Coulometric Study of Recovery Rates for Karl Fischer Titration of Water in Aldehydes and Ketones Using Rapidly Reacting Methanolic and 2-Methoxyethanolic Reagents

Abstract

Factors which influence the recovery of water with use of rapidly reacting imidazole-based methanolic or 2-methoxyethanolic Karl Fischer (KF) reagents were investigated coulometrically. Using cyclohexanone and benzaldehyde as model substances, it was found that, in addition to the well-known problems associated with low recoveries of water due to the bisulfite addition reaction, errors may also be caused by side reactions involving iodine. Such interferences are largest at the end point of a titration, where an excess of iodine is present, and will lead to an overcompensation when correcting the results for the endpoint drift. The extent of the sum of all the side reactions was found to decrease with the concentration of iodide for all types of KF reagent investigated. The high reaction rates for the imidazole-based KF reagents, despite the high concentration of iodide, make it possible to use a very low endpoint concentration of iodine. In this way, side reactions involving iodine can be further suppressed. However, for titrations carried out in 2-methoxyethanolic reagents, a relatively high concentration of iodine must be present at the end point in order to decrease the time needed for the reversal of the bisulfite addition reaction. For small amounts of water, i.e., less than 30 °g, a reagent based on methanol can be recommended, since the titrations are rapid and the recovery was found to be 100% for both cyclohexanone and benzaldehyde. For large amounts of water or slow titrations, the recoveries tend to decrease for all reagent mixtures investigated. One reason for this seems to be related to a change in stoichiometry for the KF reaction involved in the reversal of the bisulfite addition reaction. © 1995, American Chemical Society. All rights reserved.