Metrohm 756 KF Coulometer - Manual (Page 18)

2.7 Sample addition
756/831 KF Coulometer, Instructions for Use 10
12. Close the syringe and put it back on the balance.
13. Read the indicated value off the balance and feed it at your
Coulometer as sample size.
14. As soon as the determination has finished and the titration
cell is conditioned again, you can start with the next deter-
mination.
2.7 Sample addition
This section contains some information about sample addition. A de-
tailed description of this topic is not possible here. You can find fur-
ther information in the reagent manufacturer's documentation and in
Metrohm Application Bulletins.
Metrohm Application Bulletins:
No. 142: Karl Fischer water determination in gaseous samples
No. 145: Determination of small amounts of water in plastics
No. 209: Water determination in insulating oils, hydrocarbons and
their products
No. 273: Validation of KF Coulometers according to GLP/ISO 9001.
2.7.1 Sample size
The sample size should be small so that as many samples as possi-
ble can be titrated in the same electrolyte solution and the titration
time kept short. However, take care that the sample contains at least
50 µg H
2
O. The following table provides guidelines for the sample
weight.
Content of sample Sample weight H
2
O to be determined
100000 ppm = 10 %
10000 ppm = 1 %
1000 ppm = 0.1 %
100 ppm = 0.01 %
10 ppm = 0.001 %
50 mg
10 mg... 100 mg
100 mg... 1 g
1 g
5 g
5000 µg
100 µg...1000 µg
100 µg...1000 µg
100 µg
50 µg
2.7.2 Liquid samples
Liquid samples are added with the aid of a syringe. Either a syringe
with a long needle is used with the needle being immersed beneath
the surface of the reagent during injection or a short needle is used
with the last drop being sucked back into the needle.
The best way of determining the actual sample weight is by weighing
the syringe before and after injection.
Volatile or low-viscosity samples should be refrigerated before
that sample is taken in order to prevent handling losses. In contrast,
the syringe itself should not be directly refrigerated as this could
cause the formation of condensate. For the same reason aspirating
air into a syringe which has been cooled by taking up a refrigerated
sample should be avoided.