Determination of Water Content in 2-Benzoylbenzoic acid Using Karl Fischer Titration

Product Group

Carboxylic acids, Ketones

General Information concerning the product group

Carboxylic acids

Most carboxylic acids can be analysed relatively easily following standard KF titration methods. Due to their weakly acidic properties they exhibit only a slight effect on the pH of the KF solution. Only the stronger acids require neutralisation or buffering of the solvent prior to measuring. This is achieved by the use of organic amines. In some cases esterification with methanol results in the formation of water. This potential interference should especially be taken into account with coulometric determination. In the event of solubility problems solubilisers may be added. With some substances, only the adherent moisture can be determined by volumetric titration, or alternatively KF oven technology may be employed.

Ketones

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.

Special Information concerning the sample and the methods

Aromatic ketones are less reactive than aliphatic ones. The influence of disturbing side reactions resulting from the keto function is less characteristic for this substance. KF titration is carried out with methanol-free reagents, whereby two alternatives are at hand: CombiTitrant 5 Keto + CombiSolvent Keto or CombiTitrant 2 + CombiSolvent.

Titration one component system

Reagents:

Titrant
188006 Aquastar™ - CombiTitrant 5 Keto - One component reagent for volumetric Karl Fischer titration of aldehydes & ketones, 1 mL = approx. 5 mg water
or
188002 Aquastar™ - CombiTitrant 2 - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 2 mg water

Solvent
30 mL 188007 Aquastar™ - CombiSolvent Keto - Solvent for volumetric Karl Fischer titration with one component reagents for aldehydes & ketones
or
30 mL 188008 Aquastar™ - CombiSolvent - Methanol-free solvent for volumetric Karl Fischer titration with one component reagents

Titration Parameters:
Stirring time: 60 sec.
Titration settings for methanol-free solvents:
I(pol) = 20 µA, U(EP) = 100 - 250 mV
Stop criterion: drift < 20 µL/min

Sample size:
2 g

Procedure:
The titration medium is first placed into the cell and titrated dry by means of the titrant. Then the sample is added from a weighing boat (exact sample weight determination by weighing of weighing boat before and after addition) and the titration is started. For complete dissolution of the sample a stirring time of 60 seconds is recommended.

Materials

     
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