Chemistry Reference
In-Depth Information
Time (s)
0
500
1000
1500
2000
2500
3000
3500
0
1
7
6
-10
5
4
-20
-30
2
-40
-50
-60
-70
3
-80
12.14
Chronoamperometric current signal of the Ce(IV) reduction at
E
=
0.25 V vs. Ag|AgCl, recorded as a function of time at a gold-
modified PPy-polyaramide woven textile structure with a
2.36 ml min
-
1
continuous flow of 1.0 mol l
-
1
H
2
SO
4
containing
(1) 0, (2) 2
¥
10
-
4
, (3) 5
¥
10
-
4
, (4) 1
¥
10
-
4
, (5) 6
¥
10
-
5
, (6) 3
¥
10
-
5
and (7) 1
¥
10
-
5
mol l
-
1
Ce(IV) at 298.0 K.
configuration used, the numerical value of k
1
was 14 800 if the currents are
expressed in microampere, with the Ce concentration in mol l
-1
:
I
=
k
C
+
()
I
[12.20]
1
Ce IV
bg
The decay of the background current with time, not visible in segment 1 of
Fig. 12.14 owing to current scale, is shown in Fig. 12.15. It is clear that the
time dependence of the background current will have a greater effect on
the detection limit of the method at shorter times. When the detection limit
is defined as the Ce(IV) concentration corresponding to a current signal
that is twice as high as the background current
I
bg
(segment 1, Fig. 12.14),
limits of 3 ¥ 10
-6
,1¥ 10
-6
and 1.5 ¥ 10
-7
mol l
-1
are obtained after 60 s, 3600 s
and 14 h, respectively. For longer times, the background current remains
nearly constant (75 nA cm
-2
), resulting in a constant detection limit of about
1.5 ¥ 10
-7
mol l
-1
.
The influence of solution flow rate on the steady-state limiting current at
a constant potential of 0.25 V vs. Ag|AgCl was also investigated, using a
1.0 mol l
-1
H
2
SO
4
solution containing 6 ¥ 10
-5
,2¥ 10
-4
and 5 ¥ 10
-4
mol l
-1
Ce(IV). A logarithmic plot shows that the current is proportional to the
square root of the flow rate. However, this is not valid for flow rates lower
than 0.15 ml min
-1
, where a less-stable current signal is obtained. It is likely
that a steady-state transport of Ce(IV) to the electrode surface is no longer
