ag/AgCl reference electrode platinum wire counter electrode

Initial Measurements Measure the CV of the 2.0 mM K4Fe(CN)6 solution. The potentostat will increase the working electrode potential up to the high limit of +0.80 V in a linear manner. During this sweep, anodic current corresponding to the oxidation of K4Fe(CN)6 will be observed. The potentiostat will reverse direction and sweep the potential negative to -0.2 V. During this sweep, a cathodic current corresponding to the reduction of K3Fe(CN)6 generated during the first sweep will be observed. After the run, replace the solution and measure the CV twice more to ensure you have three replicates. Note: this requires 24 mL of the 25 mL that you have prepared, so be careful. The sample should be stirred for 30s and allowed to rest for 60s between replicates. Remove the working electrode and polish it between runs. Saving Data After a CV is finished, save the recorded curve in a file. File®Save will save it as a binary (.bin) file which the Chi620a program can read. After it is saved, you can convert it to a comma separated value text file that MS Excel can read by File®Convert to Text. Scan Rate Response Record the CV of the 2.0 mM sample with scan rates of 0.050, and 0.100 V/s. Only a single replicate need be recorded at each scan rate. The sample should be stirred for 30s and allowed to rest for 60s between measurement, but there is no need to replace the sample solution or polish the electrode. Using MS Excel, overlay the cathodic wave from a representative voltammagram at each scan rate on a single plot to discuss in your report. Plot the average K4Fe(CN)6 anodic and cathodic peak currents (ipa and ipc) at each scan rate vs. the square root of the scan rate (V/s) to discuss in your report in relationship to the Randles – Sevcik equation. Plot the absolute value of the difference in the potential between the anodic and cathodic peaks (DE = |Epa - Epc|) and discuss its significance using the Nernst equation. Analysis of Electrode Area from the Scan Rate Response Using the Randles – Sevcik equation that describes the peak height in CV, use the concentration of the standards and the diffusion constant of K4Fe(CN)6 (6.5 x 10-6 cm2/s) to calculate the area of the electrode surface at each scan rate. Discuss any deviation from ideality in your report.

Construction of a Calibration Curve and Determination of the Unknown Concentration Construct a calibration curve by measuring each of the external standards once using the original scan rate of 0.020 V/s. In between samples, rinse the cell with the solution to be measured next. To determine the unknown sample concentration, measure the CV of your unknown solution at this scan rate. As before, the sample should be stirred for 30s and allowed to rest for 60s between replicates. Measure two replicates for the unknown. Remove the unknown K4Fe(CN)6 solution and rinse the cell thoroughly with the supporting electrolyte solution. Measure the CV of the supporting electrolyte once to provide a blank. Determine the oxidation peak current (ipa) from each CV. This can be done automatically, however sometimes it requires manual correction if the automatically detected baseline or peak maximum / minima is incorrect. Plot a calibration curve of oxidation current (ip) vs. K4Fe(CN)6 concentration in MS Excel using your external standards (2.0 – 8.0 mM K4Fe(CN)6) and the blank you measured. Perform a non-linear least squares fitting of your data to a line and determine the best-fit calibration curve for your data. Use this curve to determine the concentration of your K4Fe(CN)6 unknown. Report molar concentrations of K4Fe(CN)6 in the unknown solution and its standard deviation in concentration (see Eqn 1-2 in the text) in the Abstract and Results sections along with the number/symbol of the unknown. Analysis of Electrode Area from the Analyte Concentration Using the Randles – Sevcik equation that describes the peak height in CV, use the concentration of the standards and the diffusion constant of K4Fe(CN)6 (6.5 x 10-6 cm2/s) to calculate the area of the electrode surface. Tabulate these values and discuss any deviations from ideality in your report. Report Follow the outline of a regular laboratory report as provided on the course web site. Make sure to provide the number of the unknown, the concentration of the unknown, and its error, in the Abstract. Provide all data in the Results and Discussion along with a brief explanation of how the data were collected and the error analysis. Use the Nernst and Randles – Sevcik equations as the basis from which to discuss your results. Discuss the advantages/disadvantages of the External-Standard Calibration Method used here, and Standard-Addition Method used in the ASV lab....