Metrohm 899 Coulometer - Manual (Page 106)

10.4 Control parameters and polarization current
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96 ￭￭￭￭￭￭￭￭
899 Coulometer
Water / µg
Measur
ed value / mV
Endpoint
Control range
I = 30 µA
pol
I = 2 µA
pol
I = 5 µA
pol
I = 10 µA
pol
I = 20 µA
pol
Figure 26 Control parameters and polarization current
The diagram shows Karl Fischer titration curves at different polarization
currents (Reagent Coulomat AD). It is clear to see that the position of the
endpoint varies with the polarization current. The curves have different
slopes, i.e. the dynamics must also be adapted. Polarization currents < 10
µA are not suitable for this application. The following table gives you an
idea of the optimal control parameters for various polarization currents.
Table 7 Optimum control parameters at different polarization currents
I(pol) (polari-
zation cur-
rent) 10 µA 20 µA 30 µA
Endpoint at 50 mV 100 mV 150 mV
Dynamics 70 mV 100 mV 120 mV
Max. rate Default value Default value Default value
Min. rate Default value Default value Default value
Stop drift Default value Default value Default value
The indicator electrode is activated after a certain period of use in the
same reagent, i.e. the titration curve becomes steeper. If the titration
curve is too steep, then slowly fluctuating drift values could occur during
conditioning. You can solve this problem by setting the endpoint lower.
Endpoints that have been set too low could however prolong the titration.
This could have unfavorable effects on the measuring error.