CH 4510: CRE Lab

Cyclic Voltammetry

\ Aim:    To become familiar with using an electrochemical potentiostat  To determine the concentration of potassium ferricyanide, K3Fe(CN)6 in an unknown solution using cyclic voltammetry and analyzing the current vs potential graphs 

Apparatus Required:      Potentiostat and electrodes  Nitrogen gas for mixing  Test tubes  Standard flask 

Reagents required:   KNO3 solution  10mM K3Fe(CN)6 solution 

Theory: Cyclic Voltammetry: Cyclic Voltammetry (CV) is an electrochemical technique which measures the current that develops in an electrochemical cell under conditions where voltage is in excess of that predicted by the Nernst equation. Cyclic voltammetry (CV) is perhaps the most versatile electro-analytical technique for the study of electroactive species. Its versatility dined with ease of measurement has resulted in extensive use of CV in the fields of electrochemistry, inorganic chemistry, organic chemistry and biochemistry. Cyclic voltammetry is often the first experiment performed in an electrochemical study of a compound, biological material, or an electrode surface. The effectiveness of CV results from its capability for rapidly observing the redox behavior over a wide potential range. The resulting voltammogram is analogous to a conventional spectrum in that it conveys information as a function of an energy scan. Instrumentation: The main components of a Cyclic Voltammeter are the Reference electrode, Auxillary electrode, Working electrode and Potentiostat. The Potentiostat is an instrument that controls the potential of the working electrode with respect to the reference electrode while also measuring the current flow between the working electrode and counter electrode. In CV, the potential of a working electrode is cycled linearly between two potential values at which the oxidation and reduction of a solute occurs. The resulting current-potential curve is called a cyclic voltammogram.

CV involves cycling the potential of a working electrode in a solution between two pre-set is then recorded as a function of applied potential, with positive current indicating anodic (oxidative) processes and negative current cathodic (reductive) processes.

Working principle of a potentiostat: A potentiostat measures the potential difference between the working and the reference electrode, applies the current through the counter electrode and measures the current as an over a series resistor ( in Fig. 1). voltage drop

Fig. 1 : Schematic of a potentiostat.

The control amplifier CA is responsible for maintaining the voltage between the reference and the working electrode as closely as possible to the voltage of the input source operation is best understood using the equations below. Prior to observing the following equations, one may note that, from an electrical point of view, the electrochemical cell and the current measurement resistor (Fig. 2). includes solution resistance between the counter and the reference. may be regarded as two impedances represents the interfacial impedance in series with the interfacial impedance of the counter electrode and the . It adjusts its output to automatically control the cell current so that a condition of equilibrium is satisfied. The theory of

of the working electrode in series with the solution resistance between the working and the reference electrodes.

Fig. 2 : Schematic of a potentiostat, with electrochemical cell replaced by two impedances.

The role of the control amplifier is to amplify the potential difference between the positive (or noninverting) input and the negative (or inverting) input. This may be translated mathematically into the following equation: . (1) where is the amplification factor of the CA. At this point the assumption may be made that a

negligible amount of current is flowing through the reference electrode. This correlates to physical phenomenon since the reference electrode is connected to a high impedance electrometer. Thus, the cell current may be described in two ways:

, (2) and

. (3) Combining Eqs. (2) and (3) yields Eq. (4):

(4) where is the fraction of the output voltage of the control amplifier returned to its

negative input; namely the feedback factor:

. Combining Eqs. (1) and (4) yields Eq. (6):

. (6) When the quantity becomes very large with respect to one, Eq. (6)

reduces to Eq. (7), which is one of the negative feedback equations: . (7) Eq. (7) proves that the control amplifier works to keep the voltage between the reference and the working close to the input source voltage.

Experiment details: In this experiment, FeIII(CN)63-/ FeII(CN)64- couple is used as the electrochemically reversible redox system. A typical current response signal obtained when the potential excitation signal in above figure is applied to a platinum electrode immersed in 2 mM K3Fe(CN)6 as the electroactive species in aqueous 1 M KNO3 as the supporting electrolyte is as below.

