Chemistry Reference
In-Depth Information
5.3.3
Stability of the sensor electrode in time
In this part of the research work, the stability was checked of the ampero-
metric signal of a calibrated sensor electrode in the presence of additives
as a function of time. To this end, during a longer period of time, without
re-calibrating the sensor electrode, the amperometric pseudo-limiting-
current was measured continuously, and the hydrogen peroxide concentra-
tion was periodically determined by means of titration of a sample of the
cell solution. In the course of the experiment, the hydrogen peroxide con-
centration deteriorates by catalytic decomposition, and the concentration
was either periodically reduced by means of dilution of the cell solution, or
increased by addition of hydrogen peroxide. Two different bleaching solu-
tions were selected that contain products from Henkel and Sandoz as can
be seen below.
Bleaching solution 1:
•20mll
-1
10 mol l
-1
NaOH solution
•50mll
-1
11 mol l
-1
H
2
O
2
solution
•15mll
-1
commercial water-glass solution
•12mll
-1
Cottoclarin
®
SV (Henkel)
•1m
-1
Defindol
®
E (Henkel)
•
5gl
-1
Na
2
S
2
O
8
Bleaching solution 2:
•20mll
-1
10 mol l
-1
NaOH solution
•50mll
-1
11 mol l
-1
H
2
O
2
solution
•12mll
-1
Sandopan
®
540 (Sandoz)
•2m
-1
Sandozin
®
(Sandoz)
5gl
-1
•
Na
2
S
2
O
8
Some of the results obtained with bleaching solution 1 are represented in
Fig. 5.5. The first point of the curve corresponds to the calibration point. It
is observed that the concentrations obtained by means of Equation 5.6 (full
line) on the one hand and titration on the other (discrete measuring points,
represented by small crosses) correspond with each other very well. The
deviations are limited to less than 2%, which is sufficient for a possible
application as a sensor in industry. During this experiment, water was added
at different stages (1-3 on Fig. 5.5) in order to modify pH as well as hydro-
gen peroxide and additive concentration. In stage 4, the hydrogen peroxide
concentration was increased and the pH reduced to its starting value, and
Stabilol
®
K, which serves as auxiliary stabiliser for water glass, was pro-
gressively added (spread over stage 5). No interference by this additive was
observed. In between the dilutions and additions, the decrease of the hydro-
gen peroxide concentration is to be ascribed to natural deterioration and,
