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The determination of pH value in paper mills is a critical component of quality control, and the testing procedures typically involve multiple standardized steps and specialized instruments. The following outlines a comprehensive and systematic approach to pH measurement for paper or pulp, based on established methodologies:
1. Preparatory Procedures
Instrument Preparation: A precision pH meter capable of accurate readings to 0.1 pH units and equipped with a temperature compensation function should be used. A flat-surface electrode assembly-comprising a glass electrode and a reference electrode-or a pH meter with an integrated flat electrode configuration should be selected depending on the specific method. Additional tools include a wash bottle, dropper, beaker (e.g., 50 mL), and stopwatch.
Sample Preparation: Depending on the type of paper or pulp, an appropriate sample quantity must be prepared. For paper samples, cut several pieces measuring 5–10 mm², totaling approximately 2 g in mass (accurate to 0.1 g).
2. Analytical Methods
Three primary methods are commonly employed for pH determination in paper mills: thermal extraction, cold extraction, and non-destructive surface testing. Detailed procedures are as follows:
a) Thermal Extraction Method
Sample Treatment: Place the prepared paper samples into a conical flask. Add 100 mL of distilled water that has been previously boiled, ensuring complete submersion of the sample. Heat the mixture gently under reflux for 1 hour, then allow it to cool to 20–25 °C, permitting full precipitation of fibers.
Measurement: Transfer the supernatant liquid to a small beaker. Measure the pH using the calibrated pH meter. Record the reading once the value stabilizes.
b) Cold Extraction Method
Sample Treatment: Introduce the cut paper samples into a conical flask and add 100 mL of distilled water at 20–25 °C. Allow the mixture to stand for 1 hour, agitating the flask at least once during this period to ensure thorough wetting of the fibers.
Measurement: Decant the solution into a clean beaker and measure the pH with the pH meter. Record the stable reading.
c) Non-Destructive Surface Testing Method (suitable for valuable artifacts or irreplaceable samples)
Sample Mounting: Securely fix the paper sample onto a rubber backing plate.
Measurement Setup: Apply a single drop of neutral distilled water onto the paper surface. Insert the flat electrode into the droplet so that it makes contact with the paper. Maintain this configuration for a specified duration (e.g., 10 minutes) to achieve equilibrium.
Measurement: Using a pH meter with a flat electrode, measure the surface pH. Repeat measurements at multiple designated points across the sample and calculate the average value to enhance accuracy.
3. Key Considerations
- Ensure electrodes are clean and dry prior to use to prevent contamination that could compromise measurement accuracy.
- Adjust testing parameters according to the specific characteristics of the paper or pulp type.
- Adhere strictly to relevant national or industry standards to ensure data reliability and reproducibility.
4. Conclusion
Accurate pH assessment of paper and pulp is essential for maintaining product quality and process consistency in paper manufacturing. By selecting the appropriate analytical method and adhering to standardized protocols, reliable and repeatable results can be achieved, thereby supporting effective production management and continuous quality improvement.
