Karl Fischer Determination of Trace Water in Polyethylene Terephthalate
Polyethylene terephthalate trace moisture determination relies on Karl Fischer coulometric oven extraction at 170 to 180 degrees Celsius to prevent chain scission.
Anode Reagent
Meaning
Chemical solutions utilized in coulometric titration determine the trace water content of hygroscopic polymer pellets before thermal processing. An anode reagent provides the iodide ions and sulfur dioxide necessary to react with moisture in a solvent-filled cell. Proper drying verification prevents polymer chain scission during moulding.
Chemical Reactivity
Electrochemical oxidation occurs within the generator electrode to convert iodide into active iodine. The anode reagent contains specific organic bases and sulfur dioxide to facilitate this reaction smoothly. When moisture is introduced from the dissolved or heated plastic sample, it consumes the generated iodine in a stoichiometric one-to-one ratio.
Moisture Quantification
Current measurements within the titration cell calculate the total electrical charge needed to regenerate the iodine. This quantitative method measures water levels as low as ten parts per million in engineering polymers. Accurate coulometric measurement provides a precise benchmark for resin dryness that standard loss-on-drying instruments cannot match.
In practice, a heating vaporiser accessory drives moisture out of the polymer matrix and carries it via a dry carrier gas into the titration vessel, protecting the chemicals from direct contamination by molten resin.
Degradation Prevention
Inadequate moisture control leads to cosmetic and structural defects in moulded components. If the anode reagent is depleted or contaminated by polymer additives, the titration yields incorrect moisture readings. Using fresh chemical solutions ensures that polyesters and polyamides do not undergo severe strength loss during melt processing.