Development of Karl Fischer Reagents

II-1 Organic Compounds
6. Ketones
82
Key Points
If the Karl Fischer reagent and the titration solvent contain methanol,
ketones react with the methanol (ketal bonding) as shown in the
following formula. As it produces water this causes an interference
reaction.
R2CO+2CH3OH � R2C(OCH3)2+H2O
This may increase the time needed to reach the end point, or may mean
that it is never reached. Excessively high moisture readings are among
other phenomena that can adversely affect accuracy and precision.
However, 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) The degree of interference varies according to the type of ketone
compound.Determine the size of the sample to use with reference to
the specific type of compound.
In general, acetone and cyclohexanone show the strongest reactivity
among the aliphatic ketone compounds and reactivity decreases in
proportion to the carbon number. Aromatic ketone compounds are
reported to be less reactive than the aliphatics. Either volumetric or
coulometric titration can be used with these ketones, but a reagent
formulated for ketones should be chosen.
Karl Fischer Reagent SS-Z (or SS) is used for volumetric titration. It
contains absolutely no methanol and can be used safely with ketones.
Use Dehydrated Solvent KTX (or CP), which is formulated for ketones.
For coulometric titration, use a combination of Aquamicron AKX/CXU.
Reference:
(1)Reference: Muroi, M, et al., J. Japan Petrol. Inst.,26, 97
(1983)."Determination of Water in Ketones and Silicone Oils
by Karl Fischer Titration Method"
(2) Kato, H.: "Determination of Moisture in Ketones and Lower
Carboxylic Acids by New Karl Fischer Coulometric Reagent"
(in Bunseki Kagaku 34, 147, 1984).