Metrohm 840 PC Control 5.0 / Touch Control - Manual (Page 210)

4.1 Titrations
198
PC Control / Touch Control
The User-defined Parameters differ for dynamic equivalence point ti-
trations (DET) and monotonic equivalence point titrations (MET).
User-defined titration parameters for DET and MET
• DET: Measuring point density (d): A small value means small
volume increments, i.e. a high measuring point density. The curve
then shows all the finest details which also include noise; this could
cause unwanted equivalence points to be found. A larger value, i.e.
a smaller measuring point density, permits quicker titrations. If you
are using a dosing device with a small cylinder volume then a
smaller measuring point density value may be beneficial. However,
you should also set a smaller measured value drift and a higher EP
criterion at the same time.
• DET: Minimum (volume) increment (d): This smallest permitted vol-
ume increment is added at the start of the titration and with steep
curves in the equivalence point region. Very small values should
only be used if a low titrant consumption is expected, as unwanted
equivalence points could be evaluated.
Note!
It is not advisable to select similar volumes for the minimum and
maximum increments. For these applications monotonic equivalence
point titration (MET) is more suitable.
• DET: Maximum (volume) increment (d): A maximum volume incre-
ment should be selected when the titrant consumption up to the
equivalence point is expected to be very small, a start volume is to
be added until just before the equivalence point is reached or if the
change of direction in the potential jump region is very abrupt, as
otherwise it is easy to add too large a volume in the equivalence
point region. The value should not be less than 1/100 cylinder vol-
ume.
• MET: Volume increment (d): Volume to be added at each dosing
step. The choice of the correct volume increment is a basic re-
quirement for achieving high accuracy. A good guideline is 1/20 of
the expected EP volume. For steep jumps the volume increment
should tend toward 1/100 and for flat jumps toward 1/10 of the EP
volume. Small volume increments are used for determining blank
values or with very asymmetrical curves. The accuracy of the
evaluation cannot be increased by using smaller increments as the
measured value alterations between two measuring points are then
of the same order of magnitude as the noise.
• Dosing rate (d): Rate at which the volume increment is added. The
maximum dosing rate depends on the cylinder volume of the ex-
change or dosing unit used (see Section 3.8.6).
• Signal drift (d): Drift, i.e. the alteration in the measured value per
minute that has to be reached for measured value acceptance. This
type of titration is often known as an equilibrium titration. off means
that the measured value acceptance takes place when the maxi-
mum waiting time has elapsed. This may be advisable if the titration
reaction occurs slowly or if the electrode has a slow response.