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.
Polycarbonate is poor soluble in alcoholic media. Direct volumetric or coulometric determination is not recommended. At first water is liberated by external extraction of the sample. Suitable solvents are chloroform, trichlorethylene or a mixture of chlorobenzene/1,2-dichlorobenzene. An aliquote of the sample is analysed by volumetric titration. In order to reduce the risk of precipiation in the titration cell, solubilisers should be added to the titration medium. A mixture of CombiSolvent oils and trichloroethylene is recommended. The result must be corrected for the blank value of the solvent. As alternative method the KF oven technique in combination with coulometry can be used. The recommended temperature range needed to release the water lies between 150 and 180 °C.
188005 Aquastar® - CombiTitrant 5 - One component reagent for volumetric Karl Fischer titration, 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 188020 Aquastar® - CombiSolvent oils - Solvent for volumetric Karl Fischer titration with one component reagents for oils
30 mL 111872 Trichloroethylene - as solubiliser
Default titration settings, e.g.:
I(pol) = 20 - 50 µA, U(EP) = 100 - 250 mV
Stop criterion: drift < 20 µL/min
5 - 10 mL aliquote of the sample solution (approx. 5 g powdered sample dissolved in 50 mL trichloroethylene)
The sample (exact weight) is dissolved externally in trichloroethylene (exact weight). The titration medium is 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.
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