Chemistry Reference
In-Depth Information
Ta b le 6.4
Sequence of applied potentials for simultaneous detection method
Solution detected
Applied potential
Applied time(s)
(vs. Ag|AgCl)
Cleaning
-
0.90
10
Indigo
-
0.55
1
Cleaning
-
0.90
10
Dithionite
0.45
1
Cleaning
-
0.90
10
Sulphite
0.80
1
oxidation potentials. The times applied for measurement of the current
signals are the shortest times possible in order to obtain a steady state
without a drop of the output signal due to indigo precipitation. However,
the period to clean the surface is made 5 s longer than the shortest time
observed to be necessary to clean the surface completely. Figure 6.16 shows
the results of such a potential multistep amperometric experiment obtained
with a cell solution in which the concentrations of indigo and dithionite
were increased periodically by repeated additions of the reagents. In Fig.
6.16a, the experimental current signals for sodium dithionite (1), sulphite
(2) and indigo (3) are shown. After each period of about 240 s, the concen-
tration of these compounds was increased by the addition of a solution of
indigo and dithionite with a concentration ratio of 1 : 2. After reduction of
indigo in this solution, a ratio of about 1 : 1 was obtained. The current signals
of the cleaning steps are not shown for clarity of the figure and because
they do not contribute to the analytical purpose of the method. The con-
centrations of the appropriate compounds could be calculated from these
data using Equations 6.29, 6.30 and 6.31 and are shown in Fig. 6.16b (con-
tinuous line). The symbols in the same figure represent the concentration
of dithionite, sulphite and indigo obtained by titration after withdrawing an
aliquot from the solution at appropriate times. It can be seen that there is
no significant discrepancy between the results of both methods. From Fig.
6.16b, it can also be seen that after reduction of indigo in the stock solu-
tion, a ratio of about 0.8 is obtained for
c
ind
/
c
dit
.This is sufficiently close to
1 regarding the continuous decomposition of dithionite as a function of
time:
c
= ¢
-055
I
[6.29]
ind
.
V
(
)
c
=
B
I
-
I
[6.30]
dit
045
.
V
-
055
.
V
(
)
c
=¢
I
-
3
I
-
I
[6.31]
sul
08
.
V
045
.
V
-
055
.
V
where B≤ is 0.62 A l mol
-1
.
