Chemistry Reference
In-Depth Information
Time (s)
0
10
000
20
000
30
000
40
000
50
000
60
000
0
-0.02
-0.04
-0.06
-0.08
-0.1
-0.12
-0.14
-0.16
-0.18
-0.2
12.15
Decay of chronoamperometric background-current signal with
time recorded at a gold-modified PPy-polyaramide woven
textile structure (
E
=
0.25 V vs. Ag|AgCl) in a flow-through cell
with a continuous flow (2.36 ml min
-
1
) of a 1.0 mol l
-
1
H
2
SO
4
solution.
attained. At higher flow rates (>3.7 ml min
-1
), the risk of leakage of the flow-
through cell increases. Therefore an optimal flow rate of 2.36 ml min
-1
was
further used.
12.4.4 Stability of the Ce(IV) solution as a function of pH
It is well known from redox titrimetry that oxidations with Ce(IV) require
a strongly acidic solution, as was used in the experiments described above
(1.0 mol l
-1
H
2
SO
4
). At higher pH values, the risk of precipitation of basic
salts increases
88
.The influence of increased pH on the chronoamperomet-
ric current signal in the flow-through cell is shown in Fig. 12.16. When the
pH is in excess of about 1.5, the Ce(IV) reduction current at 0.25 V vs.
Ag|AgCl starts decaying a short time after applying the potential, possibly
because of precipitation or adsorption of basic salts.
A possible solution to the pH problem is to make use of the FIA princi-
ple. Samples of Ce(IV) solutions of different concentration (1 ¥ 10
-5
,3¥
10
-5
,6¥ 10
-5
,1¥ 10
-4
and 2 ¥ 10
-4
mol l
-1
) and pH (0, 1.0, 2.0, 3.0, 4.0, 5.0 and
6.0) were injected in a 1.0 mol l
-1
H
2
SO
4
continuous-flow solution and
analysed by chronoamperometric detection at 0.25 V vs. Ag|AgCl. Figure
12.17 shows the amperometric responses by injection of Ce(IV) samples at
a pH of 3.0. Solutions with pH values higher than 6 were not investigated
