Platinum Ring Method for Measuring Interface Tension: The Complete Physical Process from Liquid Film Formation to Fracture

The circular ring method (platinum ring method) is one of the most classic methods for measuring surface tension and interfacial tension, with a history dating back over a hundred years. It is still a testing method specified in the national standard GB/T 6541. Only by understanding its physical principles can we understand why the operational details are so meticulous.

Basic Principles

Place a horizontally placed platinum ring below the liquid surface (or oil-water interface) and slowly lift it upwards. When the platinum ring is pulled upwards, a downward extending liquid column membrane will form below the ring - like pulling apart a liquid ring. As it continues to rise, the liquid column membrane is pulled thinner and thinner until a critical point at which the liquid film suddenly ruptures.

During the entire lifting process, the platinum ring is subjected to two directions of force: upward lifting force and downward liquid film tension. The maximum lifting force corresponding to the moment of liquid film rupture is the key reading for measurement.

Calculation formula

The most basic calculation relationship: M=mg/2L

Among them, M is the tension value, mg is the maximum force when the liquid film ruptures, and L is the total circumference of the platinum wire (the inner and outer circles of the platinum ring, so it is 2L).

However, the actual liquid column membrane is not a standard cylindrical shape and will undergo some bending deformation, so a correction factor F needs to be introduced. The correction formula provided by Zuidema and Waters takes into account the average radius of the platinum wire (Rs), the average radius of the platinum ring (Rh), the lower liquid density (P ₀), and the upper liquid or gas density (P ₁).

The instrument of HZZL-3 will automatically complete this correction calculation. The operator only needs to correctly input these four parameters in the parameter settings, and the final displayed tension value is the accurate result after correction.

Four factors that affect measurement accuracy

The manual clearly lists four factors that affect the measurement results:

① The average radius of the platinum wire ring and the radius of the platinum wire - precise input parameters are required. ② Density difference of the liquid - the larger the density difference between light and heavy liquids, the more significant the correction. ③ Purity of the liquid - electrolyte impurities will seriously reduce the tension value, and sample purity is a prerequisite. ④ Environmental temperature - temperature directly affects the thermal motion of liquid molecules, which in turn affects the tension value. HZZL-3 has a built-in temperature sensor that automatically compensates

By mastering these four factors, one can understand why platinum rings must be kept clean, why precise measurement of current temperature is necessary, and why parameter input must be accurate.