Cyclic Voltammetry of Ferricyanide PDF

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Introduction: The purpose of this lab was to reduce ferricyanide to ferrocyanide through cyclic voltammetry (CV), and learn more about the theory and instrumentation associated with CV in the process. The reduction of ferrycyanide to ferrocyanide is given by the following equation: Fe(CN)63 + e



Fe(CN)64

Cyclic voltammetry is an electrochemical measurement of the current that develops in an electrochemical cell. During CV, the potential is scanned linearly in both directions, from an initial value to a second value and the back. Figure 1 represents a potential scan from 0.8 to 0.15 V.

Figure 1: Triangular waveform used in cyclic voltammetry

Figure 2b shows the current response signal. This is obtained when the potential excitation signal is applied to a Pt electrode. In this case the Pt electrode was immersed in 6.0mM K3Fe(CN)6, the electroactive species, and in 1.0M KNO3, the supporting electrode. Figure 2a shows the potential excitation signal. This cyclic voltammogram shows that the cathodic current increases as the ferricyanide is reduced during the negative scan of the potential.

Figure 2: Cyclic voltammogtam of fericyanide Equation 1 shows the half-wave potential for a reversible couple. It is centered between the anodic peak potential (Epa) and the cathodic peak potential (Epc):

E1 = 2

E pa + Epc 2

Equation 1

The distance between the peak potentials determines the number of electrons transferred in the electrode reaction (n) for a reversible couple as shown in equation 2:

ΔE p = E pa − E pc =

59 mV n

Equation 2

Irreversibility causes the peak separation to increase. The Randles-Sevcik equation (equation 3) describes the peak current for a reversible system. ip = 2.69  105 n3/2AD1/2C1/2

Equation 3

ip = peak current (A) A= electrode (cm2) D = diffusion coefficient (cm2/s) N = electron stoichiometry (eq/mol) C = concentration (mol/cm3) V = scan rate (V/s) This equation shows that concentration and ip are directly proportional and they increase as v1 increases. The peak current of the anode and the peak current of the cathode should be close and are represented by equation 4. Chemical reactions can influence the ratio of the peak currents: ipa/ipc = 1

Equation 4

In this experiment, we measured the concentration and scan rake on the peak current of voltammograms of ferricyanide and determined the concentration of an unknown solution of ferricyanide.

Procedure: There were no changes to the procedure and it followed the lab handout as described below:

A. Experimental Setup 1. Pretreatment of the glassy carbon working electrode a. Polish surface with a few drops of alumina on a felt pad b. Thoroughly rinse with deionized water c. Wipe dry with a soft tissue 2. Prepare 1.00 L of 1.0 M KNO3 3. Prepare 250 mL of 10mM potassium ferricyanide in 1 M KNO3 4. Prepare standard solutions of 2,4,6, and 8 mM by serial dilution in 1,0M KNO3 5. Assemble the cell as shown in figure 3 6. Fill cell with 1 M KNO3 so that the ends of the electrodes are immersed 7. Purge with N2 for approximately 5 minutes to deoxygenate the cell 8. Direct N2 over the solution to prevent oxygen from reentering the cell during the experiment 9. Set scan parameters a. Turn on potentiostat b. Click on the EpsilonEC icon on the desktop c. Select “experiment” – “new experiment” d. Under potentiostat select cyclic voltammetry (CV) e. Set parameters as: Initial Potential: 800 mV Switching Potential: 200 mV Final Potential: 800 mV # of Segment: 2 (each half scan, cathodic and anodic, is a segment)

Scan Rate 10 mV/s Quiet Time: 2 s Full Scale: 100 A (this is the sensitivity and might need to be adjusted. You must use the same one throughout the experiment.)

