Chronoamperometry (CA) and chronocoulometry (CC) have the same potential wave form - the potential step - which is one of the simplest potential wave forms. As shown below (Fig1), the potential is changed instantaneously from the Initial Potential to the First Step Potential, and it is held at this value for the First Step Time. This is a single potential step experiment. In a double potential step experiment, the potential is changed to the Second Step Potential after the First Step Time, and it is then held at this value for the Second Step Time. In CA, the current is monitored as a function of time, whereas in CC, the charge is monitored as a function of time. It is important to note that the basic potential step experiment on the epsilon is CA; that is, during the experiment, the current is recorded as a function of time. However, after the experiment, the data can also be displayed as charge as a function of time (the charge is calculated by integrating the current). Hence, chronocoulometry data can be obtained. CA is a standard technique on the epsilon.
CA is different from other constant potential techniques (constant potential electrolysis (CPE) and DC potential amperometry (DCA)) in that the time scale of CA is shorter (milliseconds and seconds) than those of CPE and DCA (seconds and minutes).
Figure 1. Potential wave form for chronoamperometry and chronocoulometry.
As shown above, five parameters are used in the epsilon software to define the potential wave form for CA.
The values of these parameters are set using the Change Parameter dialog box (Fig2) in either the Experiment menu or the pop-up menu.
Figure 2. Change Parameters dialog box for chronoamperometry/chronocoulometry.
Sample Interval = Step Time/Maximum # of points
However, it should be noted that only certain values are allowed for each of these parameters, as is shown in the table below:
|Max. # of points||1000||2000||4000||8000||16000|
Let us consider the effect of a single potential step on the reaction R = O + e-. At potentials well negative of the redox potential (Enr), there is no net conversion of R to O, whereas at potentials well positive of the redox potential (Ed), the rate of the reaction is diffusion-controlled (i.e., molecules of R are electrolyzed as soon as they arrive at the electrode surface). In most potential step experiments, Enr is the Initial Potential, and Ed is the First Step Potential. The advantage of using these two potentials is that any effects of slow heterogeneous electron transfer kinetics are eliminated. In double potential step experiments, (Enr) is often used as the Second Step Potential.
It is instructive to consider the concentration profiles of O and R following the potential step (Fig3). Initially, only R is present in solution (a). After the potential step, the concentration of R at the electrode surface decreases to zero, and hence a concentration gradient is set up between the interfacial region and the bulk solution (b). As molecules of R diffuse down this concentration gradient to the electrode surface (and are converted to O), a diffusion layer (i.e., a region of the solution in which the concentration of R has been depleted) is formed. The width of this layer increases with increasing time (b-d). There is also a net diffusion of O molecules away from the electrode surface.
Since the current is directly proportional to the rate of electrolysis, the current response to a potential step is a current 'spike' (due to initial electrolysis of species at the electrode surface) followed by time-dependent decay (Fig4) (due to diffusion of molecules to the electrode surface).
Figure 4. Current-time response for a double-potential step chronoamperometry experiment.
For a diffusion-controlled current, the current-time (i-t) curve is described by the Cottrell equation:
i = nFACD½p-½t -½
n = number of electrons transferred/molecule
F = Faraday's constant (96,500 C mol-1)
A = electrode area (cm2)
D = diffusion coefficient (cm2 s-1)
C = concentration (mol cm-3)
The charge-time (Q-t) (the Anson equation) is obtained by integrating the Cottrell equation with respect to time, and can be displayed in the epsilon software by selecting Q vs T from Select Graph in the pop-up menu (the original i vs. t plot can be recovered by selecting Original from Select Graph in the pop-up menu). A typical charge-time plot is shown in Fig5:
Q = 2nFACD½p-½t ½
Figure 5. Charge-time response for a double-potential step chronocoulometry/chronoamperometry experiment.
Charge is the integral of current, so the response for CC increases with time, whereas that for CA decreases. Since the latter parts of the signal response must be used for data analysis (the finite rise time of the potentiostat invalidates the early time points), the larger signal response at the latter parts for CC makes this the more favorable potential step technique for many applications (in addition, integration decreases the noise level).
The analysis of CA and CC data is discussed elsewhere.