As the potential is scanned in a positive direction (A), the electrode becomes an increasingly stronger oxidizing agent. An anodic current (B) occurs when Fe2+ is oxidized to Fe3+ ions The anodic current increases rapidly until the concentration of Fe2+ ions approaches zero at the electrode surface. At this point, the current reaches a maximum value (C). The current then decays at a rate of t-1/2 as the solution adjacent to the electrode surface is depleted of Fe2+ ions, having been electrochemically oxidized to Fe3+ ions. The scan direction is switched in the opposite direction at (D) for the reverse scan. As the applied potential becomes negative, the electrode becomes powerful reducing agent. Fe3+ ions, which have been generated at the electrode surface by the preceding oxidation reaction, can now be reduced to Fe2+ ions (E). This generates cathodic current, which rapidly increases until the surface concentration of Fe3+ ions approaches zero. The current then peaks (F) and decays as the solution adjacent to the electrode is depleted of Fe3+ ions. The peak potentials supply information about the identity of the analyte and the kinetics of the oxidation/reduction process. The peak currents supply information about analyte concentration and the stability of the electrogenerated species. Experimental Setup:

Schematic Representation of the apparatus

Electrode details: Counter electrode: Platinum Reference electrode: Silver-Silver Chloride (Ag-AgCl) Working electrode: Platinum Other applications: Cyclic Voltammetry can also be used to determine the electron stoichiometry of a system, the diffusion coefficient of an analyte, and the formal reduction potential, which can be used as an identification tool. In addition, because concentration is proportional to current in a reversible, Nernstian system, concentration of an unknown solution can be determined by generating a calibration curve of current vs. concentration (we would be using this in the current experiment). Experimental Procedure:     Using the solutions of 10mM K3Fe(CN)6 and KNO3, diluted solutions of K3Fe(CN)6 (in KNO3) of concentrations of 2, 4, 6, 8 mM are prepared.  One of the solutions prepared above is used as the electrolyte in the electrochemical voltammetric cell (say the 2mM solution)  Immerse the 3 electrodes – Counter, Reference and Working in the solution carefully. 

        

Open the installed software and enter the value for scan rate as 0.15 V/s. Set the ranges of the potential as well. Here the range is chosen as 0-0.65 since we know from experience that the peak will occur in this range  Select “Start Run” in the ‘Control’ menu to start experiment. The cyclic voltammogram  (CV) will appear on the screen as it is generated.  Run the software and note the peak (cathodic and anodic) potentials and currents.  Now, purge the system with nitrogen for about a minute before changing the scan rate to 0.1V/s. 
Repeat the above steps for scan rates 0.01, 0.02, 0.05, 0.10 and 0.15 V/s for all the solutions prepared and tabulate the readings

Observations and Results:
The peak currents and potentials can be tabulated as below: scan rate (mV/s) Anode 150 100 50 20 10 Ep (V) 0.293 0.298 0.298 0.298 0.3 Ip (A) -1.527E-05 -1.275E-05 -9.352E-06 -6.318E-06 -4.825E-06 Concentration = 2 mM Cathode Ep (V) Ip (A) 0.215 2.083E-05 0.222 1.661E-05 0.225 1.182E-05 0.227 7.581E-06 0.232 5.468E-06

∆E vs Scan Rate Concentration = 2mM
0.08 0.078 0.076 ∆E 0.074 0.072 0.07 0.068 0.066 0 20 40 60 80 Scan Rate 100 120 140 160

Peak current vs square root of scan rate Concentration = 2 mM
0.000025 0.00002 0.000015 Peak current 0.00001 0.000005 0 -0.000005 0 -0.00001 -0.000015 -0.00002 (scan rate)0.5 5 10 15 Anodic Cathodic

scan rate Ep(V) 0.308 0.309 0.308 0.304 0.304

150 100 50 20 10

Concentration = 4 mM Anode Cathode Ip(A) Ep(V) Ip(A) -3.176E-05 0.225 3.728E-05 -2.694E-05 0.228 3.074E-05 -1.981E-05 0.231 2.223E-05 -1.324E-05 0.234 1.445E-05 -9.732E-06 0.233 1.061E-05

∆E vs Scan Rate Concentration = 4mM
0.084 0.082 0.08 0.078 0.076 0.074 0.072 0.07 0.068 0 20 40 60 80 Scan rate 100 120 140 160

∆E

Peak current vs square root of scan rate Concentration = 4 mM
0.00005 0.00004 0.00003 Peak current 0.00002 0.00001 0 -0.00001 0 -0.00002 -0.00003 -0.00004 (scan rate)0.5 5 10 15 Anodic Cathodic

scan rate Ep(V) 0.311 0.309 0.308 0.304 0.305

150 100 50 20 10

Concentration = 6mM Anode Cathode Ip(A) Ep(V) Ip(A) -4.988E-05 0.225 5.629E-05 -4.159E-05 0.228 4.617E-05 -3.063E-05 0.231 3.361E-05 -2.051E-05 0.233 2.207E-05 -1.525E-05 0.232 1.614E-05

