Development of Karl Fischer Reagents
Development of Karl Fischer Reagents
Development of Karl Fischer Reagents
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8. Organic Acids<br />
Key Points<br />
Chapter II: Applications - 1<br />
With most organic acids, moisture content can be measured directly<br />
and easily. If a dehydrated solvent with methanol as its main ingredient<br />
is used, however, interference will occur as the result <strong>of</strong> an esterification<br />
reaction between methanol and lower carboxylic acids such as formic<br />
acid, acetic acid and adipic acid. These reactions produce water and<br />
will be evidenced by a delayed end point or an unusually high<br />
analytical result.<br />
Esterification reactions vary, depending on the individual acid. Formic<br />
acid shows a strong esterification reaction and will react very quickly<br />
with methanol. Reactivity lessens as the carbon number increases.<br />
Normally, during volumetric titration, propionic acid will not trigger<br />
interference attributable to an esterification reaction. Aromatic<br />
carboxylic acid is another substance that does not cause interference<br />
related to esterification.<br />
Interference can also result from a reaction with iodine. Formic acid<br />
appears to be affected by the oxidation <strong>of</strong> iodine and L-ascorbic acid<br />
reacts quantitatively with <strong>Karl</strong> <strong>Fischer</strong> reagent.<br />
For volumetric titration, a ketone-type dehydrated solvent is used with<br />
lower carboxylic acids. A general-use dehydrated solvent can be used<br />
with higher carboxylic acids and aromatic carboxylic acid. If problems<br />
are encountered with the methanol solubility <strong>of</strong> substances with high<br />
carbon numbers, a dehydrated solvent formulated for oils (such as<br />
stearic acid) should be used.<br />
For measurements based on coulometric titration, a ketone-type <strong>of</strong><br />
anolyte is used with the lower carboxylic acids. Coulometric titration<br />
cannot be used for formic acid, however, as an end point cannot be<br />
reached. A general-use electrolyte can be used with higher carboxylic<br />
acids and aromatic carboxylic acid.<br />
Examples <strong>of</strong> substances that can be measured directly:<br />
Acetic acid, mono-, di-, trichloroacetic acid, trifluoroacetic acid, propionic<br />
acid, acrylic acid, methacrylic acid, trimethylacetic acid, caproic acid,<br />
stearic acid, oleic acid, linolenic acid, cyclohexane carboxylic acid,<br />
cyclohexyl acetic acid, camphoric acid, abietic acid, 3,5-dinitrobenzoic<br />
acid, cinnamic acid, anisic acid, benzilic acid, 2,4-dichlorophenoxy acetic<br />
acid, benzoic acid<br />
89
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