Metrohm 796 Titroprocessor - Manual (Page 118)

7.6 KFT Karl Fischer titrations
796 Titroprocessor, Instructions for use 112 112
In IPOL measurements steep curves are obtained; in UPOL measurements
the curves tend to be flatter.
For standard KF titrations the
IPOL mode with the preset standard pa-
rameters is recommended. The UPOL mode should be selected for back
titrations.
7.6.2 Reagent addition
Reagent addition takes place in three different phases. The initial addition
takes place with increasing dosing speed. It begins with the defined 'Min.
Rate' and increases up to the 'Max. Rate'.
Dosing is carried out at the defined 'Max. Rate' until the control range is en-
tered.
Within the control range the dosing speed depends on the control differ-
ence (actual value – preset endpoint) and becomes slower and slower as
the endpoint is approached. In this (final) phase addition is carried out in
single dosing steps at the minimum dosing rate.
Three predefined sets of parameters, which can be further optimized, are
available in the 796 Titroprocessor setting the titration control and the stop
criterion.
7.6.3 Measuring point list
Entering the measured values and their corresponding data in the measur-
ing point list is carried out at predefined measuring intervals and can be
switched off. However, a measuring point list is required if a titration curve is
to be shown.
7.6.4 Switch-off criteria at the endpoint
The recognition of the endpoint and therefore the end of the titration can
take place in two ways.
Stop criterion 'Time' means that the endpoint must remain exceeded during
the preset switch-off time.
Stop criterion 'Drift' means that after the endpoint has been reached the
predefined volume drift must not be exceeded. The volume drift can be in-
terpreted as being the volume of reagent which is theoretically required to
be added per minute to maintain the endpoint.
As it must be assumed that the first time the endpoint is reached the se-
lected endpoint criterion has not yet been met, the smallest possible volume
increment (
1 /10 000 of the buret volume) is added at the minimum dosing
rate until the endpoint criterion has been fulfilled.
Three predefined sets of parameters, which can be further optimized, are
available in the 796 Titroprocessor setting the titration control and the stop
criterion. The preset standard values provide correct and easily reproducible
results in the vast majority of cases.
Titer determinations should always be carried out with the same parameters
as were used for the sample titrations.
The various parameters for the KFT modes are described in detail
on pages 158 to 166.