B. Effect of Scan Rate 1. Observed by using the deoxygenated 4mM K3Fe(CN)6 in 1M KNO3 at rates 10, 20, 50, 100, and 200 mV/s 2. Between each scan stir the solution for 30s to restore initial conditions at the electrode surface 3. Make sure there are no bubbles on the electrodes 4. Wait 60s after stirring before obtaining a voltammogram

Figure 3: Experimental setup for cyclic voltammetry C. Effect of Concentration and Determination of an Unknown 1. Obtain voltammograms of 2, 4, 6, 8, and 10 mM K3Fe(CN)6 (prepared in 1 M KNO3 and deoxygenated) by using a scan rate of 20 mV/s

2. Obtain a voltammogram of the unknown K3Fe(CN)6 solution

D. Convert to Text Files 1. Save all data to a folder on the desktop 2. Do to “file” and then “convert to text” to convert all your files to .dat files a. You will have to change the file extension to find files other than CV

Instrumentation:

Working electrode Reference electrode Auxiliary electrode Potentiostat Stirring Unit

Model Number MF-2012 MF-2052 MW-4130 CV-50W Version 2,0 58945-016

Company BAS BAS BAS BAS Van Waters & Krogers Mangestir

Reagents: Chemicals

Supplier

Lot Number

Nitrogen

Airgas Nitrogen Company Spectrum Spectrum

Potassium Ferricyanide Potassium Nitrate

Expiration Date

Un1066

Molecular Weight (g/mol) NA

OV0185

329.25

NA

JD202

101.10

NA

NA

Calculation 1: Determine grams of KNO3 needed to make 1.0M stock solution 1.0 mol/L * 1.0 L * 101.10 g/mol = 101.10 g KNO3 needed Amounts of reagents used:

Chemical K3Fe(CN)6 KNO3 Unknown K3Fe(CN)6

Amount Used (g) 0.8316 101.1034 Prepared by TA

Calculation 2: Determine the volume of K3Fe(CN)6 stock solution needed to make diluted solutions of 2, 4, 6, 8, and 10mM K3Fe(CN)6 (10mM * 50mL)/10mM = 50mL K3Fe(CN)6 needed to make a 10mM solution (8mM * 50mL)/10mM = 40mL K3Fe(CN)6 needed to make a 8mM solution (6mM * 50mL)/10mM = 30mL K3Fe(CN)6 needed to make a 6mM solution (4mM * 50mL)/10mM = 20mL K3Fe(CN)6 needed to make a 4mM solution (2mM * 50mL)/10mM = 10mL K3Fe(CN)6 needed to make a 2mM solution

Preparation of Standards: Concentration (mM) 2 4 6 8 10

Volume of K3Fe(CN)6 (mL) 10 20 30 40 50

Results and Discussion:

Volume of KNO3 (mL) 40 30 20 10 0

8.00E-05

6.00E-05

Current (A)

4.00E-05

2.00E-05

10mM 8mM 6mM 4mM 2mM

0.00E+00

-2.00E-05

-4.00E-05 1.00E+00 8.00E-01 6.00E-01 4.00E-01 2.00E-01 0.00E+00 -2.00E-01 -4.00E-01 -6.00E-05

Potential (mV)

Figure 3. Cyclic voltammograms of 2, 4, 6, 8, and 10 mM Ferricyanide at 20mV/s scan rate, working electrode was glassy carbon, reference electrode Ag|AgCl, and a platinum wire auxiliary electrode

Table 1. Cathodic and anodic peak currents for 2, 4, 6, 8, and 10mM Ferricyanide Concentration (mM)

Cathodic Peak Current (A) 2 4 6 8 10

6.72E-05 5.38E-05 4.34E-05 3.06E-05 1.84E-05

Anodic Peak Current (A) 4.38E-05 3.67E-05 3.05E-05 2.28E-05 1.37E-05

0 0 f(x) = 0 x + 0 R² = 1

Peak Current (A)

0 0

cathodic Linear (cathodic) anodic Linear (anodic)

f(x) = 0 x + 0 R² = 0.99

0 0 0 0 0

1

2

3

4

5

6

7

8

9

10

11

Scan Rate(mV/s)

Figure 4. Calibration curves; cathodic and anodic peak current versus concentration of Ferricyanide 4.00E-05 3.00E-05

Current (A)

2.00E-05 1.00E-05 0.00E+00 -1.00E-05 -2.00E-05 -4.00E-01 -2.00E-01 0.00E+00 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+00 -3.00E-05

Potential (V)

