Introduction
The pH of water-based inks is one of the key physical and chemical indicators, which directly affects the stability of the printing process, the drying speed, and the durability of the final print. Accurate measurement with a pH meter is the core means of controlling this indicator. This article will systematically explain the technical points of using pH meters to measure the pH of water-based inks, covering the principles, calibration, measurement steps, maintenance and influencing factors, aiming to provide professional and practical operation guidelines for relevant technicians.
Measurement principle
pH meters determine the hydrogen ion activity of a solution by measuring the potential difference between the glass electrode and the reference electrode, and the relationship follows the Nernst equation: E = E0 - (2.303RT/F) * pH。 where E is the measurement potential, E0is the standard potential, R is the gas constant, T is the absolute temperature, and F is the Faraday constant. For water-based inks, their composition is complex, often containing resins, pigments, additives, etc., and may have the characteristics of high viscosity and easily polluting electrodes. Therefore, choose a laboratory-grade pH meter with a rugged, anti-contamination composite electrode and ensure that it is temperature compensated.
Calibration Points:
Calibration is a prerequisite for ensuring measurement accuracy. A two-point calibration method is recommended, and the selection of calibration buffer should cover the expected pH range of the sample being tested.
| Calibration Steps: | Key operations and instructions |
| Prepare the buffer | Use pH 4.01, 7.00, 9.21 (25°C) standard buffers, with pH 4.01 and 7.00 preferred. The buffer should be fresh and free of contamination. |
| Electrode pretreatment | Rinse the electrode with deionized water and gently blot it with filter paper to avoid wiping the sensitive glass film. |
| Sequential calibration | Calibrate the neutral point (e.g., pH 7.00) first, then the acidic or basic point. Stir well during calibration and confirm when the reading is stable. |
| Calibration verification | After calibration, validate with another buffer (e.g., pH 9.21) with an error of no more than ±0.05 pH units. |
Sample measurement steps
The preparation and measurement methods of water-based ink samples had a significant impact on the results.
| Step by step | Specific operations and precautions |
| Sample preparation | Stir the ink sample well to avoid air bubbles. If necessary, it can be diluted with deionized water in a fixed ratio and the dilution status is noted in the report. |
| Measurement conditions | The control measurement ambient temperature is stable, and it is recommended to perform at 25±1°C. Use a magnetic stirrer to stir the sample evenly at low speed. |
| Electrode immersion | The calibrated electrode is immersed in the sample to ensure that the liquid network (reference electrode salt bridge interface) is completely submerged. Avoid the electrode touching the container wall. |
| Reading records | Record the pH value after the displayed value stabilizes (usually 30-60 seconds). Record the sample temperature at the same time. |
| Follow-up cleaning | Rinse the electrode thoroughly with deionized water immediately after measurement to prevent the ink residue from drying and clogging the liquid network. |
Electrode maintenance
Proper maintenance extends electrode life and ensures measurement accuracy. After each use, rinse with deionized water. If the electrode is contaminated with ink, it can be soaked in a mild special cleaning solution according to the manufacturer's recommendation. When not used for a long time, the electrode head should be stored in an electrode protection solution containing potassium chloride, and should not be allowed to dry out.
Main influencing factors and data interpretation
The following factors need to be paid attention to when measuring the pH value of water-based inks: First, the temperature has a significant impact, and the automatic temperature compensation function of the instrument must be enabled. Secondly, organic solvents or polymers in inks may cause the electrode response to slow down, and sufficient stabilization time is required. Furthermore, changes in the ionic strength of the sample may lead to small changes in the "junction potential". Finally, the measurement data should be interpreted in conjunction with the production process standards, and the pH value of water-based inks is usually controlled within a narrow range (e.g., 8.0-9.5) to maintain the stability and drying performance of the resin. Accidental errors may occur in a single measurement, and parallel sample determination is recommended.
Conclusion
Accurate pH measurement of water-based inks using a pH meter is a systematic technical effort that involves specification calibration, rigorous sample handling, correct measurement operations, and ongoing electrode maintenance. A deep understanding of measurement principles and influencing factors, strict adherence to operating procedures, is the basis for obtaining reliable data and effectively guiding production and quality control.
References
GB/T 9724-2007, General principles for pH determination of chemical reagents.
ASTM E70, Standard Test Method for Determination of pH in Aqueous Solutions Using Glass Electrodes.
Printing Technology - Process Control - Part 5: Inks for Screen Printing, Related Technical Reports.