Hanna Instruments HI 904 - Manual (Page 121)

9-9
METHODS OPTIMIZATION
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 904 provides options for the automatic calculation of samples prepared normally,
using external extraction and external dissolution.
9.2.5.1 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 5% should therefore be diluted with a dry solvent before being introduced into the
titration vessel. Dilutions are carried out using the ‘external dissolution’ sample type option.
Anhydrous methanol is the solvent of choice for sample dilutions. If the sample contains fats
or oils, then a mixture of methanol and chloroform can be used to promote solubility of the
sample.
The following outlines a generic dilution procedure:
1. Determine the mass of a dry flask equipped with a septum stopper.
2. Transfer approximately 1 g of sample to the flask and measure the mass of the
flask and the sample together.
3. Add 30 grams of dilution solvent to the flask. Re-seal and mix the flask contents.
4. Determine the moisture content of the dry solvent used as the diluent in a separate
titration.
5. Add the diluted sample as per the instructions for adding liquid samples in this
section.
9.2.5.2 External Dissolution
External dissolutions are used for all solid samples or mixed-phase samples that will dissolve
in a solvent mixture.
Sample preparation and choice of solvent or solvent mixture is sample specific. Consult an
applicable standard method for procedural details.
The HI 904 will conduct the necessary calculations automatically when ‘external dissolution’
is selected from the sample type menu.
9.2.5.3 External Extraction
External extraction is used for all insoluble solid samples.
The HI 904 will conduct the necessary calculations automatically when ‘external extraction’
is selected from the sample type menu.