Determination of Water Content in Polystyrene (EPS) 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

The solubility of polystyrene in methanolic solvents is limited. Therefore an external dissolution in xylene is necessary. Following to the external dissolution, Methanol is added to precipitate the polystyrene again. The measurement result has to be corrected for the blank value of the solvent mixture.

Titration one component system

Reagents:

Titrant
188002 Aquastar™ - CombiTitrant 2 - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 2 mg water
or
188005 Aquastar™ - CombiTitrant 5 - One component reagent for volumetric Karl Fischer titration, 1 mL = approx. 5 mg water

Solvent
50 mL 188008 Aquastar™ - CombiSolvent - Methanol-free solvent for volumetric Karl Fischer titration with one component reagents

Titration Parameters:
stirring time: 10s
Default titration settings, e.g.:
I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV
Stop criterion: drift < 20 µL/min

Sample size:
blank value: 5 g sample: 4 g aliquot of the external dissolution (external dissolution: 10 g polystyrene in 40 g solvent)

Procedure:
The titration medium is first placed into the cell and titrated dry by means of the titrant. Then the externally dissolved sample is added with a syringe (exact sample weight determination by weighing of syringe before and after injection) and the titration is started. It is possible to analyse approx. 10 g solution before the KF solvent must be changed.

Coulometry without diaphragm

Reagents:

Working medium
100 mL 109257 Aquastar™ - CombiCoulomat fritless - Coulometric Karl Fischer reagent for cells with or without diaphragm

Titration Parameters:
Default coulometer settings for cell without diaphragm:
For end point indication, e.g.:
I(pol) = 5 - 10 µA, U(EP) = 50 - 100 mV
Stop criterion: drift < 10 µg/min

Sample size:
blank value: approx. 1,5 g sample: approx. 2 g aliquot of the external dissolution (external dissolution: 10 g polystyrene in 40 g solvent)

Procedure:
The Karl-Fischer reagent is placed into the titration cell without diaphragm. The coulometer is started and the solvent is titrated dry. After preliminary titration and stabilisation of drift the sample is injected into the titration cell with a syringe (exact sample weight determination by weighing of syringe before and after injection) and the water determination is started.

Materials

     
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