Determination of Water Content in 2-Aminophenol Using Karl Fischer Titration

Product Group

Amines, Phenols

General Information concerning the product group

Amines

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.

Phenols

In most phenols water determination can be carried out according to the standard methods after Karl Fischer. Some substances, however, are oxidised by the Karl Fischer reagent. This side reaction leads to unstable or dragging end points. During coulometry, interferences can accumulate to the point where analysis becomes impossible. Because the side reaction is dependent on the pH, its influence can be reduced through modification of the solvent. In a weakly acidic titration environment (e.g. through the addition of Aquastar™ buffer solution for strong bases, benzoic or salicylic acid) easily oxidisable substances can be determined nevertheless.

Special Information concerning the sample and the methods

With this substance a side reaction is observed in the Karl Fischer solution. Through titration in methanol-free media and under acidic conditions its influence can be reduced to the extent that volumetric analysis is possible. The coulometric method is not recommended.

Titration one component system

Reagents:

Titrant
188005 Aquastar™ - CombiTitrant 5 - One component reagent for volumetric Karl Fischer titration, 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 188008 Aquastar™ - CombiSolvent - Methanol-free solvent for volumetric Karl Fischer titration with one component reagents
and
5 g 100635 salicylic acid - as buffer substance

Titration Parameters:
Default titration settings, e.g.:
I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV
Stop criterion for fast titration: drift < 30 µL/min

Sample size:
0.5 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. Titration is carried out to fast end point.

Materials

     
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