During water determination after Karl Fischer (KF) aldehydes and ketones cause side reactions with the normally utilized methanol. Both side reactions (i.e. the formation of acetals and ketals) as well as bisulphite addition, can often be recognized by dragging end points and lead to wrong results. During acetal or ketal formation, water is released which simulates water contents that are too high. Bisulphite addition, however, consumes water, leading to results that are too low. The extent of the interferences depends on the reactivity of the substances as well as the reactivity of the utilized KF solutions. Through utilization of specific KF reagents for aldehydes and ketones, it is possible to largely suppress the side reactions. The coulometric method is not recommended.
Acetone is one of the most reactive ketones. The formation of ketals with methanol and the addition of bisulphite are very characteristic here and require KF titration with specific reagents for aldehydes & ketones.
188006 Aquastar® - CombiTitrant 5 Keto - One component reagent for volumetric Karl Fischer titration of aldehydes & ketones, 1 mL = approx. 5 mg water
30 mL 188007 Aquastar® - CombiSolvent Keto - Solvent for volumetric Karl Fischer titration with one component reagents for aldehydes & ketones
Titration settings for methanol-free solvents:
I(pol) = 20 µA, U(EP) = 100 - 250 mV
Stop criterion for fast end point: drift < 30 µL/min
The titration medium is first placed into the titration cell and dried by means of the titrant. Then the sample is added with a syringe (exact sample weight determination by weighing of syringe before and after injection) or volumetric pipette and the titration is started. Titration is carried out to fast end point.