Chemical Quantitation of Moisture
Barbara Kanegsberg And Mantosh Chawla with Ed Kanegsberg, October 2002
We have discussed several approaches to determining water content including
gravimetrc methods, electrical impedance techniques, and spectroscopic methods.
In addition, for specificity and sensitivity, a chemical method, the Karl Fischer
titration (KF) is the method of choice for many applications. KF is a chemical
reaction of water with iodine in the presence of base (e.g., pyridine), solvent
(typically methanol), sulfur dioxide, and buffering. KF has the appeal of a
molecularly quantitative chemical reaction; if the number of molecules of iodine
used in the reaction is known, the number of molecules of water can be determined.
KF is used to determine water content in an immense variety of materials
such as solvents, oils, gasses, natural products, and lyophilized
material. Typical
uses in critical areas of contamination control include lyophilized materials
and assorted solid reagents for pharmaceutical applications and organic solvents
used in aerospace and wafer fabrication. KF is appealing in that it is specific
for water and is a direct (extractive) measurement. Gravimetric methods,
on the other hand, while direct, are non-specific. That is, weight
loss after
drying the sample could be due to outgassing of other solvents or of plasticizers,
not only to water content.
KF determinations may be coulombic or volumetric. Coulombic measurements
are typically used for microgram to milligram levels; volumetric measurements,
for higher levels of water content, up to 100%. Coulombs are the product
of
current (amps) and titration time in seconds; these can be quantitatively
related to the number of molecules of iodine. Volumetric determinations
measure, as
one might expect, the volume of iodine added. In typical analytical systems,
the volumetric endpoint (i.e., the absence of unreacted water and presence
of free iodine) is determined not by an operator peering intently at a
glass burette but rather by a decrease in voltage needed to maintain
a pre-specified
current.
Automatic KF titrators, for which there are several commercial sources,
improve convenience and consistency. KF determinations, however, are
by no means
trivial. Samples must be collected and extracted appropriately; analysis
requires a
skilled, thoughtful, and often innovative chemist. To avoid interferences,
the analyst must be aware of the sample composition. The minutiae of
ramifications of collection and extraction techniques, standardization,
titration methods,
temperature, agitation, and reagent provides for ongoing spirited discussion
and a host of technical papers; all are of great fascination to analytical
chemists.
For the rest of us, a few illustrative highlights will suffice. Water
itself, when inadvertently introduced, interferes with accuracy. Humidity
must
be controlled during testing, and spurious, atypical humidity must
be controlled during sample
collection and handling. Complete extraction of water can be a challenge.
If the water is bound (as in some biological samples), it may not be
available to react with the KF reagent. One approach, sometimes used
with lyophilized
samples, is to subject the sample to vigorous agitation or to ultrasonic
action
prior to analysis.
Side reactions impact accuracy. For example, ketones (such as acetone)
and aldehydes can react with methanol to form acetals or ketals,
with the release
of water. Falsely elevated water levels can also result from the
presence of a host of readily oxidized chemicals; copper and tin
salts, for
example, can
react with iodine. In addition, the KF reaction must be held within
a limited pH range, to avoid factors which overpower the buffering
system
and may
skew the results.
Because KF requires an understanding of sample composition and potential
interferences, partnering with your analytical chemist is imperative.
It is crucial to communicate
pertinent factors such as the type of sample (solid, liquid, gas),
approximate level of water expected, and materials of construction
or chemical components
of the mixture. If there are unknowns, initial testing can sometimes
reveal biased determinations (high or low); steps can then be taken
to correct
the problem. For example, if there are excess oxidizing agents,
water can be
evaporated from the solvent, then collected for analysis in an
inert stream of gas. For
ongoing analysis protocols, it is important to explain changes
in the product or process variables, because an apparent change in
water
could
actually
signal some other product or process modification.
The authors appreciate the comments of Eric Andersen, Manager,
Materials and Processes Department of Northrop-Grumman, Navigation
Systems
Division.In the
next column, we will discuss details of the primary Karl-Fisher
technique and two additional secondary on-line techniques, near
infrared reflectance
and
millimeter wave.
In the next column, we will discuss more of the common secondary
methods for moisture measurement.
Next month: A discussion of the most common techniques for
moisture measurement.
Contact Barbara Kanegsberg and Ed Kanegsberg at BFK Solutions;16924
Livorno Dr., Pacific Palisades, CA 90272,310-459-3614;barbara@bfksolutions.com;contact
Mantosh Chawla at Photoemission Tech., (PET), 3255 Grade
Vista Dr., Newbury
Park, CA, 91320;805-499-7667
