Chemistry Reference
In-Depth Information
0.0008
y
= 0.6244x + 2
E
- 06
R
2
= 0.9995
0.0007
0.0006
0.0005
0.0004
0.0003
0.0002
0.0001
0
0
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
Concentration (mol l
-1
)
6.15
Calibration plot of indigo at a platinum electrode positioned in a
wall-jet with a flow rate of 1.5 l min
-
1
, a diameter of the electrode
of 3.0 mm, a diameter of the nozzle of 2.0 mm and a NES gap of
2.5 mm. Currents are measured at
-
0.55 V vs. Ag|AgCl. (Reprinted
from
Analytica Chimica Acta
, Vol 486, No 1, Gasana
et al
., 'A
wall-jet disc . . .' pp 73-83, Copyright 2003, with permission from
Elsevier.)
indigo oxidation overlaps with the onset of the oxidation wave of sodium
dithionite. Therefore, first the interference signal of sodium dithionite on
the indigo oxidation signal was investigated. It was found that the amper-
ometric signal of indigo is independent of dithionite concentration in the
region from -0.6 to -0.45 V vs. Ag|AgCl. Therefore, indigo can be detected
selectively in this potential range.
Figure 6.15 shows a calibration plot obtained by repeated additions of
indigo and dithionite to the cell solution, keeping the dithionite/indigo con-
centration ratio constant (to guarantee that all added indigo is in the
reduced form). A linear relationship was found with satisfactory sensitivity
and reproducibility at a potential of
E
=-0.55 V vs. Ag|AgCl. The repro-
ducibility was studied by repeating the calibration experiment eight times,
resulting in the error flags shown in Fig. 6.15. Note that this plot was
obtained by measuring the signal for reduced indigo during 1 s at -0.55 V
vs. Ag|AgCl, followed by a cleaning step of 10 s at -0.9 V vs. Ag|AgCl.
For the simultaneous detection of sodium dithionite, sulphite and indigo,
a multistep amperometric method was worked out and optimised, as out-
lined in Table 6.4. It is clear that after each measuring step, the electrode
surface should be cleaned to remove indigo that is oxidised at all applied
