Hanna Instruments HI 903 - Manual (Page 140)

9-12
METHODS OPTIMIZATION
vessel or other internal components of the cell may not be titrated with the rest of the sample.
It is equally important that no drops remain on the tip of the needle. ‘Hanging drops’ will end
up on the bottom of the septum. This will result in false low results for the determination.
Liquid samples with high viscosity like honey can be added via a syringe without needle
through the sample port following the steps outlined above.
In some cases liquid samples may require one of the additional preparatory steps listed in
the sections that follow. Specific sample preparation instructions are included with each
standard method.
9.2.5 Sample Preparation Techniques
While many samples can be introduced directly into the titration vessel (see section 6.5
Sample Addition ), others require preparatory steps. It is critical that samples are not
contaminated with additional water or lose water during the preparation phase.
The steps required for the most common sample preparation techniques are outlined below.
For detailed application-specific instructions, consult the instructions included with applicable
standard methods.
The HI 903 provides options for the automatic calculation of samples prepared normally,
using external extraction and external dissolution.
9.2.5.1 Internal Extractions
Internal extractions are carried out using the ‘normal’ sample type option within the ‘sample
parameters menu’. This type of sample preparation is suitable for solid samples which release
their water relatively quickly (during the pre-analysis stir time) and exhibit limited or no
solubility in Karl Fischer solvents. Internal extraction should be used preferentially over
external extraction techniques because the extracted water is titrated immediately, which
favors complete extraction by Le Chatlier’s principle.
An outline of the general procedure follows:
1. Add methanol or an appropriate solvent to the titration cell and pre-titrate to
dryness.
2. Adjust the pre-analysis stir time to be sufficiently long enough to complete the
extraction. Appropriate set times will be sample and solvent specific. Consult an
applicable standard method or experiment by increasing the pre-analysis stir time
and titrating samples until the resulting water content no longer increases.
3. Reduce the samples to as fine of a powder as possible to ensure that sample water
is extracted quickly.
4. Add the sample to the titration vessel using the back weighing method.
9.2.5.2 Dilutions
It is very difficult to accurately add very small amounts of sample to the titration vessel. In
order to produce accurate and reproducible results, samples having water content greater
than 50% should therefore be diluted with a dry solvent before being introduced into the