Cyclic Voltammetry: Electrochemistry of a Complex Ion

Ryan Geelan
3/23/2015

What is the diffusion coefficient of the hexammineruthenium(III) cation?

The diffusion coefficient obtained from the Randles-Secvik plot between the cathodic peak current and the square root of the sweep rate was determined to be closer to the literature value than the diffusion coefficient that came from the calibration curve (Table 1). The reliability of the Randles-Secvik plot demonstrates that it should be employed instead of a calibration curve when the diffusion coefficient is unknown.

Table 1: Tabulation of Diffusion Coefficients Calculated from the Slopes of Different Calibration Curves
Method of Obtaining Coefficient Value
Calibration Curve
Randles-Secvik Plot
Literature Value I
Diffusion Coefficient (cm2/s)
1.28 x 10-4
4.32 x 10-5
9.0 x 10-6

What is the formal potential, Eo, for the ruthenium(III/II) redox couple?

The tabulated results in Table 2 demonstrate that the carbon electrode produced a formal potential closer to the literature value than the platinum electrode. Despite this departure from the literature formal potential, platinum electrodes are more commonly used because of their inert nature.

Table 2: Formal Potential Values for the Various Electrodes Compared to the Literature Value for the Ruthenium (III/II) Redox Couple
Electrode Used
Carbon Electrode
Platinum Electrode
Literature Value II
Formal Potential in V/(Ag/AgCl)
-0.1994
-0.1696
-0.1950

Which electrode results in the most reversible electrochemistry?

The cyclic voltammogram is created through initially applying a reducing potential that gives electron to the reducible analyte, increasing the cathodic current. When the reduction potential is reached the cathodic current will decrease as the concentration of the reducible analyte is decreased. Eventually equilibrium will be reached between new analyte diffusing to the surface of the electrode and the reduction of analyte, at which point the voltammogram levels off. During the reverse scan s reversible redox reaction will show the re-oxidation of the reduced analyte, producing an anodic current. A perfectly reversible reaction will produce an oxidation peak that is symmetrical to the reduction peak. Deviations from this shape, as indicated by a peak potential separation other than 59mV or a peak current ratio (ipa/ipc) other than 1, reveal a reduction in reversibility. The carbon electrode in this experiment presented a -1.56 peak current ratio, while the platinum electrode had a -1.41 ratio (Figures 1,2 ). This variability may have come from a reaction between the reduced analyte and the matrix. The loss of reduced analyte will