Metrohm 787 KF Titrino - Manual (Page 32)

2.6. Parameters, key <PARAM>
787 KF Titrino
28
Relation between the stop criteria "time" and "drift"
The stop criterion "time", t(delay), means that the end point must be exceeded for a
certain period of time. In other words, after the last dosed increment, time t is allowed to
elapse before the titration is stopped. The size of this last increment depends on the
volume of the Exchange Unit used. With a 20 mL Exchange Unit, the smallest possible
increment is 2 µL. With a t(delay) = 5 s, the last 2 µL reagent must thus suffice for 5 s or
longer. This results in a drift of ≤ 2 µL/5 s = 24 µL/min (the drift can be less than 24
µL/min as it is not known whether the last increment would also suffice for 10 s). If you
have been working up to this point with a 20 mL Exchange Unit and a t(delay) = 5 s, you
can set a value of ≤ 24 µL/min as stop drift. The following table shows several values for
the maximum stop drift.
t(delay)
min.incr. (Exchange Unit)
5 s 10 s 20 s
0.5 µL (5 mL) 6 µL/min 3 µL/min 1.5 µL/min
1 µL (10 mL) 12 µL/min 6 µL/min 3 µL/min
2 µL (20 mL) 24 µL/min 12 µL/min 6 µL/min
5 µL (50 mL) 60 µL/min 30 µL/min 15 µL/min
Same t(delay) with a range of extremely small volume increments means different switch-
off points. In case the stop criterion "drift" is used, the stopping point remains the same.
The determination of the free water is easily done, as far as the specifications of the
reagent manufacturer regarding the "water capacity" of the reagents are considered.
Problems may occur with specific sample matrices. The relevant literature contains many
precise analysis instructions. Here, we give you some useful hints for the sample addition.
Sample addition
For sample addition various accessories are available, for example injection syringes,
weighing spoons etc.
It is a good idea to apply the back-weighing technique, except when you are disposing
small amounts of liquids with a micro-syringe. Fill the syringe or the weighing spoon with
your sample just before the titration (e.g. during conditioning) and tare it on a balance
with an appropriate precision, which ideally is connected to the Titrino. Sample size
request should always be switched on (see page 24). Add the sample during the request
for the sample size, and weigh the syringe or weighing spoon again to evaluate the
weight difference. Use the transfer of balance data to enter the correct sample size if
possible. The weight has to be transmitted in gram. If you transfer the sample size from
the balance, the sample size request is confirmed automatically, and the titration is
started at once. Otherwise use the keypad to enter the weight.
If the sample size request is switched off, and preconditioning is set, you are explicitly
prompted to add your sample. For six seconds, the message "add sample" is displayed
after which the titration is started by the Titrino itself.