Chemistry Reference
In-Depth Information
0
-0.6
-0.4
-0.2
0
0.2
0.4 0.6
E
(V) vs. Ag
|
AgCl
-0.1
-0.2
-0.3
-0.4
1
2
3
4
5
6
-0.5
-0.6
-0.7
-0.8
-0.9
I
(mA)
6.12
Current-potential curves of the reduction of 2.0
¥
10
-
3
mol l
-
1
K
3
[Fe(CN)
6
] in 0.8 mol l
-
1
KNO
3
recorded at a wall-jet platinum-disc
electrode with a diameter of 3.0 mm, a nozzle diameter of 2.0
mm, a NES gap of 2.1 mm and a flow rate of (1) 0.5, (2) 1.0, (3)
1.6, (4) 2.0, (5) 2.7 and (6) 3.0 l min
-
1
. (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.)
was changed by variation of the vessel opening (see Chapter 1, pp 19-21)
and was measured with a flow meter (again, see Chapter 1, pp 19-21). The
results of these experiments are shown in Fig. 6.12. It can be seen that the
limiting current of the Fe(III) reduction increases with flow rate. A loga-
rithmic plot of the limiting current versus the flow rate proved that the lim-
iting current is proportional to the square root of the flow rate. Note also
that for smaller flow rates a larger noise signal is detected.
Finally, the diameter of the platinum electrode and the diameter of the
nozzle were varied. Similarly shaped curves were obtained as shown in Fig.
6.12, and therefore they are not given. A linear relationship between limit-
ing current
I
L
(mA) and electrode area
A
(cm
2
) was obtained.
Investigation of the influence of the diameter of the nozzle was not
simple. The nozzle diameter was varied from 1.25 to 3.0 mm, and the
obtained results were not always reproducible. This can possibly be
explained by the fact that diminishing the nozzle diameter also has an influ-
ence on the flow rate. With the smallest nozzle diameter, the maximum flow
rate that could be obtained was 0.5 l min
-1
. As can be seen from the results
in Fig. 6.12, this is not an optimal value for the flow rate, resulting in rela-
tively small signals with a weak signal-to-noise ratio. From a qualitative
analysis, it was found that the limiting current increases with decreasing
nozzle diameter. This can be explained by the fact that a smaller nozzle
diameter in combination with a constant flow rate causes a jet of solution
under higher pressure. This gives rise to smaller values for the thickness of
the diffusion layer.
