The core of determining whether the platinum ring/platinum plate of the interfacial tension meter needs to be replaced is based on three dimensions: its physical state, performance stability, and reliability of the test results, combined with observation, testing verification, and standard calibration in daily use for comprehensive judgment. Platinum ring/platinum plate, as the core sensor for measuring interfacial tension (calculated by the force of pulling up the liquid film), directly determines the measurement accuracy based on its integrity, cleanliness, and physical properties. The specific judgment method is as follows:
1、 Judging by "physical appearance observation": visually identify obvious damage
Before daily use/after cleaning, use a strong light (such as a flashlight) or microscope (10-20 times) to observe the surface and structure of the platinum ring/platinum plate. If the following situations occur, it usually needs to be replaced directly:
1. "Structural integrity damage" of platinum rings/platinum plates
Fracture or gap:
The circular structure of platinum rings (especially the edges and welding points of the rings) may have fractures, gaps, or local defects (such as a broken section of the ring or obvious notches at the edges).
Mechanism of influence: Fracture can cause a change in the effective circumference of the pulled liquid film (reducing the actual area in contact with the liquid), directly resulting in a smaller measured "force value" and a lower calculated interfacial tension value (such as an actual tension of 50mN/m, which may be measured as 40mN/m after fracture), and cannot be corrected through calibration.
Deformation or distortion:
The platinum ring shows obvious deformation (such as the ring surface not being horizontal, the ring becoming elliptical or flat), or the platinum plate shows bending or curling edges (such as the edge of the plate curling upwards and unable to make parallel contact with the liquid surface).
Mechanism of influence: Deformation can cause abnormal "liquid film contact angle" (non-standard 90 ° contact), rendering the conversion formula for "force tension" (such as the Wilhelmy hanging plate method and the DuNo ü y ring method formula) ineffective, and resulting in extremely poor repeatability of measurement results (differences of more than 5% in multiple measurements of the same sample).
Welding point detachment or corrosion:
The welding point between the platinum ring and the connecting rod may become loose or detached, or the welding point may have holes or oxide layer peeling due to corrosion (such as long-term exposure to strong acidic and alkaline solutions).
Mechanism of influence: Loose welding points can cause the ring to shake during measurement, resulting in "noise" in the signal captured by the force sensor; Corrosion can alter the effective mass of platinum, interfere with "buoyancy correction" (the buoyancy of the ring/plate itself needs to be deducted during measurement), and lead to increased errors.
2. Surface "permanent pollution or corrosion"
Stubborn stains that cannot be cleaned:
After the standard cleaning process (such as "soaking in ethanol → rinsing with deionized water → burning with an alcohol lamp until red hot → cooling"), stubborn stains such as black and brown still remain on the surface (such as long-term measurement of solutions containing grease and high molecular weight polymers, where the residue adheres after carbonization).
Mechanism of influence: Stains can change the "hydrophilicity/hydrophobicity" of platinum surfaces (such as the originally hydrophilic platinum surface becoming hydrophobic), causing the liquid film to not adhere evenly, and the pulled up liquid film is prone to rupture, resulting in large fluctuations in measurement values (such as continuous measurements of 3 times, with results of 48, 55, and 42mN/m).
Surface oxidation or corrosion marks:
A gray white or black oxide layer appears on the surface (such as long-term exposure to air without cleaning, or contact with strong oxidizing solutions such as potassium permanganate and nitric acid), or pitting corrosion holes appear (contact with strong acidic and alkaline solutions such as hydrochloric acid and sodium hydroxide).
Mechanism of influence: The oxide layer will increase the surface roughness of platinum, causing additional "adhesion force" when the liquid film comes into contact, resulting in measured force values being overestimated and tension values being overestimated; Corrosive pores can disrupt the uniformity of the surface and also lead to unstable adhesion of the liquid film.
2、 Judging through "performance testing verification": Quantitative evaluation accuracy deviation
If there is no obvious damage to the appearance, but the measurement results are abnormal, quantitative methods such as "standard sample testing", "repeatability verification", "calibration comparison" should be used to determine whether replacement is needed:
1. The measured value of the standard solution deviates from the allowable range
Testing method: Select a standard solution with known interfacial tension (such as a surface tension of 72.0 ± 0.5mN/m for deionized water at 25 ℃); The surface tension of n-butanol solution is 24.6 ± 0.3mN/m, and the platinum ring/platinum plate to be judged is measured three times according to the standard procedure, and the average value is calculated.
Judgment criteria: If the deviation between the measured average value and the standard value is greater than ± 2% (or exceeds the allowable error specified in the instrument manual, such as ± 1mN/m), and the deviation cannot be eliminated after excluding factors such as "instrument calibration failure, operational errors, and abnormal environmental temperature and humidity", replacement is required.
Example: When measuring deionized water at 25 ℃, the standard value is 72.0mN/m. If the average measurement value is 70.0mN/m (deviation -2.8%) and the instrument is recalibrated and deionized water is replaced, it still remains at 70.0mN/m, indicating that the performance of the platinum ring/platinum plate has deteriorated.
2. Unstable repeatability test results
Test method: Measure the same sample (such as deionized water or sample solution from the same batch) continuously for 5 times and calculate the "relative standard deviation (RSD)".
Judgment criteria: If the RSD is greater than 2% (the repeatability requirement of a conventional interfacial tension meter is RSD ≤ 1%), and after excluding factors such as "liquid level fluctuations, instrument vibration, and inconsistent operating methods", the repeatability is still poor, then replacement is necessary.
Mechanism of influence: Poor repeatability is usually caused by the uneven surface state of platinum rings/plates (such as local stains and small deformations), resulting in different liquid film states and large fluctuations in force signals each time they are pulled up.
3. Calibration comparison with "spare platinum ring/platinum plate"
Testing method: If there are spare and verified qualified platinum rings/plates in the laboratory, measure the same standard solution using the "to be judged" and "spare" parts respectively, and compare the measurement results of the two.
Judgment criteria: If the difference between the two is greater than ± 1.5mN/m (or exceeds the repeatability error range of the instrument), and the measured value of the spare part is consistent with the standard value, it indicates that the part to be judged has failed and needs to be replaced.
Example: The spare part measured deionized water to be 72.1mN/m (compliant with the standard), while the pending part measured 69.5mN/m, with a difference of 2.6mN/m, indicating that the performance of the pending part no longer meets the requirements.