Weakly alkaline amines pose no problems in the water determination after Karl Fischer and can be titrated according to standard methods. For strongly alkaline amines the buffer capacity of the Karl Fischer solution is not sufficient. A shift of pH into the alkaline range leads to a side reaction of the iodine. This can be recognised by the dragging end points, or a continuous consumption of iodine solution, respectively. If turn-off criteria are reached nevertheless, the result is too high. This side reaction, caused by the disproportionation of iodine molecules above pH 8, can be avoided through additional buffering of the working medium with a weak acid. Aquastar® buffer solution for strong bases, benzoic or salicylic acid are the most suitable because, unlike acetic acid, they do not show any tendency for esterification in the Karl Fischer solution. The acids are added to the solvent prior to titration and can even be employed in excess. Some amines react with the Karl Fischer solution. Measures to suppress or overcome this side reaction are, for example, titration in methanol-free working media, utilisation of small sample sizes, as well as titration to a fast end point.
During water determination after Karl Fischer (KF) aldehydes and ketones cause side reactions with the normally utilised methanol. Both side reactions, i.e. the formation of acetals and ketals, as well as bisulphite addition, can often be recognised 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 utilised KF solutions. Through utilisation of specific KF reagents for aldehydes and ketones it is possible to largely suppress the side reactions. The coulometric method is not recommended.
For heterocyclic ketones the formation of ketals only plays a minor role. Bisulphite addition, however, may cause interferences. KF titration is carried out with methanol-free reagents, whereby two alternatives are at hand: CombiTitrant 5 Keto + CombiSolvent Keto or CombiTitrant 2 + CombiSolvent.
188006 Aquastar® - CombiTitrant 5 Keto - One component reagent for volumetric Karl Fischer titration of aldehydes & ketones, 1 mL = approx. 5 mg water
188002 Aquastar® - CombiTitrant 2 - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 2 mg water
30 mL 188007 Aquastar® - CombiSolvent Keto - Solvent for volumetric Karl Fischer titration with one component reagents for aldehydes & ketones
30 mL 188008 Aquastar® - CombiSolvent - Methanol-free solvent for volumetric Karl Fischer titration with one component reagents
Titration settings for methanol -free solvents:
I(pol) = 20 µA, U(EP) = 100 - 250 mV
Stop criterion for fast titration: drift < 30 µL/min
5 - 10 mL
The titration medium is first placed into the cell and titrated dry 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.
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