Development of Karl Fischer Reagents

17. Acid Anhydrides
Key Points
Chapter II: Applications - 1
Acid anhydrides and acylates can be measured directly using
volumetric titration. The sample is subjected to Karl Fischer titration
after it has been dissolved in either a general-use dehydrated solvent or
a compound solvent made up of one part pyridine to four parts
methanol by volume.
Examples of substances that can be titrated directly:
Acid Anhydrides
Acetic anhydride, propionic anhydride, butyric anhydride, n-valeric
anhydride, succinic anhydride, benzoic anhydride, phthalic anhydride
18. Acid Chlorides
Key Points
The following acid chlorides cause interference through oxidation of the
hydriodic acid in Karl Fischer reagent and the release of free iodine. For
this reason, water content is measured using volumetric titration as
follows.
To suppress the interference, neutralize the sample by adding 1-3g of
the sample to a methanol solvent that includes 20% pyridine/sulfur
dioxide (SO2 32g/pyridine 100ml). Then carry out the Karl Fischer
titration.
1) Potassium dichloroisocyanurate
C3N3O3Cl2K+2SO2+3C5H5N+CH3OH �
C3N3OH(OCH3)2+C5H5N•HCl+KCl+2C5H5N•HSO4CH3
2) Aryl sulfonyl chloride
ArSO2Cl+CH3OH+C5H5N � ArSO3CH3+C5H5N•HCl
Coulometric titration cannot be used because of the interference.
Examples of substances that can be titrated directly:
Acid Chlorides
Propionyl chloride, butyryl chloride, valeryl chloride, caproyl chloride, capryl
chloride, stearoyl chloride, benzoyl chloride
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