Both aldehydes and ketones pose problems with Karl Fischer titration because they form acetals and ketals respectively with conventional KF reagents (Figure 9.6.a). The reaction forms water, which is also titrated, resulting in vanishing end points and erroneously high water content. With aldehydes a second side reaction, the bisulfite addition, can also occur (Figure 9.6.b). This reaction consumes water and leads to an erroneously low water content.
Figure 9.6.a. The formation of acetals or ketals.
Figure 9.6.b. The bisulfite addition.We have investigated the behavior of certain aldehydes and ketones toward the KF titration. The reactivity of aliphatic ketones decreases with increasing chain length. Aromatic ketones are less reactive than aliphatic ketones. Aldehydes are much more reactive than ketones and their tendency to undergo the bisulfite addition is particularly strong. The formation of acetals and ketals can be suppressed by replacing methanol in the titrating agent with another solvent, typically pyridine or 2-methoxyethanol (methylglycol). However, we found both of these solvents to be unsatisfactory. Pure pyridine alters the stoichiometry of the KF reaction, enhances the bisulfite addition, and leads to a falsely low water content. 2 methoxyethanol does not sufficiently inhibit the formation of both ketals and acetals and results in a slow titration rate. The levels of water are too high and, because only small samples can be analyzed, the accuracy of the titration is negatively affected. Our research identified suitable solvents that permit determination of water in aldehydes and ketones without adverse side reactions. These solvents are the basis of the Hydranal™ K-type reagents.
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