∆E vs Scan Rate Concentration = 6mM
0.1 0.08 ∆E 0.06 0.04 0.02 0 0 20 40 60 80 Scan rate 100 120 140 160

Peak current vs square root of scan rate Concentration = 6 mM
0.00008 0.00006 Peak current 0.00004 0.00002 0 -0.00002 -0.00004 -0.00006 (scan rate)0.5 0 5 10 15 Anodic Cathodic

scan rate Ep(V) 0.309 0.309 0.308 0.304 0.304

150 100 50 20 10

Concentration = 8mM Anode Cathode Ip(A) Ep(V) Ip(A) -6.556E-05 0.224 7.23E-05 -5.489E-05 0.226 6.03E-05 -4.051E-05 0.227 4.41E-05 -2.712E-05 0.232 2.91E-05 -2.003E-05 0.231 2.14E-05

∆E vs Scan Rate Concentration = 8mM
0.086 0.084 0.082 0.08 0.078 0.076 0.074 0.072 0.07 0 20 40 60 y = -9E-07x2 + 0.0002x + 0.0698

∆E

80 Scan rate

100

120

140

160

Peak current vs square root of scan rate Concentration = 8 mM
0.00008 0.00006 0.00004 Peak current 0.00002 0 -0.00002 0 -0.00004 -0.00006 -0.00008 (scan rate)0.5 5 10 15 Anodic Cathodic

scan rate Ep(V) o.307 0.305 0.304 0.300 0.299

unknown Anode Ip(A) -4.579E-05 -3.846E-05 -2.844E-05 -1.907E-05 -1.411E-05 Ep(V) 0.22 0.22 0.224 0.227 0.226 Cathode Ip(A) 5.137E-05 4.258E-05 3.102E-05 2.045E-05 1.495E-05

150 100 50 20 10

Calibration plot for scan rate 20 mV/sec
9 8 7 6 Concentration (mM) 5 4 3 2 1 0 0 0.000005 0.00001 0.000015 0.00002 Cathodic current 0.000025 0.00003 0.000035

Concentration vs Peak Cathodic Current Scan Rate - 20mV/sec

y = 276536x - 0.0635

From the above graph, unknown concentration = 5.59 mM. Table for number of electrons transferred: ∆E 0.078 0.076 0.073 0.071 0.068 0.083 0.081 0.077 0.07 0.071 # of electrons 0.758974359 0.778947368 0.810958904 0.833802817 0.870588235 0.713253012 0.730864198 0.768831169 0.845714286 0.833802817 concentration scan rate 2 150 2 100 2 50 2 20 2 10 4 150 4 100 4 50 4 20 4 10

0.086 0.081 0.077 0.071 0.073 0.085 0.083 0.081 0.072 0.073

0.688372093 0.730864198 0.768831169 0.833802817 0.810958904 0.696470588 0.713253012 0.730864198 0.822222222 0.810958904

6 6 6 6 6 8 8 8 8 8

150 100 50 20 10 150 100 50 20 10

Conclusions:  It is observed that the plot is linear indicating that I is directly proportional to v0.5 in accordance with the theoretical formula given as:

  

The concentration of the unknown solution is found to be 5.59 mM Effect of scan rate on ∆E is verified to be quadratic (for a given concentration = 8mM)
Theoretically the number of electrons transferred is one for this reversible redox reaction but due to irreversibility in the system the number of electrons calculated are less than 1.

Precautions and sources of error: 1. Make sure that the 3 electrodes are immersed completely in the electrolyte solution 2. Clean the electrodes gently before transferring them from one solution to another 3. There is motion of electrons after every cycle. Hence purge the solution with nitrogen after every cycle (while changing the scan rates) 4. Take care while preparing the solutions of different concentrations – clean the measuring cylinders properly and avoid parallax error. Lab experience: Working with electrochemistry lab was very interesting and easy to learn. I recommend this to continue for next batches also. The experiments were slightly

repetitive, doing the same procedure for different concentrations. Doing the experiment with different solution can widened its scope as we will get to see the effect of different solutions also. Overall the experience with the lab was good and informative.

References
1. Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications. New York: John Wiley & Sons, 1980. 2. P.T. Kissinger and W.R. Heineman, J. Chem. Ed. 60 (1983) 702. 3. P.T. Kissinger, D.A. Roston, J.J. Van Benschoten, J.Y. Lewis and W.R. Heineman, J. Chem. Ed. 60 (1983) 772.