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Determination of Water Content in Polymethylmethacrylates Using Karl Fischer Titration

Product Group

Polymers, plastics

General Information concerning the product group

Polymers, plastics

Polymers are hardly or not at all soluble in the common Karl Fischer solvents. In order to extract the water trapped within the sample the plastic must be dissolved or swollen by means of an appropriate solvent to such an extent that the water can be released through diffusion. Which solvent is appropriate depends on the properties of the plastic. Some substances can be analysed through direct volumetry under addition of a solubiliser (e.g. polyvinyl acetate). Most substances require external extraction (e.g. polyamide), whereby an aliquot of the sample solution is analysed either volumetrically or coulometrically. Here too, the addition of a solubiliser is necessary in order to avoid renewed precipitation of the sample in the KF solution. For most plastics the KF oven technology is a suitable alternative. The water released through heating of the granulated or powdered sample in an oven is subsequently determined according to the Karl Fischer methodology. Due to the very low water concentration the combination with coulometry is preferred.

Special Information concerning the sample and the methods

Polymethylmethacrylates can be dissolved externally in chloroform at 50 °C. An aliquote of the sample is analysed either volumetrically or coulometrically. The result must be corrected for the blank value of the solvent. The KF oven technique in combination with coulometry, however, is more suitable. The recommended temperature range needed to release the water lies between 170 and 190 °C.

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 188009 Aquastar™ - CombiMethanol - Solvent for volumetric Karl Fischer titration with one component reagents, max. 0.01 % water
and
20 mL 102445 Chloroform - as solubiliser

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

Sample size:
5 mL aliquote of the sample solution (2 g powdered sample dissolved in 50 mL chloroform)

Procedure:
The sample is dissolved externally in chloroform under gentle heating to about 50 °C. The titration medium is first placed into the titration cell and titrated dry by means of the titrant. Then an aliquote of the sample is added with a syringe (exact sample weight determination by weighing of syringe before and after injection) and the titration is started. Finally the blank value of the solvent is determined and subtracted from the titration result.

Titration two component system

Reagents:

Titrant
188010 Aquastar™ - Titrant 5 - Titrant for volumetric titration with two component reagents, 1 mL = approx. 5 mg water
or
188011 Aquastar™ - Titrant 2 - Titrant for volumetric titration with two component reagents, 1 mL = approx. 2 mg water

Solvent
30 mL 188015 Aquastar™ - Solvent - Solvent for volumetric titration with two component reagents
and
20 mL 102445 Chloroform - as solubiliser

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

Sample size:
5 mL aliquote of the sample solution (2 g powdered sample dissolved in 50 mL chloroform)

Procedure:
The sample is dissolved externally in chloroform under gentle heating to about 50 °C. The titration medium is first placed into the titration cell and titrated dry by means of the titrant. Then an aliquote of the sample is added with a syringe (exact sample weight determination by weighing of syringe before and after injection) and the titration is started. Finally the blank value of the solvent is determined and subtracted from the titration result.

Materials

     
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