Chemistry Reference
In-Depth Information
peroxide concentration in the bath (e.g. by addition of hydrogen peroxide
or water), the peristaltic pump was started, and the time necessary for the
measurement of the newly established hydrogen peroxide concentration in
the detection cell was established. From approximately 30 measurements,
it was concluded that the dead time of the sensor expanded with the FIA
system amounts to around 30 s for an actual volume of ca. 7 ml, a 1-mm
inside diameter of the tubes and a ca. 1-m length between bath and detec-
tion cell.
This means that one should bear in mind that the values of the process
parameters detected at the sensor correspond to a situation which occurred
in the bath of the process 30 s earlier. This is extremely important if one
wishes to control the hydrogen peroxide concentration in the considered
process. Though the dead time can be shortened by increasing the flow rate,
the problem arises then that higher flow rates generate more waste-water
and turbulent behaviour in the detection cell which affects the sensor signal.
Similarly, the dead time of the same sensor expanded with the FIA system
was verified with different flow rates (Fig. 5.18). With a flow rate of, for
example, 2 l/h, the dead time amounts to ca. 23 s.
Another important element that should be taken into account is that, in
certain processes, relatively low hydrogen peroxide concentrations are
used. This means that the sensor's detection limit can be a crucial parame-
ter for these applications. As a criterion, one speaks of useful measurements
when the sensor signal is three times higher than the noise observed in the
absence of hydrogen peroxide. As appears from the data shown in Fig. 4.9,
the sensor signal increases with increasing pH. Hence, it is interesting to
600
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100
0
0
0.5
1.0
1.5
2.0
2.5
Flow rate (l/h
-1
)
5.18
Development of the dead time as a function of the liquid flow
rate.
