Metrohm 899 Coulometer - Manual (Page 42)

4.3 Sample addition
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899 Coulometer
4.3.2 Working with liquid samples
Liquid samples are added with a syringe. The samples can be injected
two different ways:
￭ One uses a syringe with a long needle, which one immerses in the
reagent during the injection.
￭ One uses a syringe with a short needle and aspirates the last drops
back into the needle.
The best way for you to determine the injected sample amount is to
reweigh the sample.
Glass syringes should be used for the determination of traces and val-
idations. We recommend obtaining these from a specialized syringe
manufacturer.
Highly volatile samples and samples of low viscosity should be
cooled before sampling. Doing so avoids losses while working. The
syringe, however, must not be cooled directly, as condensates could form.
For the same reason, no air may be aspirated into a syringe into which a
cooled sample has been aspirated beforehand.
Samples of high viscosity can be thinned by heating. The syringe must
be heated as well. The same target can be reached by diluting with suita-
ble solvents. In this case the water content of the solvent has to be deter-
mined and subtracted as blank value.
Paste and fats can be added to the coulometer cell with a syringe with-
out needle. You can use the ground-joint opening for this. If you also wish
to aspirate, you can use the opening with the septum stopper. The best
way for you to determine the sample amount is by reweighing the sam-
ple.
If samples contain only traces of water, then the syringe has to be pre-
dried well. If possible, the syringe should be rinsed with the sample solu-
tion by filling in and discarding solution several times.
4.3.3 Working with solid samples
If possible, solid samples are to be extracted or dissolved in a suitable sol-
vent. The resulting solution is injected, during which a blank value correc-
tion for the solvent must be carried out.
If no suitable solvent can be found for a solid sample, or if the sample
reacts with the Karl Fischer reagent, then a Karl Fischer oven should be
used.
If solid samples are added directly into the coulometer cell, then the gen-
erator electrode without diaphragm should be used. The samples can be
added through the ground-joint opening or through the side opening.
While doing so, take care to ensure that