Metrohm 840 PC Control 5.0 / Touch Control - Manual (Page 220)

4.1 Titrations
208
PC Control / Touch Control
steep curves
Control
range
EP
V/mL
U/mV
flat curves
Control
range
EP
V/mL
U/mV
Fig. 4.9: Control range size
Editing the parameters for the Stop criterion:
• Stop criterion (d): The titration stops when the endpoint has
been reached and the stop criterion has been fulfilled. The titration
can be stopped when a particular (volume) drift has been
achieved or after a preset time. If no stop criterion has been se-
lected the titration will not be stopped. The Stop conditions (see
following page) will always stop the titration even if the stop criterion
has not been fulfilled.
• Stop drift (d): If the endpoint and the stop drift have been
reached the titration will be stopped.
• Delay time (d): After the endpoint has been reached the preset
time will be allowed to elapse after the last addition before the titra-
tion is stopped.
In older instructions the Delay time was usually defined as the Stop
criterion . The stop criterion time means that the endpoint must be
exceeded for a certain time: the Delay time. The same Delay time with
different very small volume increments (depending on the volume of the
exchange or dosing unit, see below) means different switch-off points.
In contrast, if drift is used as the stop criterion then the switch-off
point will always have the same curve slope dV/dt.
You can use the switch-off time in the following way to calculate the
maximum stop drift to be used: the size of the last increment to be
added depends on the volume of the attached exchange unit. With a
20 mL exchange unit (10'000 pulses per cylinder volume) – number of
impulses depends on the dosing device – the smallest possible incre-
ment is 2 μL. With a delay time of 5 s this means that the last 2 μL of re-
agent to be added must be sufficient for 5 s or longer. This results in a
drift of ≤ 2 μL/5 s = 24 μL/min. The drift can be smaller than 24 μL/min
as we do not know whether the last increment would also have been
sufficient for 10 s. This means that if you have previously worked with a
20 mL exchange unit (10'000 pulses per cylinder volume) and 5 s
switch-off delay then you should set a value of ≤ 24 μL/min as the stop
drift.
The control parameters for Endpoint 2 are the same as for Endpoint 1.