Figure 5. Cyclic voltammogram of unknown solution of ferricyanide; 20mV/s scan rate used, working electrode was glassy carbon, reference electrode Ag|AgCl, and a platinum wire auxiliary electrode Table 2. Data for determine the concentration of the unknown ferricyanide solution Peak Current (A) Cathodic

Calibration Line 3.13E-05 y=6.00E-06x+6.00E-06

Anodic

2.06E-05 y=4.00E-06x+7.00E-06

Calculation 3: Determination of the unknown concentration using the cathodic scan data: x = (3.13E-05 + 2.06E-05)/6.00E-06 = 55.6mM The concentration of ferricyanide in the unknown calculated from the anodic peak current was 47.7mM. The difference between these two concentrations is 15%, and we use the cathodic peak current because it is more accurate.

Scan Rate Study

1.00E-04 8.00E-05 6.00E-05

Current (A)

4.00E-05 2.00E-05

200 100 50 20 10

0.00E+00 -2.00E-05 -4.00E-05 -6.00E-05 1.00E+00 8.00E-01 6.00E-01 4.00E-01 2.00E-01 0.00E+00 -2.00E-01 -4.00E-01 -8.00E-05

Potential (mV)

Figure 6. Cyclic voltammogram of 4mM Ferricyanide with scan rates of 10, 20, 50, 100, and 200 mV/S; working electrode was glassy carbon, reference electrode Ag|AgCl, and a platinum wire auxiliary electrode

Table 3. Cathodic and anodic peak currents of varying scan rates and the square root of each scan

root. The last column shows ipa/ipc for each scan rate Scan Rate (mV/s) 10 20 50 100 200

SQRT

Cathodic Peak Current (A) 2.24E-05 3.06E-05 4.60E-05 6.24E-05 8.47E-05

3.16 4.47 7.07 10.00 14.14

Anodic Peak Current (A) 6.66E-05 4.90E-05 3.50E-05 2.28E-05 1.59E-05

ipa/ipc 0.786 0.785 0.761 0.745 0.710

0 f(x) = 0 x + 0 R² = 1

0

Peak Current (A)

0 f(x) = 0 x + 0 R² = 1

0 0

cathodic Linear (cathodic) anodic Linear (anodic)

0 0 0 0 0

2

4

6

8

10

12

14

16

Square Root of Scan RAte (mV/S)^1/2

Figure 7. Plot of anodic and cathodic peak currents vs. square root of the scan rate Table 3, which shows the cathodic and anodic peak currents at different scan rates and the ratio of the peak currents is calculated using equation 4.

Calculation 4: This calculation determines the half-cell potential for the ferri/ferrocyanide couple in 1.0 M KNO3 from the 10mV/ s scan rate graph E1/2 = (224mV + 159mV)/2 = 192mV The lab handout shows the half-cell value of ferricyanide in 0.1 M KNO3 to be 424 mV versus the NHE reference electrode. Versus the Ag|AgCl reference electrode this value is 225mV,

which is the reference electrode we use in this experiment. The percent error of these two values is 15%, and can be calculated using the equation (experimental – actual)/actual * 100.

Calculation 5: Using equation 2, determine n for the ferri/ferrocyanide couple in 1.0 KNO3: n = 59mV/224mV– 1594mV) = 0.908 The error in our experiment was not major.

Conclusion and Opinion: Due to the amount of people in the lab group it was a fairly simple lab, but I do not feel like I really learned much about this lab. Our TA was very good about making sure that everyone was involved and we all did at least one thing. I probably would have learned more about the lab if our lab group were smaller and we could each be more hands on.

References: 1. Experiment 1: Cyclic Voltammetry of Ferricyanide, Laboratory Handout, Chemistry 2410L, Fall Semester 2018 2. Cyclic Voltammetry of Ferricyanide Example, Example Lab Report, Chemistry 2410L, Fall Semester 2018 3. Barker, G. (1992). Comprehensive analytical chemistry, vol. XXVII: Analytical voltammetry. Journal of Electroanalytical Chemistry, 338(1-2), 373-376. doi:10.1016/0022-0728(92)80437-9...