10-13
10.2
Potential Step Techniques (CA, CC,
STEP)
In these techniques, the potential is stepped from one value to another, and the current
(CA) (or charge (CC)) response is monitored as a function of time (it should be
remembered that charge is simply the integral of current). After the potential has been
held at this value for a time
τ
, the potential can be stepped to another value (often the
original potential). Therefore, potential step experiments can be single step, or double
step.
The General Parameters are slightly different for CA and CC. For CA, an initial
potential (Initial E), and high and low potentials (High E and Low E) are required.
The potential can be stepped from Initial E to either Low E or High E (this is
determined by the Initial Direction - Positive or Negative respectively). After time
τ
(the Pulse Width), the potential can be stepped in the opposite direction (from Low
E to High E or High E to Low E), and is held at this value for time
τ
(Figure 10-4).
For CC, the only potentials are the initial potential (Initial E) and the final potential
(Final E) (Figure 10-5).
E
PULSE
WIDTH
HIGH E
INIT. E
t
PULSE
WIDTH
QUIET
TIME
LOW E
Figure 10-4. Potential wave form for CA.
The current/charge response depends on the values of the initial and final potentials.
If no faradaic reaction occurs at either potential (a faradaic reaction is the electrolysis
of any molecules in solution), then the response is due to charging of the electrode
(the charging or background current). The response is a current 'spike', which decays
exponentially.