Reduction Charge Smaller than the Deposited One in Cathodic Stripping Voltammograms of AgCl

Abstract

A silver electrode was anodized in lowly concentrated potassium chloride solution almost under the steady state to generate a deposit of silver chloride on the electrode, and the deposit was cathodically stripped by linear potential scan to evaluate the reduction charge. Then the oxidation charge was larger than the reduction one. Since the equality was valid for long term chronocoulometry at the double potential step, the inequality is not due to any irreversibility of electrode reactions, but can be attributed to the process of the cathodic potential scan. A reason for the inequality is the negative charge of the capacitance involved in the electrode reaction, which has been observed in simple, dissolved redox species like a ferrocenyl derivative. The negative capacitive currents are conspicuous for high concentrations of redox species on the electrode because they result from the orientation of the dipoles of the redox species coupled with counterion, of which direction is opposite to that of the solvent dipoles. If a silver chloride film was thin enough to be regarded as a monolayer, we found that half of the cathodic stripping charge should be lost by the negative capacitance.

A Loss of Charge at Reduction of Hydrogen Ion by Fast Scan Voltammetry

“…The proportional constant of the plot of j p vs v gives Γ = 1.2 × 10 -9 mol cm −2 or (0.3 8 nm) 2 per adsorbate, which is close to the density for a monolayer adsorption. Since this value includes the charge by the negative capacitance as has been observed for adsorbed species at stripping voltammetry, 24,25 the net redox charge should be less than this value. Because the species to be reduced is adsorbed before the voltage scan to the negative, the adsorbate should be hydrogen ion, H ad + , rather than H ad .…”

“…However, fast scan voltammetry exhibits drawbacks of deformation of voltammograms by solution resistance at enhanced currents as well as by capacitive currents of double layers. There is another source of capacitive current caused by a redox reaction, [22][23][24][25] which depresses the faradaic current. We take into account drawbacks each by each.…”

Participation in Negative Capacitance of Diffusion-Controlled Voltammograms of Hemin

“…21,26 The other possibility of both the downward deviation in Figure 2 and the potential shifts in Figure 4 is the participation in the negative capacitance associated with the redox reaction. [22][23][24]27 It is based on the following concept. 28 When a potential cathodic enough to reduce hemin is applied to the electrode, the electric field near the electrode is directed from the solution to the electrode, as illustrated in Figure 5.…”

“…However, no adsorption wave has been overlapped with the diffusion current. , Furthermore, the peak currents were lower than the diffusion-controlled ones associated with peak potential shift, ,− like in complication by heterogeneous kinetics. This behavior can also be predicted from the capacitive current brought about by the redox reactions. − It is important to clarify the true rate-determining step in order to clarify electrocatalytic criteria from enhancement of the reduction currents and potential shifts by scan rates. A key is to characterize an effect of adsorption on catalysis quantitatively.…”

Enhancement of the Negative Capacitance Associated with the Dissolution of Silver by Salt Concentrations by Means of Anodic Stripping Voltammetry

“…The imbalance is caused by the difference in techniques of time-dependent potential control. The imbalance has been found in the stripping voltammetry of silver ion [35] and silver chloride [36] to demonstrate the participation in the negative capacitance.…”