Resistance associated with measurements of capacitance in electric double layers

The behavior of electric double layers at polarized interfaces in KC1 solutions is revisited in order to examine properties of the constant phase element (CPE). We pay attention specifically to frequency dependence of both the capacitance and the resistance. Two parallel platinum wires immersed in solution are used as insulator-free electrodes. They avoid stray capacitance or irreproducibility of impedance caused by incompleteness of electric shield of electrodes. The Nyquist plot takes approximately a straight line because the in-phase component, Z1, is inversely proportional to ac-frequency, similar to the capacitance. Since Zi extrapolated to zero separation of the electrodes is non-zero, a resistance is present at the double layer in parallel form. It is not a Faradaic resistance because of absence of any electroactive species. The parallel resistance is inversely proportional to the frequency, whereas the capacitance decreases with a linear relation to logarithm of the frequency. The latter is responsible for the frequency-dependence of the former. The parallel resistance is the apparent one involved inevitably in ac-measurements of the capacitance. Values of the capacitance are independent of concentration of KC1 in the domain from 0.1 mM to 3 M

Electrochemically instantaneous reduction of conducting polyaniline-coated latex particles dispersed in acidic solution

A cathodic voltammetric wave was observed in an aqueous suspension of mono-dispersed, spherical polyaniline-coated polystyrene particles, whereas no anodic wave was detected. This irreversibility was common to particles with eight different diameters ranging from 0.2 to 7.5 μm. Such irreversibility cannot be found at polyaniline-coated electrodes, and thus is a property of the dispersion of polyaniline latex. The reduction current was controlled by diffusion of dispersed particles. The reduction, being the conversion from the electrical conducting state to the resistive one, should begin at a point of contact between the conducting particle and the electrode in order to be propagated to the whole particle rapidly. In contrast, the oxidation proceeds slowly with the propagation of conducting zone, during which Brownian motion lets the particle detach from the electrode. The number of loaded aniline units per particle, determined by weight analysis, ranged from 6×10_6 (φ 0.2 μm) to 3×10_11 (φ 7.5 μm) and was proportional to 2.9 powers of the particle diameter. The diffusion-controlled current of the cathodic wave was proportional to 2.4 powers of the diameter. The difference in these powers, 0.5, agreed with a theoretical estimation of the diffusion-controlled current, the diffusion coefficient for which was given by the Stokes-Einstein equation

Voltammetry in low concentration of electrolyte supported by ionic latex suspensions

Since ionic conductivity has a linear relation with the square of the number of charge, ionic latex particles with a huge number of the charge could provide high conductance. It is expected that addition of only a small amount of latex particles into voltammetric solution enhances the conductance so much that voltammograms can be measured, overcoming ohmic drop. Conductivity of latex suspensions of polystyrenepolystyrenesulfonic acid with volume fractions less than 0.02, which were well deionized by centrifugation, was determined by ac-impedance at two parallel wire electrodes. Since the resistance was determined by the dependence of the in-phase component on the electrode distance, it did not include participation of electric double layers or adsorption of latex. The relationship between conductivity and a diffusion coefficient stated that the conductivity of the suspension was provided mainly by diffusion of latex particles with multiple charges rather than that of the counterion. The suspension with [H^+] = 10^-5 M, corresponding to 8.9 x 10^5 number mm^-3, including hydrogen gas showed a voltammetric oxidation peak of hydrogen, whereas hydrochloric acid with [HCl] = 10^-5M showed a resistive current-potential curve

Determination of concentration of saturated ferrocene in aqueous solution

The solubility of ferrocene in aqueous solution is known to be approximately 0.04 mmol/dm3. The solubility values determined by voltammetry have been overestimated because of adsorption on electrodes. This work deals with discerning diffusion from adsorption by altering not only the voltammetric time scale but also the solvents used. Fast voltammetric responses by differential pulse voltammetry and fast scan voltammetry exhibited adsorption behavior. In contrast, quasi steady-state voltammetry showed the diffusion-control, the current of which seemed to evaluate the saturated concentration accurately. However, the currents in the solution including a small amount of organic solvent were smaller than those in the aqueous solution although the concentrations were identical. Solutions including organic solvents have often been used to obtain calibration curves. Therefore, the concentration evaluated from the calibration curve was estimated to be larger than the true concentration. The current in the organic solvent was explained in terms of the extra solvation energy by supersaturation, which was dissipated to low concentrated domains by diffusion. It was formulated in the form of diffusion coefficients. The true concentration was evaluated to be 0.01 mmol/dm3 by slow scan voltammetry in the solution without calibration curves