Metrohm 756 KF Coulometer - Manual (Page 19)

2.7 Sample addition
756/831 KF Coulometer, Instructions of Use 11
Highly viscous samples can be warmed to lower their viscosity;
the syringe must also be warmed. The same goal can also be
reached by dilution with a suitable solvent. In this case the water
content of the solvent must be determined and deducted as a blank
value correction.
Pastes, greases can be placed in the measuring cell by using a
syringe without a needle. The joint opening can be used for this
purpose. If aspiration is additionally required the opening with the
septum stopper can be used.
The best way of determining the actual sample weight is by weighing
the syringe before and after injection.
With samples containing a lot of water care must be taken that
the needle is not introduced into the measuring cell through the
septum before <START> has been pressed as otherwise the drift
and therefore the result of the analysis could be falsified.
With samples containing only a trace of water the syringe must
be thoroughly dried beforehand. If possible the syringe should be
rinsed with the sample solution by taking up the sample solution
several times and then discarding it.
2.7.3 Solid samples
Whenever possible solid samples should be extracted or dissolved
in a suitable solvent and the resulting solution injected; a blank value
correction should be made for the solvent.
If no suitable solvent can be found for a solid sample or if the
sample reacts with the Karl Fischer solution the drying oven should
be used.
If solid samples have to be placed in the measuring cell directly then
the generator electrode without diaphragm should be used. The
sample can be added through either the joint opening or through the
opening at the side. Take care that:
• The sample releases its moisture completely
• No side reaction occurs with the Karl Fischer solution
• The surface of the electrodes is not covered by the sample
substance (incomplete KF reaction!)
• The Pt grid of the generator electrode is not damaged
• The Pt wires of the indicator electrode are not damaged