Voltammetry uncompensated resistance

D. Garreau and J.-M. Saveant, Linear Sweep Voltammetry. Compensation of Cell Resistance and Stability. Determination of Residual Uncompensated Resistance, J. Electroanal. Chem. 35 309 (1972). 

Cyclic-voltammetry measurements of quasireversible systems yield more easily to interpretation. Both the cathodic and anodic peak potentials shift as a function of scan rate, resulting in an increasing AE as v increases. This dependence of AEp on electron-transfer rate is used to measure the k value of the system, but AE also increases monotonically with v from the effects of uncompensated resistance, and the two effects are difficult to separate. The absence of appreciable resistance effects must be insured when making these measurements. Many reported rate constants are erroneous because of improper attention to this problem ... 

At UMEs, the picture is quite different, because the currents are extremely small consequently, the error in potential control in a voltammetric experiment is often much smaller than in the same experiment with an electrode of conventional size. Consider, for example, a disk UME with radius tq at which we desire to carry out sampled-current voltammetry. What are the conditions that will allow the recording of a voltammogram in which the half-wave potential is shifted less than 5 mV by the effect of uncompensated resistance ... 

OTTLE Cell Electrochemical and Spectroscopic Response The electrochemical response of the OTTLE cell should of course ideally be of the thin film variety with a peak separation close to zero and a symmetrical shape. The thin layer has restricted diffusion and exchange with the bulk solution, which leads to high uncompensated resistance and distorted responses at all but the lowest scan rates. Thus, cyclic voltammetry waves are often distorted with greater peak separations in the CVs than expected [134]. On the other hand, edge effects may mean that the edges are not at equihbrium so it is better to mask the area interrogated by the beam. 

The specific features of voltammetry at microelectrodes (absence of interferences arising from charging currents, uncompensated resistance and instrumental imperfections, high signal/noise ratio) were emphasized in chapter 2 (section 2.3) of this volume. Various construction modes and application of (ultra)-microelectrodes in chemical and biochemical practice will be treated in chapter 7 of this volume. In this subsection the exploitation of microelectrodes (under steady-state conditions) to investigation of electrode mechanisms and homogeneous reaction kinetics is discussed. 

Mashkina, E., Peachey, T., Lee, C.-Y. et al. (2013) Estimation of electrode kinetic and uncompensated resistance parameters and insights into their significance using Fourier transformed ac voltammetry and e-science software tools. Journal of Electroanalytical Chemistry, 690,... 

The uncompensated resistance was analyzed by potential step chronoam-perometry. The initial potential was set the same as the initial potential in cyclic voltammetry experiments, and then a step of — 50 mV to the initial potential was applied. The analysis of the RC decay curve was performed as described by He and Faulkner and in Chapter 2. 

Pulse voltammetry techniques are characterized by a succession of potential steps. During the sequential potential steps, the rates of current decay of the capacitive (If.) and the faradaic currents (7p) are essentially different (specifically, while 4 in Equation 2.1 decays exponentially with time. Ip decreases as a function of t 2, characteristic of a diffusion-controlled electrochemical reaction). In this way, the rate of decay of If. is significantly faster than that of Ip, and thus I. is negligible at a time of 57 uQ after the potential step is imposed (where 7 uQ is the time constant, Tj-gy, for the electrochemical cell having values from microseconds to milliseconds, and R is the uncompensated resistance between reference and working electrodes). Consequently, Ip is the main contribution to the measured current I when its value is measured at the end of a potential step. The detection limits of these techniques therefore fall around 10 M making them suitable for quantitative analysis. 

In typical Au MFC ET experiments with SAMs, the clusters are linked to a metal electrode surface through a self-assembled alkyl chain monolayer with specific linking sites. The electron is presumed to be transferred to/from the tethered MFC through the chain linker to/from the electrode surface. Techniques used to measure ET rates in Au MFCs attached to SAMs are potential step voltammetry and electrochemical AC impedance. Rate constants determined through impedance measurements have the virtue of less distortion due to uncompensated resistance. Also, rate constants determined using cyclic voltammetry are explicitly based on the kinetic behavior of the subpopulation of MFCs that produces the most prominent current peaks. Fotential step voltammetry resolves more peaks and thus gives a better indication of the diversity of rate constants of the redox species in the monolayer. ... 

Cyclic voltammetry at different applied potential scan rates is normally the first technique employed to study the properties of polymer-modified electrodes. From current-potential curves recorded with solutions containing supporting electrolyte only and at scan rates sufficiently low for thin layer conditions to prevail both the formal potentials and the amounts of redox species incorporated in the surface coating can easily be measured. In addition, deviations from ideal behaviour provide information on interactions within the polymer phase and, when uncompensated resistance effects can be accounted for, on the reversibility of electron transfer at the electrode-coating interface. Figure 4 shows a typical cyclic voltammogram obtained from a glassy-carbon electrode covered with... 

The imposition of a rapid, controlled change in an electrode s interfacial potential, a requirement of many electrochemical techniques (e.g., chronoamperometry and cyclic voltammetry [42]), is limited by the system s RC time constant, aR C, where a (units cm ) is the electrode area, (units ohms) is the uncompensated solution resistance, and Q (units F/cm is the interfadal capacitance. 

Imbeaux JC, Saveant JM (1970) Linear sweep voltammetry. Effect of uncompensated cell resistance and double layer charging on polarization curves. J Electroanal Chem 28 325-338... 

Different transient techniques have also been suggested for the measurement of corrosion rate. Pulse techniques can be used to eliminate from the polarization data the effects of uncompensated solution resistance and mass transport, or to minimize the effect of time-dependent phenomena. However, these techniques must be used with caution because the classical electrode kinetic theory can be used in the data evaluation only if /corrA/<0.9. The square-wave techniqueand ac impedance techniquehave also been used to measure the polarization resistance. The linear potential scan (potentiodynamic) technique has been used to obtain the polarization curve or the polarization resistance (small-amplitude cyclic voltammetry and exponential scan techniques were also proposed to determine the polarization curve. 

Consider the effect of the uncompensated solution resistance on a cyclic voltammetry experiment, in which a cyclic linear sweep is applied between the working and the reference electrodes. A typical current-time response is shown in Figure 4.4. 

---