Hanna Instruments HI 904, Быстрый старт

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9-10
METHODS OPTIMIZATION
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.
6. Facilitate extraction by shaking the solution or placing the solution on a stirring
plate or in a sonicator.
7. Allow the insoluble portion of the sample to settle to the bottom of the extraction
bottle.
8. Titrate an appropriately sized sample of the supernatant (solvent above the settled
solid sample).
9.2.5.4 Homogenization
Homogenization is recommended for non-aqueous or mixed phase liquid samples as well as
solids with inhomogeneous distributions of water. Water can be evenly distributed throughout
a collected sample by the use of high speed, high shear mixers called homogenizers.
In mixed phase (oil and water) non-aqueous samples, water tends to migrate to the surface
of the sample solution, adhere to the inner walls of or sink to the bottom of the sample
bottle. This is particularly problematic when sampling is done at high temperatures and the
specimen is subsequently allowed to cool to room temperature prior to analysis.
Solid samples typically exhibit inhomogeneous water distributions and must therefore be
thoroughly reduced to powder or homogenized. The procedure for homogenization depends
upon the characteristics of the specific sample.
Homogenization is particularly suited for semi-solid samples and suspensions and is the only
method that can disrupt plant and tissue cells in order to release water present inside the
cells. Homogenization is typically carried out externally in a dry flask with the addition of a
suitable solvent, preferably methanol.
9.2.5.5 Heating
Sample heating is used for the analysis of solid or liquid samples that cannot be extracted or
that interfere with the Karl Fischer reaction. These include plastics, minerals, petrochemical
products which contain additives, and starting materials for pharmaceutical products.
Samples are heated in a special oven while a dry stream of carrier gas passes through the
sample chamber or, for liquid samples, the sample itself. The carrier gas is introduced into
the titration vessel.