Development of Karl Fischer Reagents

7. Aldehydes
Key Points
Chapter II: Applications - 1
If the Karl Fischer reagent and the titration solvent contain methanol,
aldehydes react with the methanol (acetal bonding) as shown in the
following formula. As it produces water, this causes an interference
reaction.
RCHO+2CH3OH � RCH(OCH3)2+H2O
This may cause end point fluctuation, or may mean that the end point is
never reached. Other phenomena include excessively high moisture
readings.
As shown in the following formula, aldehydes may also react with an
ingredient in the Karl Fischer reagent (sulfur dioxide), causing an
interference reaction that consumes water (bisulfite addition) and
reduces the moisture content reading.
C5H5NSO2+RCHO+H2O � C5H5NHSO3CH(OH)R
Furthermore, since aldehydes are more reactive than ketones, the
reaction speed will vary considerably according to the specific
aldehyde involved. Because of these complex problems it is very
difficult to obtain accurate analytical readings for aldehydes.
In general, Karl Fischer titration can be carried out if the following
counteracting steps are taken.
a) Use a Karl Fischer reagent and titration solvent that do not contain
methanol.
b) Keep the size of the sample used for titration to a minimum to
suppress the reaction that consumes water.
c) Acetaldehydes are highly reactive and must be titrated under special
conditions.These are described later in this manual.
With coulometric titration, sulfur dioxide in the electrolytic solution
causes interference that leads to low moisture readings. Although an
end point may be reached the analytical reading is unlikely to be
accurate and using this method is difficult. In practice, this method
should never be used with acetaldehydes.
Reference:
Muroi, K, et al., Bull. Chem. Soc. Japan, 38,1176 (1965).
"The Determination of the Water Content in Acetaldehyde by
Means of Karl Fische Reagent"
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