Describe important features of cyclic voltammogram of k3fecn6
Answers
Cyclic Voltammetry
Early contributions to cyclic voltammetry were made by investigators including Randles,3Nicholson and Shain,4,5 and Kalthoff and Tomsicek.6 Cyclic voltammetry is one of the most versatile electroanalytical techniques for the study of electroactive species. Cyclic voltammetry has the capability for rapidly observing redox behavior over a wide potential range. Cyclic voltammetry involves the cycling of the potential of an electrode, which is immersed in an unstirred solution, and measuring the resulting current. The controlling potential applied across these two electrodes is an excitation signal. For cyclic voltammetry, the excitation signal is a linear potential scan with a triangular waveform8 (Figure 14). A cyclic voltammogram is obtained by measuring the current at the working electrode during the potential scan. The voltammogram is a display of current versus potential.7
Figure 15 is a cyclic voltammogram of 6 mM K3Fe(CN)6 in 1 M KNO3. The scan was initiated at 0.80 V (applied at a) vs. SCE in negative direction at 50 mV/s. The area of the platinum electrode is 2.54 mm2. The cathodic current at b to due to the electrode process:
[FeII(CN)6]3- + e- Þ [FeIII(CN)6]4-
The cathodic current rapidly increases (b-d) until the concentration of [Fe(CN)6]3- at the electrode surface approaches zero, and the current peaks at d. The current then decreases (d-g) as the solution surrounding the electrode is depleted of [Fe(CN)6]3-. The scan direction is switched to positive at -0.15V (f) for the reverse scan. Anodic current is generated (i-k) when the electrode becomes a sufficiently strong oxidant, and [Fe(CN)6]4- can be oxidized by the electrode process:
[FeII(CN)6]4- Þ [FeIII(CN)6]3- + e-
The anodic current increases until the surface concentration of [Fe(CN)6]4- approaches zero and the current peaks (j). The current decays (j-k) as the solution surrounding the electrode is depleted of [Fe(CN)6]4-. The magnitudes of parameters including the anodic peak current, (ipa), cathodic peak current (ipc), anodic peak potential (Epa), and cathodic peak potential (Epic) are elucidated from the cyclic voltammogram.
The formal reduction potential E° ¢ for an electrochemically reversible couple is:
E° ¢ = Epa +Epc
2
For a reversible redox couple, the number of electrons transferred in the electrode reaction can be determined by the separation between the peak potentials:
D Ep = Epa - Epic @ 0.059
n
The Randles-Sevcik equation for the forward sweep of the first cycle is:
ip = 2.69 X 105n3/2AD1/2Cv1/2
where ip = peak current, A
n = electron stoichiometry
A = electrode area, cm2
D = diffusion coefficient, cm2/s
C = concentration, mol/cm3
v = scan rate, V/s
Furthermore, ip increases with v1/2 and is directly proportional to concentration. This relationship becomes particularly important in the study of electrode mechanisms. The ratio of ipa to ipc should be close to one; however, chemical reactions coupled to the electrode process can significantly alter the ratio of peak currents:
ipa @ 1
ipc