4/9/98
A-34 Application of the Hygrometer (900-901D1)
Empirical Calibrations
For those liquids in which a Henry’s Law type analysis is not
applicable, the absolute moisture content is best determined by
empirical calibration. A Henry’s Law type analysis is generally
not applicable for the following classes of liquids:
• liquids with a high saturation value (2% by weight of water
or greater)
• liquids, such as dioxane, that are completely miscible with
water
• liquids, such as isopropyl alcohol, that are conductive
For such liquids, measurements of the hygrometer dew point
readings for solutions of various known water concentrations
must be performed. Such a calibration can be conducted in either
of two ways:
• perform a Karl Fischer analysis on several unknown test
samples of different water content
• prepare a series of known test samples via the addition of
water to a quantity of dry liquid
In the latter case, it is important to be sure that the solutions have
reached equilibrium before proceeding with the dew point
measurements.
Note: Karl Fisher analysis is a method for measuring trace
quantities of water by titrating the test sample against a
special Karl Fischer reagent until a color change from
yellow to brown (or a change in potential) indicates that
the end point has been reached.
Either of the empirical calibration techniques described above
can be conducted using an apparatus equivalent to that shown in
Figure A-3. The apparatus pictured can be used for both the Karl
Fischer titrations of unknown test samples and the preparation of
test samples with known moisture content. Procedures for both of
these techniques are presented below.
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