Chemistry Reference
In-Depth Information
6.3
Kinetic limitations in oxidation of dithionite
and sulphite
As described in the theoretical section of this topic, an electrochemical
reaction consists of different steps, and each of these steps (transport and/or
charge-transfer steps) can be rate determining. In this section, it is explained
why it is not possible to obtain a purely transport-controlled oxidation reac-
tion for sulphite as outlined in the previous section. This is caused by the
platinum electrode surface condition that has a large influence on the
electron-transfer rate. Therefore, the electrochemical behaviour of the elec-
trode surface itself is described first and limited to observations made
during oxidation of dithionite and sulphite.
6.3.1
Electrochemical behaviour of platinum
electrode surface
First, the electrochemical behaviour of the substrate material, which is plat-
inum, was investigated as a function of applied potential by cyclic voltam-
metry. For that, the potential was swept in positive direction from -0.5 V vs.
Ag|AgCl to a first vertex potential of 0.9 V vs. Ag|AgCl, followed by a scan
in reversed direction to a second vertex potential of -0.8 V and back to
-0.5 V vs. Ag|AgCl. The curves obtained in solutions of different pH values
are shown in Fig. 6.5. Similarly shaped curves were obtained by variation of
I
(mA)
0.05
8
0.03
1-7
0.01
-0.01
-1.1
-0.6
-0.1
0.4
0.9
-0.03
E
(V) vs. Ag
|
AgCl
-0.05
1-8
-0.07
-0.09
-0.11
-0.13
6.5
Current-potential curves recorded at a platinum-disc electrode
rotating at
N
=
16.67 Hz. The pH of the solution is (1) 11.79, (2)
12.04, (3) 12.13, (4) 12.29, (5) 12.42, (6) 12.61, (7) 12.76 and (8)
13.11. (Reprinted from
Electrochemistry Communications
, Vol 2,
No 10, Gasana
et al
., 'Influence of changes of . . .' pp 727-732,
Copyright 2000, with permission from Elsevier.)
