EN 14370 Surfactants - Determination of Surface Tension

The European standard "EN 14370 Surfactants - Determination of surface tension" describes test methods for determining the surface tension of liquids, especially surfactant solutions. These methods are suitable for determining the static surface tension of liquids, e.g. inorganic and organic liquids and solutions.

In principle, it is measured by the surface tension on a plate placed vertically in contact with the liquid and completely wetted by the liquid (static method) or by the force F required to withdraw a horizontally suspended stirrup or loop from the liquid surface (quasi-static method). The surface tension is obtained by calculation.

In the static method, the plate is stationary so that an equilibrium value is obtained. The quasi-static method requires the ring or stirrup to move. Deviations from equilibrium are minimized by using sufficiently small and slow changes in the ring or stirrup position during measurement.

The equipment used in these tests includes:

• Tensiometer: This device shall be designed for a plate, stirrup or ring and shall consist essentially of two parts: a support for the sample container in the form of a small horizontal platform which can be moved up and down (for example using a micrometer screw) and a device for measuring the force exerted on the plate, stirrup or ring. The measurement uncertainty shall not exceed plus/minus 10 N, which corresponds to a maximum error of plus/minus 0,1 mg in determining the mass. Instead of a torsion balance, a lever balance or an electronic balance (laboratory, analytical or microbalance) may be used as an alternative. To obtain higher efficiency and repeatability, an automatic tensiometer shall be used, consisting of a balance, a motor-driven platform and an evaluation unit.
• Plate: The plate consists of a platinum or platinum/iridium sheet. It is approximately 20 mm long, 0,2 mm thick and at least 10 mm high and is roughened to increase wetting power. Satisfactory roughening can be achieved by sandblasting, sandpapering or sintering a layer of platinum black onto the platinum sheet. The plate is suspended from a point on the axis of symmetry. The platinum, platinum/iridium sheet should have an effective height of at least 10 mm to keep wetting errors as low as possible.
• Stirrups: The stirrups should be made of platinum or platinum/iridium wire. The measuring wire should be 0,1 plus/minus 0,01) mm in diameter and 10 mm to 40 mm long. The two branches should be at least 10 mm long. Two small platinum balls can be attached to the lower ends of these branches as counterweights or alternatively the lower ends can be connected with a platinum wire.
• Ring: The ring should be made of platinum or platinum/iridium wire with a thickness not greater than 0,4 mm and an average circumference of 60 mm (e.g. inner diameter 18,7 mm, outer diameter 19,5 mm). It should be suspended horizontally from a wire loop on a metal rod connecting it to the tensiometer.
• Sample container: The sample container for the liquid tested should be a glass container suitable for use in a thermostat. It should have sufficient capacity for the liquid and, if necessary, be protected from evaporation. It should be manufactured in such a way that the temperature of the gas phase above the liquid can also be kept constant by the thermostat. A thermostat is not required for measurements at room temperature. The sample container should consist of a cylindrical glass container with an internal diameter of at least 45 mm for use by the ring method, stirrup method or plate method. If a container with a diameter smaller than 45 mm2 has to be used, for example if only a small amount of sample is present, wall effects may cause an error in the surface tension measurement. The magnitude of this error depends on the distance between the wall of the container and the edge of the ring or stirrup.

Among the numerous testing, measurement, analysis and evaluation studies carried out for businesses in various sectors, our organization also provides testing services defined in the "EN 14370 Surfactants - Determination of surface tension" standard with its trained and expert staff and advanced technological equipment.