Metrohm 851 Titrando - Manual (Page 46)

4.3 Sample addition
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38 ￭￭￭￭￭￭￭￭
851 Titrando
15 The next determination can be started as soon as the determination
has been finished and the coulometer cell has been conditioned (drift
stable) again.
4.3 Sample addition
This chapter contains a few notes concerning sample addition. An exhaus-
tive discussion of this topic is not possible here. Further notes can be
found in the publications from the reagent manufacturers and in the fol-
lowing Metrohm Application Bulletins:
Bulletin No. Title
No. 142 Karl Fischer water content determination in non-explosive gases
No. 145 Determination of low water contents in plastics using the KF oven method
No. 209 Coulometric water content determination by the Karl Fischer method in
insulating oils, hydrocarbons and their products
4.3.1 Size of the sample size
The sample weight should be small in order to be able to titrate as many
samples as possible in the same electrolyte solution and in order to keep
the titration time short. However, ensure that the sample contains at least
50 µg of H
2
O. The following table helps you determine the appropriate
sample size.
Table 2 Recommended sample sizes
Water content of the sam-
ple Sample size Resulting water content
10,000 ppm = 1% 10 mg - 100 mg 100 µg - 1,000 µg
1,000 ppm = 0.1% 100 mg - 1 g 100 µg - 1,000 µg
100 ppm = 0.01% 1 g 100 µg
10 ppm = 0.001% 5 g 50 µg
4.3.2 Working with liquid samples
Liquid samples are added with a syringe. The samples can be injected in
two different ways:
￭ You can use a syringe with a long needle, which is immersed in the
reagent during the injection.
￭ You can use a syringe with a short needle and aspirate the last drop
back into the needle.