Chemistry Reference
In-Depth Information
same time that the electrical-current response is followed continuously
as a function of time. The use and advantages of this approach are
multifunctional:
•The method is useful for the evaluation of diffusion coefficients.
•The method enables study of the rate of electrode reactions, and the
rates of coupled chemical reactions, as well as the study of adsorption
phenomena.
•For analytical purposes, the method can be used to obtain time-
independent signals when a constant thickness of the diffusion layer is
obtained by convection or using ultramicro electrode configurations.
The latter use is particularly appropriate for the work and results shown
and discussed in this topic (see Chapters 4, 5 and 6).
2.6
Other voltammetric methods
2.6.1
Introduction
In this chapter, some commonly known electrochemical methods are
described to complete the set of methods described in Chapter 1. In con-
trast to the methods discussed in Chapter 3, the methods described here
are not used in the further work considered and evaluated in this topic,
though they have been included for general information.
2.6.2
Fast direct-current voltammetry
In fast direct-current voltammetry
67
, also known as linear sweep voltam-
metry, scan rates of 0.01-20 V s
-1
are used and result in a peak-shaped
current response at a stationary electrode. The resolving and separating
capacity can be up to 40 mV and 1 ¥ 10
3
/1, resulting in a detection limit of 1
¥ 10
-7
mol l
-1
. However, a drawback of using fast scan rates is the increasing
background current due to the capacitive charging effect of the electrical
double layer. In practice, this results in a limitation to the Faraday current
with increasing scan rate because this current is superponated at an increas-
ing base line. At high scan rates, this increase in capacitive current is larger
than the increase of the Faraday current, and therefore an increasing per-
turbation of the Faraday current is observed having a negative effect on its
shape, nature and detection limit.
In fact this method is identical to cyclic voltammetry, except that the
potential is swept from a certain starting potential to an end potential
without returning to the initial starting potential. In other words, only the
forward sweep is recorded in linear sweep voltammetry. However, rela-
tionships for the measured current and its relation to applied potential, and
