Chemistry Reference
In-Depth Information
prove this statement. In order to activate the electrode surface in a repro-
ducible and repeatable way, it should be subjected to mechanical treatment
comprising scouring the electrode surface to remove a few micrometers fol-
lowed by polishing to smoothen the surface. A typical procedure is given
here: the electrode surface was scoured briefly on 1200-grit SiC-emery
paper to obtain a fresh surface. To smooth this relatively rough surface, it
was further subjected to sequential polishing on a polishing cloth covered
with alumina (Buehler) powder of 1, 0.3 and 0.05 mm for respectively 5, 10
and 20 min. To remove any adherent Al
2
O
3
particles, the electrode surface
was rinsed thoroughly with doubly deionised water and cleaned in an ultra-
sonic bath (Branson 3210) for 2 min. An alternative polishing procedure is
the use of diamond paste instead of Al
2
O
3
polishing powder.
After the mechanical pre-treatment, the electrode can be subjected to a
chemical and/or electrochemical pre-treatement. Chemical treatment con-
sists mainly of etching of a few layers of the metal, including possible con-
taminants. A well-known etching solution for gold is the so-called 'piranha'
solution. Other reported solutions are kali, which is a concentrated KOH
solution to remove fats and oils and etch off a layer of platinum.
Finally, electrochemical pre-treatment is performed to obtain a repro-
ducible surface. This is done mainly by cycling the applied potential over
the entire potential window limited by the hydrogen and oxygen evolution
reaction. Such a treatment has two functions: first, removal of adsorbed
species and, second, altering the microstructure of the electrode, the latter
being caused by the repetitive dissolution and deposition of a metal mono-
layer in the scanning procedure.
Electrode configurations
Rotating-disc
3
and ring-disc electrodes
44
Disc electrodes are commonly used in voltammetry as stationary and as
rotating electrodes. The diffusion of electroactive species towards the
surface of these electrodes is linear, as shown in Fig. 1.6a. The advantage of
the second configuration is that rotation of the electrode causes convection
in solution that compensates for the increase of the diffusion layer thick-
ness with time after a period of about 200 ms. This results in a limiting
current instead of a peak-shaped current (see also section 3.2) according to
the Levich equation
3
:
I
=
062
.
n
FD uw
A
23
-
16
12
c
[1.15]
lim
where
I
lim
is the limiting current (mA),
n
is the number of electrons, F is the
Faraday constant (C mol
-1
),
A
is the surface of the electrode (cm
2
), D is the
diffusion coefficient (cm
2
s
-1
), n is the kinematic viscosity (cm
2
s
-1
), w=2 p
N
