Measurement principle
Karl Fischer moisture determination is a classical moisture quantification technique based on electrochemical reactions, the core principle of which is that iodine and sulfur dioxide react quantitatively with water in the presence of alcohols and alkalis. The reaction equation can be expressed as:
I₂ + SO₂ + 3 C₅H₅N + H₂O → 2 C₅H₅N·HI + C₅H₅N· SO₃
C₅H₅N· SO₃ + ROH → C₅H₅N· HSO₄R
In the determination of moisture content of resin particles, the instrument accurately calculates the moisture content in the sample by measuring the volume of Karl Fischer reagent consumed during titration. This method has high sensitivity and accuracy for trace and trace moisture in resin.
Measurement points
The physical morphology and chemical properties of resin particles had a significant impact on the moisture determination results. Samples need to be representative and it is usually recommended to seal and store them immediately after sampling to prevent interference from ambient humidity. For partially hydrophobic or high-viscosity resins, dissolution/dispersion may be required by heat-assisted injection or with a specialized solvent such as anhydrous methanol to ensure that the water is completely released and exposed to the reagent. During the measurement, it is necessary to control the ambient humidity and ensure that the injection system is well sealed to prevent the intrusion of ambient moisture.
Measurement conditions
To obtain reliable resin moisture content data, instrument parameters need to be adjusted according to the resin type. The diplatinum electrode potential method is usually used to determine the end point of titration, and the endpoint potential value needs to be calibrated according to the reagent and solvent system. The stirring speed should be sufficient to disperse the sample evenly without air bubbles. The titration speed can be initially used in fast mode and switched to slow speed near the end to improve accuracy. For resin samples with low water content, the sample weighing amount can be appropriately increased to improve the detection signal-to-noise ratio.
| Parameter categories | Recommended settings and considerations |
| Sample volume | Adjust according to the estimated moisture content, usually 1-5 grams |
| Titration speed | Dynamic control, slow titration before the end |
| Stirring conditions | Stir evenly at medium speed to avoid splashing |
| Solvent system | The appropriate solvent is selected according to the solubility of the resin |
| Endpoint determination threshold | Set according to standard calibration |
Method validation
To ensure the accuracy of measurement results, the instrument is regularly calibrated and validated using standard hydrates such as sodium tartrate or standard samples with known water content. It is recommended to perform repeatable experiments and spike recovery experiments to evaluate the precision and accuracy of the method. For the same batch of resin samples, the number of parallel assays should be no less than three times, and the relative standard deviation should be calculated. Any changes in the sample preparation steps will need to be re-validated for method suitability.
Common distractions
Some components in the resin particles may interfere with the Karl Fischer reaction. For example, resins containing aldehyde ketone groups may react with methanol in the reagent as a by-product, resulting in high results. Unsaturated bonds may consume iodine. For such interference, consider changing to a specialized reagent (e.g., aldehyde and ketone special reagent without methanol) or using a Coulomb Karl Fischer instrument for determination. In addition, the presence of volatile alkaline or acidic substances in the sample may also affect the pH environment, which needs to be overcome by adjusting the buffer capacity of the reagent or using a buffer solvent system.
The results show
The moisture content of resin is usually expressed as a percentage of mass (%) and is calculated as:
Moisture content (%) = (V × T / W) × 100
Where V is the volume of reagent consumed (ml), T is the titration of the reagent (mg H₂O/ml), and W is the mass of the sample (g). Accurate moisture data is critical for controlling the resin synthesis process, evaluating storage stability, predicting processing properties (e.g., injection molding bubble generation), and the mechanical properties of the final product.
References
General method for Karl Fischer moisture determination. GB/T 6283-2008.
Plastics Polymer Dispersions and Resins Moisture Determination Karl Fischer method. ISO 15512:2019.
Handbook of Analytical Chemistry, Electrochemical Analysis and Moisture Determination. Chemical Industry Press.