Metrohm 851 Titrando - Manual (Page 47)

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4 Coulometric titration
851 Titrando
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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
must, however, not be cooled directly, as condensation 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 effect can also be achieved by diluting with a
suitable solvent. In this case, the water content of the solvent has to be
determined and subtracted as blank value.
Pastes 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 to reweigh the sample.
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, and a blank value correction for the
solvent has to be made.
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
￭ the sample releases its moisture completely.
￭ no side reaction with the Karl Fischer reagent takes place.
￭ the surfaces of the electrodes are not covered by the sample substance
(incomplete KF reaction).
￭ the Pt grid of the generator electrode does not become damaged.
￭ the Pt wires of the indicator electrode do not become damaged.