COULOMETRIC STUDY OF THE INFLUENCE OF ALDEHYDES AND KETONES ON THE FISCHER,KARL TITRATION OF WATER USING STANDARD PYRIDINE/METHANOL REAGENTS

Conditions for the determination of water in active carbonyl compounds have been studied coulometrically for standard pyridine/methanol Karl Fischer (KF) reagents. The presence of active carbonyl compounds in the sample is considered to be one of the serious problems associated with the KF method, since they lead to a continuous formation of water through reaction with methanol. This study confirms that such side reactions cannot normally be adequately corrected for simply by subtracting the end-point drift. For high water formation rates, large negative errors were found to arise mainly for two reasons: (i) unreacted water at the end point of the titration as a consequence of the slow kinetics of the KF reaction in pyridine/methanol and (ii) formation of a reaction product between water, sulfur dioxide, and the carbonyl compound present, the so-called bisulfite-addition reaction. It is shown that the rate of water production can be kept at relatively low levels simply by keeping the quotient [PyH(+)]/[Py] as low as possible. Fortunately, it was possible to do this without affecting the reaction rate of the KF reaction too much. By using a reagent having a quotient [PyH(+)]/[Py] of 0.05 which corresponds to a spent reagent made up of 0.05 mol/L I-2, 0.15 mol/L SO2, and 5 mol/L pyridine in methanol, the end-point drift could be decreased by a factor of 10 compared to the value given in the literature for a pyridine/methanol reagent. By using such a reagent in combination with the choice of an end-point concentration of iodine of at least 10(-4) mol/L, it was found that the recovery of water in benzaldehyde (50 mu L) and cyclohexanone (10 mu L) samples is near 100% for water amounts in the range 0-100 mu g (0-1% water). The lowest concentration of water in benzaldehyde which gives a relative standard deviation equal to or less than 10% was about 0.008%.