Hanna Instruments HI 903 - Manual (Page 141)

9-13
METHODS OPTIMIZATION
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.
9.2.5.3 External Dissolution
External dissolutions are recommended for titrations which require a large amount of soluble
solid sample due to inhomogeneous water distribution or very low water content. External
dissolution reduces the error typically associated with the titration of low water content solids
by collecting the water released by a large amount of solid sample by dissolving it in a
relatively small amount of solvent. A small portion of the solvent can then be injected into
the titration vessel.
Sample preparation and choice of solvent or solvent mixture is sample specific. Consult an
applicable standard method for procedural details.
The HI 903 will conduct the necessary calculations automatically when ‘external dissolution’
is selected from the sample type menu.
9.2.5.4 External Extraction
External extraction is recommended for insoluble solid samples which release water slowly.
The HI 903 will conduct the necessary calculations automatically when ‘external extraction’
is selected from the sample type menu.
An outline of a general procedure follows:
1. Determine the mass of an extraction bottle or flask equipped with a septum.
2. Add the extraction solvent to the bottle and determine the mass of the bottle and
the solvent. In order to maximize the effectiveness of the extraction, the water
content of the solvent should be as low as possible. When choosing an extraction
solvent, one must carefully consider the limit of water saturation for a possible
solvent.
3. Determine the water content of the solvent.
4. Determine the mass of the solvent remaining in the extraction bottle.
5. Add a finely crushed sample to the solvent in the extraction bottle. The amount of
sample added should be large enough so that the amount of water in the sample is
much greater than that in the solvent before the extraction.