Metrohm 796 Titroprocessor - Manual (Page 114)

7.4 SET Titration to a preset endpoint
796 Titroprocessor, Instructions for use 108 108
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 for setting the titration control and the
stop criterion.
7.4.3 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.
7.4.4 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 for setting the titration control and the
stop criterion.
The various parameters for the SET modes are described in detail
on pages 148 to 153.
What to do if ...
Observation Possible causes and remedies
Final addition lasts too
long and
increments are too
small.
"Will never end!"
• Increase 'Min.Rate'. Carry out a test at a much higher Min.Rate.
• Alter switch-off criterion, e.g. try increasing the stop drift or use a shorter
switch-off time as the stop criterion.
• Try leading an inert gas through the titration vessel.