Chemistry Reference
In-Depth Information
establish a high pH in the detection cell in order to obtain the detection
limits as low as possible. Experiments have been performed to determine
these detection limits analogously as in Chapter 4, section 4.5.2 (Figs
4.7-4.10) and with a pH of ca. 12.5; the lowest detectable concentration
equals 5 ¥ 10
-4
mol l
-1
(ª 10 ppm). This concentration corresponds to a sensor
signal of ca. 8 mA, which is more than three times higher than the signal in
the absence of hydrogen peroxide (2.5 mA). The deviation in relation to the
result obtained through titration of a solution with this hydrogen peroxide
concentration amounts to ca. 10%, and is clearly wider than the error
obtained when determining higher concentrations. The upper limit of the
hydrogen peroxide concentration, which can be determined with the clas-
sical sensor system (2 mol l
-1
) is maintained when using the FIA system.
For the calculation of the concentration, it appeared that the same equa-
tion can be used as the one used for the sensor which was directly applied
(Equation 5.6) if the dilution factor was taken into account. A first possible
approach to take the dilution factor into account is the following: Equation
5.6 is connected to the hydrogen peroxide concentration in the process
through sampling of process-bath solution, and the obtained hydrogen per-
oxide concentration should be divided by the dilution factor, 1.98. The
hydrogen peroxide concentration simultaneously obtained via titration has
to be connected to the sensor signal which is obtained in the detection cell
ca. 30 s after the sampling, which can cause errors, even with an efficient
timing.
Hence another strategy was followed, where the calibration is done by
means of a solution from the detection cell. With this solution, the sensor
signal, as well as the concentration of hydrogen peroxide through titration,
was obtained. In order to be able to calculate subsequently the original con-
centration in the bath where the process occurs, the dilution factor should
be taken into account, and for the control of the hydrogen peroxide con-
centration one should bear in mind a 30-s delay. In this way, a possible dif-
ference in process-bath composition is eliminated. The equation for the
calculation of the hydrogen peroxide concentration in the process bath is:
(
)
6 607
.
È
-
0 0791
.
[
(
)
]
c
-
-
0 0791
.
I
I
32 120050 71
-
.
c
-
Í
Í
HO
m
HO
2,cal
2,cal
c
=
198
.
c
-
-
HO
(
)
6 607
.
HO
2m
-
0 0791
.
2,cal
cal
c
Î
-
HO
2,m
1
[
(
)
]
È
Í
Ê
Á
ˆ
˜ -
˘
˙
˘
-
-
0 0791
.
-
0 0791
.
32 12005071
-
.
c
23 0050 71
.
c
1 2
-
HO2,m
c
HO2,cal
˙
˙
˙
˙
-
OH
(
)
0 03049
.
TT
-
cal
e
cal
m
[
(
)
-
]
-
-
0 0791
.
320050
.
771
c
1 2
HO
2,m
c
-
˚
OH
m
[5.7]
